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NDT

1. Non-Destructive Testing (NDT)
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2. Introduction to Non-Destructive Testing Techniques
Defectology Page 1 of 12
Defectology
The aim of non-destructive inspection is to determine if the object being inspected is
to be accepted or rejected. During the inspection, the inspector looks for
discontinuities in the object and idenefies their nature and size. Then, those
discontiouities are evaluated according to an acceptance criterion to determine if they
are considered to be defects (the presence of defects mans that the object will be
rejected).
A Discontinuity is defined as an imperfection or interruption in the normal physical
characteristics or structure of an object (crack, porosity, inhomogeneity, etc.). On the
other hand, a Defect is defined as a flaw or flaws that by nature or accumulated effect
render a part or product unable to meet minimum applicable acceptance standards or
specifications (defect designates rejectability).
It should be clear that a discontinuity is not necessarily a defect. Any imperfection that
is found by the inspector is called a discontinuity until it can be identified and
evaluated as to the effect it will have on the service of the part or to the requirements
of the specification. A certain discontinuity may be considered to be a defect in some
cases and not a defect in some other cases because the defenetion of defect changes
with the type of component, its construction, its materials and the specifications or
codes being used.
Types of Discontinuities
Discontinuities are generally categorized according to the stage of the manufacturing
or use in which they initiate.
Therefore, discontinuities are categorized in four groups which are:
 Inherent discontinuities
 Primary processing discontinuities
 Secondary procession discontinuities
 Service discontinuities

INHERENT DISCONTINUITIES
This group refers to the discontinuities that originate during the initial casting process
(when the metal is casted into ingots for further processing) and also it includes the
discontinuities that are produced when metal is casted as parts of any given shape. The
initial casting discontinuities are usually removed by chopping the ingots but some of
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Defectology Page 2 of 12
them remain and further change their shape and nature during the subsequent
manufacturing operations.
Cold Shut
Cold shut occurs usually during the casting of parts because of
imperfect fusion between two streams of molten metal that
converged together. It could be on the surface or subsurface. It
could be attributed to sluggish molten metal, surging or
interruption in pouring, or any factor that prevents the fusion of
two meeting streams.
Pipe
During solidification of molten material it shrinks causing an
inverted-cone shaped cavity in the top of the ingot. It could be
on the surface or subsurface. If this defected region is not cut
out completely before further processing (rolling or forging) it
will show up in the final product as an elongated subsurface
discontinuity. Also, pipe could occur during extrusion when the
oxidized surface of the billet flows inwards toward the center of
the extruded bar.
Shrinkage Cavities
Shrinkage cavities are subsurface discontinuities that are found in
casted parts. They are caused by the lack of enough molten metal
to fill the space created by shrinkage (similar to pipe in an ingot).
Micro-shrinkage Cavities
Micro-shrinkage cavities are aggregates of subsurface discontinuities that are found in
casted parts. They are usually found close to the gate and they occur if
metal at the gate solidifies while some of the metal beneath is still
molten. Also, micro-shrinkage could be found deeper in the part when
molten metal enters from the light section into heavy section where
metal could solidify in the light section before the heavy section.
Hot Tears
Hot tears occurs when low melting point materials segregate during
solidification and thus when they try to shrink during solidification
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Defectology Page 3 of 12
cracks and tears will develop because the surrounding material has already solidified.
Also, hot tears occur at the joining of thin
sections with larger sections because of
the difference of the cooling rate and thus
solidification.
Blowholes and Porosity
Blowholes and porosity are small rounded cavities found at the
surface or near surface of castings and they are caused by the
entrapped gasses that could not escape during solidification.
Blowholes are caused by gases released from the mold itself
(external gases) while porosity is caused by gases entrapped in the
molten material (internal gases). During subsequent manufacturing
operations these gas pockets get flattened or elongated or fused
shut.
Nonmetallic Inclusions
Nonmetallic (or slag) inclusions are usually oxides, sulfides or
silicates that remained with the molten metal during original
casting. The properties of those inclusions are different from the
metal and usually they have irregular shapes and discontinuous
nature therefore they serve as stress raisers that limit the ability
of the material to withstand stresses.
Segregation
Segregation is localized differences in material composition (and thus mechanical
properties) caused by the concentration of some alloying elements in limited areas.
These compositional differences may be equalized during subsequent hot working
processes but some still remain.

PRIMARY PROCESSING DISCONTINUITIES
This group refers to the discontinuities that originate during hot or cold forming
processes (extrusion, forging, rolling, drawing, welding, etc.). Also, some of the
inherent discontinuities in the material could propagate and become significant.

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Seams
Seams are elongated surface discontinuities that occur in bars
during rolling or drawing operations. They result due to under-filled
areas that are closed shut during rolling passes. Those under-filled
areas may result because of blowholes or cracks in the material.
Also seams may result from the use of faulty, poorly lubricated or
oversized dies.
Lamination
Laminations are thin flat subsurface separations that are parallel to the surface of
plates. They may result from inherent discontinuities (pipe, inclusions, porosity, etc.)
that are flattened during the rolling process.
Stringers
Stringers are elongated subsurface discontinuities that are found in bars (they run in
the axial direction). They result from the flattening and lengthening of nonmetallic
inclusions during the rolling process.
Cupping
Cupping is a subsurface discontinuity that may occur in bars
during extrusion or sever cold drawing. It is a series of cone-
shaped internal ruptures that happen because the interior
of the material cannot flow as fast as the surface where that
causes stress buildup and thus rupture.
Cooling Cracks
Cooling cracks may occur on the surface of bars after rolling operations due to stresses
developed by uneven cooling. They run in the axial direction (similar to seams) but
unlike seams, they do not have surface oxidation.
Forging and Rolling Laps
Laps are elongated surface discontinuities that occur during
rolling or forging operations due to the presence of some
excessive material (fin) that is folded over. They may result
because of oversized blanks or improper handling of the material
in the die.
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Internal or External Bursts
Internal bursts are found in bars and forgings formed
at excessive temperatures due to presence of
inherent discontinuities that are pulled apart by the
tensile forces developed during the forming
operation.
External bursts occur when the forming section is too severe or the sections are thin.
Slugs
Slugs are surface discontinuities found on the inner surface of
seamless (extruded) tubes. They occur when some metallic
pieces that are stuck on the mandrel, are torn and fused back
on the inner surface of the tube.
Gouging
Gouging is surface tearing found on the inner surface of
seamless (extruded) tubes and it is caused by excessive friction
between the mandrel and the inner surface of the tube.
Hydrogen Flakes
Hydrogen is available during manufacturing operations (from decomposition of water
vapor or hydrocarbons “oil”, atmosphere, etc.) and it dissolves in material at
temperatures above 200° C. Hydrogen flakes are thin subsurface discontinuities that
develop during cooling of large size parts produced by forging or rolling because of the
entrapment of hydrogen resulting from rapid cooling.

Welding Discontinuities
Several types of discontinuities result from welding operations. Only the discontinuities
associated with fusion welding processes (arc welding, gas welding, etc.) are presented
here.
Cold Cracks
Cold cracks, also known as delayed cracks, are hydrogen induced surface
or subsurface cracks that appear in the heat affected zone or the weld
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metal during cooling or after a period of time (hours or even days). The sources of
hydrogen which leads to this type of cracks may include moisture in the electrode
shielding, the shielding gas or base metal surface, or contamination of the base metal
with hydrocarbon (oil or grease).

Hot Cracks
Hot cracks include several types of cracks that occur at elevated temperatures in the
weld metal or heat affected zone. In general, hot cracks are usually associated with
steels having high sulfur content. The common types of hot cracks include:
Solidification Cracks: This type occurs near the solidification temperature of the
weld metal. They are caused by the presence of low melting point constituents
(such as iron sulfides) that segregate during solicitation then the shrinkage of the
solidified material causes cracks to open up.
 Centerline Crack is a longitudinal crack along the centerline of the weld bead.
It occurs because the low melting point impurities move to the center of the
wield pool as the solidification progresses from the
weld toe to the center, then shrinkage stresses of
the solidified material causes cracking along the
centerline. The likelihood of centerline cracking
increases when the travel speed is high or the
depth-to-width ratio is high.
 Crater Crack which occurs in the crater formed at
the termination of the weld pass. Crater cracks are
mostly star shaped and they are caused by three
dimensional shrinkage stresses. The likelihood of
crater cracks increases when welding is terminated
suddenly.
Liquidation Cracks: This type, also known as hot tearing, occurs in the heat affected
zone when the temperature in that region reaches to the melting temperature of
low melting point constituents causing them to liquidate and segregate at grain
boundaries. As the weld cools down, shrinkage stresses causes the formation of
small micro-scale cracks which later might link up due to applied stresses to form a
continuous surface or subsurface crack.

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Lamellar Tearing
Lamellar tearing is a subsurface discontinuity that occurs in rolled
plates having high content of nonmetallic inclusions. Those
inclusions have low strength and they are fattened during roiling,
thus they can be torn underneath the welds because of shrinkage
stresses in the through thickness direction.
Lack of Fusion
Lack of fusion is the failure of the filler metal to fuse with the
adjacent base metal (or weld metal from previous pass) because the surface of base
metal did not reach to melting temperature during welding. This
typically occurs when welding large components that could
dissipate heat rapidly especially when it is at a relatively low
temperature before welding. Lack of fusion is often seen at the
beginning of the first pass and in such case it is commonly called a
cold start. Also, lack of fusion could occur when the surface a
previous pass is not properly cleaned from slag where slag reduces
the heating of the under-laying surface.
Lack of Penetration
Lack of penetration is insufficient (less than specified)
penetration of the weld metal into the root of the joint. This is
mostly caused by improper welding parameters such as; low
amperage, oversized electrode or improper angle, high travel
speed, or inadequate surface pre-cleaning. Also, lack of
penetration could happen when the root face is too large, the
root opening is too narrow, or the bevel angle is too small.
Porosity
Porosity is small cavities or bores, which mostly have
spherical shape, that are found on the surface of the weld
or slightly below surface. Porosity occurs when some
constituents of the molten metal vaporize causing small gas
pockets that get entrapped in the metal as it solidifies.
These small bores could have a variety of shapes but mostly
they have a spherical shape. The distribution of bores in
weld metal could be linear (linear porosity) or they could be
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clustered together (cluster porosity). In general, porosity can result from the presence
of dirt, rust or moisture on the surface of base or filler metal. Also, it could result from
high sulfur content in the base metal or excessive arc length.
Inclusions
Inclusions refer to the presence of some material, that is not supposed to be present,
in the weld metal.
Slag Inclusions: This type of inclusions mostly happens in shielded metal arc welding
(SMAW) and it occurs when the slag cannot float to the surface of the molten metal
and get entrapped in the weld metal during solidification. This could happen when;
the solidification rate is high, the weld pool viscosity is high, an oversized electrode
is used, or slag on the previous pass was not properly removed.
Tungsten Inclusions: This type of inclusions can be found in weld
metal deposited by gas tungsten arc welding (GTAW) as a result
of allowing the tungsten electrode to come in contact with the
molten metal.
Oxide Inclusions: This type of inclusions results from the presence of high melting
point oxides on the base metal which mixes with the molten material during
welding.
Undercut
Undercut is a reduction in the base metal thickness at the weld
toe. This is caused by an oversized molten weld pool which may
result from excessive amperage or oversized electrode.
Overlap
Overlap is the protrusion of the weld metal over the weld toe (due
to lack of fusion). This may be caused by insufficient amperage or
travel speed.




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Defectology Page 9 of 12
SECONDARY PROCESSING DISCONTINUITIES
This group refers to the discontinuities that originate during grinding, machining, heat
treating, plating and related finishing operations.
Grinding Cracks
Grinding cracks develop at locations where there is a localized heating of the base
metal and they are usually shallow and at right angle to the grinding direction. Such
cracks might be caused by the use of glazed wheels, inadequate coolant, excessive
feed or grinding depth.
Pickling Cracks
Pickling is chemical surface cleaning operation (using acids) used to remove unwanted
scale. Picking cracks are hydrogen induced cracks caused by the diffusion of the
hydrogen generated at the surface into the base metal. Such cracks mostly occur in
materials having high residual stresses such as hardened or cold worked metals.
Heat Treatment (Quenching) Cracks
Heat treatment cracks mostly occur during quenching especially when harsh media is
used for quenching (such as cold water, oil quenching is less harsh). During quenching
the material at the surface cools immediately upon contacting the liquid while the
material inside take relatively longer time. This difference in cooling rate causes
residual stresses in the component and could also result in cracks at the surface if the
residual tensile stress is higher than the strength of the material. In steels, austenite is
transferred into ferrite and martensite upon cooling. This transformation results in
volume increase and thus causes tensile stresses at the surface layer since the material
at the surface transformed and solidified before material at the core.
Machining Tears
Machining tears result from the use of machining tools having dull or chipped cutting
edges. Such discontinuities serve as stress raisers and can lead to premature failure of
a component especially when it is subjected to fatigue loading.
Plating Cracks
Plating cracks are surface discontinuities that can develop due to the penetration of
hydrogen or hot plating material into the base metal. Also, some plating materials
(such as chromium, copper and nickel) produce residual tensile stress which can reduce
the fatigue strength of a component.
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SERVICE DISCONTINUITIES
This group refers to the discontinuities that originate or develop while the component
is in service. The service conditions (loading, mechanical and chemical environment,
maintenance) of a component affect its expected life. Although most of service
discontinuities might look somehow similar but they are caused by different failure
mechanisms.
Fatigue Cracks
When a component is subjected to fatigue stress (cyclically applied stress), fatigue
cracks can develop and grow and that will eventually lead to failure (even if the
magnitude of the stress is smaller than the ultimate strength of the material). Fatigue
cracks normally originate at the surface but in some cases can also initiate below
surface. Fatigue cracks initiate at location with high stresses such as discontinuities
(hole, notch, scratch, sharp corner, porosity, crack, inclusions, etc.) and can also initiate
at surfaces having rough surface finish or due to the presence of tensile residual
stresses.
According to Linear-Elastic Fracture Mechanics (LEFM), fatigue failure
develops in three stages:
- Stage 1: development of one or more micro cracks due to the
cyclic local plastic deformation at a location having high stress
concentration.
- Stage 2: the cracks progress from micro cracks to larger cracks
(macro cracks) and keep growing making a smooth plateau-like fracture surfaces
which usually have beach marks that result from variation in cyclic loading. The
geometry and orientation of the beach marks can help in determining the
location where the crack originated and the progress of crack growth. The
direction of the crack during this stage is perpendicular to the direction of the
maximum principal stress.
- Stage 3: occurs during the final stress cycle where the remaining material cannot
support the load, thus resulting in a sudden fracture.
The presence of the crack can (and should) be
detected during the crack growth stage (stage
2) before the component suddenly fails.

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Creep Cracks
When a metal is at a temperature greater than 0.4 to 0.5 of its absolute melting
temperature and is subjected to a high enough value of stress (lower than the yield
strength at room temperature but it is actually higher than the yield strength at the
elevated temperature), it will keep deforming continuously until it finally fractures.
Such type of deformation is called creep and it is caused by the continuous initiation
and healing of slipping dislocation inside the grains of the material.
According to the rate of progress of the deformation, three stages of creep
deformation can be distinguished:
- Initial stage (or primary creep): the strain
rate is relatively high but slows with
increasing time due to work hardening.
- Second stage (or steady-state creep): the
strain rate reaches a minimum and
becomes steady due to the balance
between work hardening and annealing
(thermal softening). The characterized
"creep strain rate" typically refers to the
rate in this secondary stage.
- Third stage (or tertiary creep): the strain
rate exponentially increases with stress because of necking phenomena and
finally the component ruptures.
Creep cracks usually develop at the end of the second stage (the beginning of third
stage) and they eventually lead to failure. However, when a component reaches to the
third stage, its useful life is over and thus creep should be detected (by monitoring the
deformation) during the second stage which takes the longest time period of the three
stages. For steels, adding some alloying elements such as molybdenum and tungsten
can enhance creep resistance. Also, heat treatments that produce coarse grains (such
as annealing) can also increase life under creep conditions.

Stress Corrosion Cracks
Stress corrosion cracks are small sharp and usually branched cracks that result from
the combined effect of a “static” tensile stress and a corrosive environment. The stress
can either be resulting from an applied load or a residual stress. Stress corrosion cracks
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Defectology Page 12 of 12
can lead to a sudden failure of ductile materials without any previous plastic
deformation. The cracks usually initiate at the surface due to the presence of
preexisting discontinuity or due to corrosive attack on the surface. Once the cracks
initiate at the surface, corrosive material enters the cracks and attacks the material
inside forming corrosion products. The formation of the corrosion products (which
have a larger volume than original metal) inside the tight cracks
causes a wedging action which increase the stress at the crack
tip and causes the crack to grow. The corrosive environment
varies from material to material; for example saltwater is
corrosive to aluminum and stainless steel, ammonia is corrosive
to copper alloys, and sodium hydroxide is corrosive to mild
steel. The resistance to corrosion can be improved by plating
the surface of a component by appropriate material which does
not react with the environment.

Hydrogen Cracks
Hydrogen cracking, also known as hydrogen embrittlement, results from the presence
of hydrogen medium and usually occurs in conjunction with the presence of applied
tensile stress or residual stress. Hydrogen can be already present in the metal due to
previous processes such as electroplating, pickling, welding in moist atmosphere or the
melting process itself. Also, hydrogen can come from the presence of hydrogen
sulfides, water, methane or ammonia in the work
environment of a component. Hydrogen can diffuse in the
metal and initiate very small cracks at subsurface cites
(usually at the grain boundaries) subjected to high values
of stress. The presence of such cracks at several locations
causes ductile materials to show brittle fracture behavior.




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14. Stringers defect:
http://www.nasaspaceflight.com/2010/11/sts-133-structural-defectcrack-found-on-et-
137/







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15. Welding



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17. http://nptel.ac.in/courses/112107144/welding/lecture13.htm
Welding Defects
The defects in the weld can be defined as irregularities in the weld
metal produced due to incorrect welding parameters or wrong
welding procedures or wrong combination of filler metal and parent
metal.
Weld defect may be in the form of variations from the intended weld
bead shape, size and desired quality. Defects may be on the surface
or inside the weld metal. Certain defects such as cracks are never
tolerated but other defects may be acceptable within permissible
limits. Welding defects may result into the failure of components
under service condition, leading to serious accidents and causing the
loss of property and sometimes also life.
Various welding defects can be classified into groups such as
cracks, porosity, solid inclusions, lack of fusion and inadequate
penetration, imperfect shape and miscellaneous defects.
1. Cracks
Cracks may be of micro or macro size and may appear in the weld
metal or base metal or base metal and weld metal boundary.
Different categories of cracks are longitudinal cracks, transverse
cracks or radiating/star cracks and cracks in the weld crater. Cracks
occur when localized stresses exceed the ultimate tensile strength of
material. These stresses are developed due to shrinkage during
solidification of weld metal.

Fig 13.1: Various Types of Cracks in Welds
Cracks may be developed due to poor ductility of base metal, high
sulpher and carbon contents, high arc travel speeds i.e. fast cooling
rates, too concave or convex weld bead and high hydrogen contents
in the weld metal.
2. Porosity
Porosity results when the gases are entrapped in the solidifying weld
metal. These gases are generated from the flux or coating
constituents of the electrode or shielding gases used during welding
or from absorbed moisture in the coating. Rust, dust, oil and grease
present on the surface of work pieces or on electrodes are also
source of gases during welding. Porosity may be easily prevented if
work pieces are properly cleaned from rust, dust, oil and
grease.Futher, porosity can also be controlled if excessively high
welding currents, faster welding speeds and long arc lengths are
avoided flux and coated electrodes are properly baked.
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18. Fig 13.2: Different Forms of Porosities
3. Solid Inclusion
Solid inclusions may be in the form of slag or any other nonmetallic
material entrapped in the weld metal as these may not able to float
on the surface of the solidifying weld metal. During arc welding flux
either in the form of granules or coating after melting, reacts with the
molten weld metal removing oxides and other impurities in the form
of slag and it floats on the surface of weld metal due to its low
density. However, if the molten weld metal has high viscosity or too
low temperature or cools rapidly then the slag may not be released
from the weld pool and may cause inclusion.
Slag inclusion can be prevented if proper groove is selected, all the
slag from the previously deposited bead is removed, too high or too
low welding currents and long arcs are avoided.

Fig 13.3: Slag Inclusion in Weldments
4. Lack of Fusion and Inadequate or incomplete penetration:
Lack of fusion is the failure to fuse together either the base metal
and weld metal or subsequent beads in multipass welding because
of failure to raise the temperature of base metal or previously
deposited weld layer to melting point during welding. Lack of fusion
can be avoided by properly cleaning of surfaces to be welded,
selecting proper current, proper welding technique and correct size
of electrode.

Fig 13.4: Types of Lack of Fusion
Incomplete penetration means that the weld depth is not upto the
desired level or root faces have not reached to melting point in a
groove joint. If either low currents or larger arc lengths or large root
face or small root gap or too narrow groove angles are used then it
results into poor penetration.

Fig 13.5: Examples of Inadequate Penetration
5. Imperfect Shape
Imperfect shape means the variation from the desired shape and
size of the weld bead.
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19. During undercutting a notch is formed either on one side of the weld
bead or both sides in which stresses tend to concentrate and it can
result in the early failure of the joint. Main reasons for undercutting
are the excessive welding currents, long arc lengths and fast travel
speeds.
Underfilling may be due to low currents, fast travel speeds and small
size of electrodes. Overlap may occur due to low currents, longer arc
lengths and slower welding speeds.




Fig 13.6: Various Imperfect Shapes of Welds
Excessive reinforcement is formed if high currents, low voltages,
slow travel speeds and large size electrodes are used. Excessive
root penetration and sag occur if excessive high currents and slow
travel speeds are used for relatively thinner members.
Distortion is caused because of shrinkage occurring due to large
heat input during welding.
6. Miscellaneous Defects
Various miscellaneous defects may be multiple arc strikes i.e.
several arc strikes are one behind the other, spatter, grinding and
chipping marks, tack weld defects, oxidized surface in the region of
weld, unremoved slag and misalignment of weld beads if welded
from both sides in butt welds.





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20. Casting Defects http://nptel.ac.in/courses/112107144/26
The following are the major defects, which are likely to occur in sand
castings
 Gas defects
 Shrinkage cavities
 Molding material defects
 Pouring metal defects
 Mold shift
Gas Defects
A condition existing in a casting caused by the trapping of gas in the
molten metal or by mold gases evolved during the pouring of the
casting. The defects in this category can be classified into blowholes
and pinhole porosity. Blowholes are spherical or elongated cavities
present in the casting on the surface or inside the casting. Pinhole
porosity occurs due to the dissolution of hydrogen gas, which gets
entrapped during heating of molten metal.
Causes
The lower gas-passing tendency of the mold, which may be due to
lower venting, lower permeability of the mold or improper design of
the casting. The lower permeability is caused by finer grain size of
the sand, high percentage of clay in mold mixture, and excessive
moisture present in the mold.
 Metal contains gas
 Mold is too hot
 Poor mold burnout
Shrinkage Cavities
These are caused by liquid shrinkage occurring during the
solidification of the casting. To compensate for this, proper feeding of
liquid metal is required. For this reason risers are placed at the
appropriate places in the mold. Sprues may be too thin, too long or
not attached in the proper location, causing shrinkage cavities. It is
recommended to use thick sprues to avoid shrinkage cavities.
Molding Material Defects
The defects in this category are cuts and washes, metal penetration,
fusion, and swell.
Cut and washes
These appear as rough spots and areas of excess metal, and are
caused by erosion of molding sand by the flowing metal. This is
caused by the molding sand not having enough strength and the
molten metal flowing at high velocity. The former can be taken care
of by the proper choice of molding sand and the latter can be
overcome by the proper design of the gating system.
Metal penetration
When molten metal enters into the gaps between sand grains, the
result is a rough casting surface. This occurs because the sand is
coarse or no mold wash was applied on the surface of the mold. The
coarser the sand grains more the metal penetration.
Fusion
This is caused by the fusion of the sand grains with the molten metal,
giving a brittle, glassy appearance on the casting surface. The main
reason for this is that the clay or the sand particles are of lower
refractoriness or that the pouring temperature is too high.
Swell
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21. Under the influence of metallostatic forces, the mold wall may move
back causing a swell in the dimension of the casting. A proper
ramming of the mold will correct this defect.
Inclusions
Particles of slag, refractory materials, sand or deoxidation products
are trapped in the casting during pouring solidification. The provision
of choke in the gating system and the pouring basin at the top of the
mold can prevent this defect.
Pouring Metal Defects
The likely defects in this category are
 Mis-runs and
 Cold shuts.
A mis-run is caused when the metal is unable to fill the mold cavity
completely and thus leaves unfilled cavities. A mis-run results when
the metal is too cold to flow to the extremities of the mold cavity
before freezing. Long, thin sections are subject to this defect and
should be avoided in casting design.
A cold shut is caused when two streams while meeting in the mold
cavity, do not fuse together properly thus forming a discontinuity in
the casting. When the molten metal is poured into the mold cavity
through more-than-one gate, multiple liquid fronts will have to flow
together and become one solid. If the flowing metal fronts are too
cool, they may not flow together, but will leave a seam in the
part. Such a seam is called a cold shut, and can be prevented by
assuring sufficient superheat in the poured metal and thick enough
walls in the casting design.
The mis-run and cold shut defects are caused either by a lower
fluidity of the mold or when the section thickness of the casting is
very small. Fluidity can be improved by changing the composition of
the metal and by increasing the pouring temperature of the metal.
Mold Shift
The mold shift defect occurs when cope and drag or molding boxes
have not been properly aligned.

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22. http://nptel.ac.in/courses/Webcourse-contents/IIT-
ROORKEE/strength%20of%20materials/lects%20&%20picts/image/lect11/lecture11.htm
MECHANICAL PROPERTIES
LECTURE 1
Mechanical Properties:
In the course of operation or use, all the articles and structures are subjected to the action
of external forces, which create stresses that inevitably cause deformation. To keep these
stresses, and, consequently deformation within permissible limits it is necessary to select
suitable materials for the Components of various designs and to apply the most effective
heat treatment. i.e. a Comprehensive knowledge of the chief character tics of the semi-
finished metal products & finished metal articles (such as strength, ductility, toughness etc)
are essential for the purpose.
For this reason the specification of metals, used in the manufacture of various products and
structure, are based on the results of mechanical tests or we say that the mechanical tests
conducted on the specially prepared specimens (test pieces) of standard form and size on
special machines to obtained the strength, ductility and toughness characteristics of the
metal.
The conditions under which the mechanical test are conducted are of three types
(1) Static: When the load is increased slowly and gradually and the metal is loaded by
tension, compression, torsion or bending.
(2) Dynamic: when the load increases rapidly as in impact
(3) Repeated or Fatigue: (both static and impact type) . i.e. when the load repeatedly
varies in the course of test either in value or both in value and direction Now let us consider
the uniaxial tension test.
[ For application where a force comes on and off the structure a number of times, the
material cannot withstand the ultimate stress of a static tool. In such cases the ultimate
strength depends on no. of times the force is applied as the material works at a particular
stress level. Experiments one conducted to compute the number of cycles requires to break
to specimen at a particular stress when fatigue or fluctuating load is acting. Such tests are
known as fatigue tests ]
Uniaxial Tension Test: This test is of static type i.e. the load is increased comparatively
slowly from zero to a certain value.
Standard specimen's are used for the tension test.
There are two types of standard specimen's which are generally used for this purpose,
which have been shown below:
Specimen I:
This specimen utilizes a circular X-section.

Specimen II:
This specimen utilizes a rectangular X-section.

lg = gauge length i.e. length of the specimen on which we want to determine the mechanical
properties.The uniaxial tension test is carried out on tensile testing machine and the
following steps are performed to conduct this test.
(i) The ends of the specimen's are secured in the grips of the testing machine.
(ii) There is a unit for applying a load to the specimen with a hydraulic or mechanical drive.
(iii) There must be a some recording device by which you should be able to measure the
final output in the form of Load or stress. So the testing machines are often equipped with
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23. the pendulum type lever, pressure gauge and hydraulic capsule and the stress Vs strain
diagram is plotted which has the following shape.
A typical tensile test curve for the mild steel has been shown below

Nominal stress – Strain OR Conventional Stress – Strain diagrams:
Stresses are usually computed on the basis of the original area of the specimen; such
stresses are often referred to as conventional or nominal stresses.
True stress – Strain Diagram:
Since when a material is subjected to a uniaxial load, some contraction or expansion always
takes place. Thus, dividing the applied force by the corresponding actual area of the
specimen at the same instant gives the so called true stress.
SALIENT POINTS OF THE GRAPH:
(A) So it is evident form the graph that the strain is proportional to strain or elongation is
proportional to the load giving a st.line relationship. This law of proportionality is valid upto a
point A.
or we can say that point A is some ultimate point when the linear nature of the graph ceases
or there is a deviation from the linear nature. This point is known as the limit of
proportionality or the proportionality limit.
(B) For a short period beyond the point A, the material may still be elastic in the sense that
the deformations are completely recovered when the load is removed. The limiting point B is
termed as Elastic Limit .
(C) and (D) - Beyond the elastic limit plastic deformation occurs and strains are not totally
recoverable. There will be thus permanent deformation or permanent set when load is
removed. These two points are termed as upper and lower yield points respectively. The
stress at the yield point is called the yield strength.
A study a stress – strain diagrams shows that the yield point is so near the proportional limit
that for most purpose the two may be taken as one. However, it is much easier to locate the
former. For material which do not posses a well define yield points, In order to find the yield
point or yield strength, an offset method is applied.
In this method a line is drawn parallel to the straight line portion of initial stress diagram by
off setting this by an amount equal to 0.2% of the strain as shown as below and this
happens especially for the low carbon steel.

(E) A further increase in the load will cause marked deformation in the whole volume of the
metal. The maximum load which the specimen can with stand without failure is called the
load at the ultimate strength.
The highest point ‘E' of the diagram corresponds to the ultimate strength of a material.
u = Stress which the specimen can with stand without failure & is known as Ultimate
Strength or Tensile Strength.
u is equal to load at E divided by the original cross-sectional area of the bar.
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24. (F) Beyond point E, the bar begins to forms neck. The load falling from the maximum until
fracture occurs at F.
[ Beyond point E, the cross-sectional area of the specimen begins to reduce rapidly over a
relatively small length of bar and the bar is said to form a neck. This necking takes place
whilst the load reduces, and fracture of the bar finally occurs at point F ]
Note: Owing to large reduction in area produced by the necking process the actual stress at
fracture is often greater than the above value. Since the designers are interested in
maximum loads which can be carried by the complete cross section, hence the stress at
fracture is seldom of any practical value.
Percentage Elongation: ' ':
The ductility of a material in tension can be characterized by its elongation and by the
reduction in area at the cross section where fracture occurs.
It is the ratio of the extension in length of the specimen after fracture to its initial gauge
length, expressed in percent.

lI = gauge length of specimen after fracture(or the distance between the gage marks at
fracture)
lg= gauge length before fracture(i.e. initial gauge length)
For 50 mm gage length, steel may here a % elongation  of the order of 10% to 40%.
Elastic Action:
The elastic is an adjective meaning capable of recovering size and shape after deformation.
Elastic range is the range of stress below the elastic limit.

Many engineering materials behave as indicated in Fig(a) however, some behaves as
shown in figures in (b) and (c) while in elastic range. When a material behaves as in (c),
the vs is not single valued since the strain corresponding to any particular ‘  ' will
depend upon loading history.
Fig (d): It illustrates the idea of elastic and plastic strain. If a material is stressed to level (1)
and then relased the strain will return to zero beyond this plastic deformation remains.
If a material is stressed to level (2) and then released, the material will recover the
amount (2 2p ), where 2p is the plastic strain remaining after the load is removed.
Similarly for level (3) the plastic strain will be3p.
Ductile and Brittle Materials:
Based on this behaviour, the materials may be classified as ductile or brittle materials
Ductile Materials:
It we just examine the earlier tension curve one can notice that the extension of the
materials over the plastic range is considerably in excess of that associated with elastic
loading. The Capacity of materials to allow these large deformations or large extensions
without failure is termed as ductility. The materials with high ductility are termed as ductile
materials.
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25. Brittle Materials:
A brittle material is one which exhibits a relatively small extensions or deformations to
fracture, so that the partially plastic region of the tensile test graph is much reduced.
This type of graph is shown by the cast iron or steels with high carbon contents or concrete.

Conditions Affecting Mechanical Properties:
The Mechanical properties depend on the test conditions
(1) It has been established that lowering the temperature or increasing the rate of
deformation considerably increases the resistance to plastic deformation. Thus, at low
temperature (or higher rates of deformation), metals and alloys, which are ductile at normal
room temperature may fail with brittle fracture.
(2) Notches i.e. sharp charges in cross sections have a great effect on the mechanical
properties of the metals. A Notch will cause a non – uniform distribution of stresses. They
will always contribute lowering the ductility of the materials. A notch reduces the ultimate
strength of the high strength materials. Because of the non – uniform distribution of the
stress or due to stress concentration.
(3) Grain Size: The grain size also affects the mechanical properties.
Hardness:
Hardness is the resistance of a metal to the penetration of another harder body which does
not receive a permanent set.
Hardness Tests consists in measuring the resistance to plastic deformation of layers of
metals near the surface of the specimen i.e. there are Ball indentation Tests.
Ball indentation Tests:
iThis method consists in pressing a hardened steel ball under a constant load P into a
specially prepared flat surface on the test specimen as indicated in the figures below :

After removing the load an indentation remains on the surface of the test specimen. If area
of the spherical surface in the indentation is denoted as F sq. mm. Brinell Hardness number
is defined as :
Bhn = P / F
F is expressed in terms of D and d
D = ball diameter
d = diametric of indentation and Brinell Hardness number is given
by
Then is there is also Vicker's Hardness Number in which the ball is of conical shape.
Goto Home
LECTURE 2
Compression Test: Machines used for compression testing are basically similar to those
used for tensile testing often the same machine can be used to perform both tests.
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26. Shape of the specimen: The shapes of the specimen to be used for the different materials
are as follows:
(i) For metals and certain plastics: The specimen may be in the form of a cylinder
(ii) For building materials: Such as concrete or stone the shape of the specimen may be
in the form of a cube.
Shape of stress stain diagram
(a) Ductile materials: For ductile material such as mild steel, the load Vs compression
diagram would be as follows

(1) The ductile materials such as steel, Aluminum, and copper have stress – strain
diagrams similar to ones which we have for tensile test, there would be an elastic range
which is then followed by a plastic region.
(2) The ductile materials (steel, Aluminum, copper) proportional limits in compression test
are very much close to those in tension.
(3) In tension test, a specimen is being stretched, necking may occur, and ultimately
fracture fakes place. On the other hand when a small specimen of the ductile material is
compressed, it begins to bulge on sides and becomes barrel shaped as shown in the figure
above. With increasing load, the specimen is flattened out, thus offering increased
resistance to further shortening ( which means that the stress – strains curve goes upward )
this effect is indicated in the diagram.
Brittle materials ( in compression test )
Brittle materials in compression typically have an initial linear region followed by a region in
which the shortening increases at a higher rate than does the load. Thus, the compression
stress – strain diagram has a shape that is similar to the shape of the tensile diagram.
However, brittle materials usually reach much higher ultimate stresses in compression than
in tension.
For cast iron, the shape may be like this

Brittle materials in compression behave elastically up to certain load, and then fail suddenly
by splitting or by craking in the way as shown in figure. The brittle fracture is performed by
separation and is not accompanied by noticeable plastic deformation.
Hardness Testing:
The term ‘hardness' is one having a variety of meanings; a hard material is thought
of as one whose surface resists indentation or scratching, and which has the ability to indent
or cut other materials.
Hardness test: The hardness test is a comparative test and has been evolved mainly from
the need to have some convenient method of measuring the resistance of materials to
scratching, wear or in dentation this is also used to give a guide to overall strength of a
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27. materials, after as an inspection procedure, and has the advantage of being a non –
destructive test, in that only small indentations are lift permanently on the surface of the
specimen.
Four hardness tests are customarily used in industry namely
(i) Brinell
(ii) Vickers
(iii) Rockwell
(vi) Shore Scleroscopy
The most widely used are the first two.
In the Brinell test the indenter is a hardened steel ball which is pressed into the surface
using a known standard load. The diameter of resulting indentation is than measured using
a microscope & scale.
Units:
The units of Brinell Hardness number in S.I Unit would have been N/mm
2
or Mpa
To avoid the confusion which would have been caused of her wise Hardness numbers are
quotes as kgf / mm
2

Brinell Hardness test:
In the Brinell hardness test, a hardened steel ball is pressed into the flat surface of
a test piece using a specified force. The ball is then removed and the diameter of the
resulting indentation is measured using a microscope.
The Brinell Hardness no. ( BHN ) is defined as
BHN = P / A
Where P = Force applied to the ball.
A = curved area of the indentation
It may be shown that
D = diameter of the ball,
d = the diameter of the indentation.
In the Brinell Test, the ball diameter and applied load are constant and are selected to suit
the composition of the metal, its hardness, and selected to suit the composition of the metal,
its hardness, the thickness etc. Further, the hardness of the ball should be at least 1.7 times
than the test specimen to prevent permanent set in the ball.
Disadvantage of Brinell Hardness Test: The main disadvantage of the Brinell Hardness
test is that the Brinell hardness number is not independent of the applied load. This can be
realized from. Considering the geometry of indentations for increasing loads. As the ball is
pressed into the surface under increasing load the geometry of the indentation charges.

Here what we mean is that the geometry of the impression should not change w.r.t. load,
however the size it impression may change.
Vickers Hardness test:
The Vicker's Hardness test follows a procedure exactly a identical with that of Brinell
test, but uses a different indenter. The steel ball is replaced by a diamond, having the from
of a square – based pyramid with an angle of 136
0
between opposite faces. This is pressed
into the flat surface of the test piece using a specified force, and the diagonals of the
resulting indentation measured is using a microscope. The Hardness, expressed as a
Vicker's pyramid number is defined as the ratio F/A, where F is the force applied to the
diamond and A is the surface area of the indentation.
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28. It may be shown that

In the Vicker Test the indenters of pyramidal or conical shape are used & this overcomes
the disadvantage which is faced in Brinell test i.e. as the load increases, the geometry of the
indentation's does not change

The Variation of Hardness number with load is given below.

Advantage: Apart from the convenience the vicker's test has certain advantages over the
Brinell test.
(i) Harder material can be tested and indentation can be smaller & therefore less obtrusive
or damaging.
Upto a 300 kgf /mm
2
both tests give the same hardness number but above too the Brinell
test is unreliable.
Rockwell Hardness Test :
The Rockwell Hardness test also uses an indenter when is pressed into the flat
surface of the test piece, but differs from the Brinell and Vicker's test in that the
measurement of hardness is based on the depth of penetration, not on the surface area of
indentation. The indenter may be a conical diamond of 120
0
included angle, with a rounded
apex. It is brought into contact with the test piece, and a force F is applied.
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29. Advantages :
Rockwell tests are widely applied in industry due to rapidity and simplicity with which they
may be performed, high accuracy, and due to the small size of the impressions produced on
the surface.
IMPACT STRENGTH
Static tension tests of the un-notched specimen's do not always reveal the susceptibility of
metal to brittle fracture. This important factor is determined in impact tests. In impact tests
we use the notched specimen's.

This specimen is placed on its supports on anvil so that blow of the striker is opposite to the
notch the impact strength is defined as the energy A, required to rupture the specimen,
Impact Strength = A / f
Where f = the cross – section area of the specimen in cm
2
at fracture & obviously at notch.
The impact strength is a complex characteristic which takes into account both toughness
and strength of a material. The main purpose of notched – bar tests is to study the
simultaneous effect of stress concentration and high velocity load application
Impact test are of the severest type and facilitate brittle friction. Impact strength values
cannot be as yet is used for design calculations but these tests as rule provided for in
specifications for carbon & alloy steels. Further, it may be noted that in impact tests fracture
may be either brittle or ductile. In the case of brittle fracture, fracture occurs by separation
and is not accompanied by noticeable plastic deformation as occurs in the case of ductile
fracture.

Impact testing:
In an ‘impact test' a notched bar of material, arranged either as a cantilever or as a simply
supported beam, is broken by a single blow in such a way that the total energy required to
fracture it may be determined.
The energy required to fracture a material is of importance in cases of “shock loading' when
a component or structure may be required to absorb the K.E of a moving object.
Often a structure must be capable of receiving an accidental ‘shock load' without failing
completely, and whether it can do this will be determined not by its strength but by its ability
to absorb energy. A combination of strength and ductility will be required, since large
amounts of energy can only be absorbed by large amounts of plastic deformation. The
ability of a material to absorb a large amount of energy before breaking is often referred as
toughness, and the energy absorbed in an impact test is an obvious indication of this
property.
Impact tests are carried out on notched specimens, and the notches must not be regarded
simply as a local reduction in the cross – sectional area of the specimen, Notches – and , in
fact, surface irregularities of many kind – give rise to high local stresses, and are in practice,
a potential source of cracks.

The specimen may be of circular or square cross – section arranged either as a cantilever
or a simply supported beam.
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30. Toughness: It is defined as the ability of the material to withstand crack i.e to prevent the
transfer or propagation of cracks across its section hence causing failures. Cracks are
propagated due to stress concentraction.
Creep: Creep is the gradual increase of plastic strain in a material with time at constant
load. Particularly at elevated temperatures some materials are susceptible to this
phenomena and even under the constant load, mentioned strains can increase continually
until fractures. This form of facture is particularly relevant to the turbines blades, nuclear
rectors, furnaces rocket motors etc.
The general from of strain versus time graph or creep curve is shown below.

The general form of  Vs t graph or creep curve is shown below for two typical operation
conditions, In each case the curve can be considered to exhibit four principal features
(a) An initial strain, due to the initial application of load. In most cases this would be an
elastic strain.
(b) A primary creep region, during which he creep rate ( slope of the graph ) dimensions.
(c) A secondary creep region, when the creep rate is sensibly constant.
(d) A tertiary creep region, during which the creep rate accelerate to final fracture.
It is obvious that a material which is susceptible to creep effects should only be subjected to
stresses which keep it in secondary (st.line) region throughout its service life. This enables
the amount of creep extension to be estimated and allowed for in design.
Practice Problems:
PROB 1: A standard mild steel tensile test specimen has a diameter of 16 mm and a gauge
length of 80 mm such a specimen was tested to destruction, and the following results
obtained.
Load at yield point = 87 kN
Extension at yield point = 173 x 16
6
m
Ultimate load = 124 kN
Total extension at fracture = 24 mm
Diameter of specimen at fracture = 9.8 mm
Cross - sectional area at fracture = 75.4 mm
2

Cross - sectional Area ‘A' = 200 mm
2

Compute the followings:
(i) Modulus of elasticity of steel
(ii) The ultimate tensile stream
(iii) The yield stress
(iv) The percentage elongation
(v) The Percentage reduction in Area.
PROB 2:
A light alloy specimen has a diameter of 16mm and a gauge Length of 80 mm. When tested
in tension, the load extension graph proved linear up to a load of 6kN, at which point the
extension was 0.034 mm. Determine the limits of proportionality stress and the modulus of
elasticity of material.
Note: For a 16mm diameter specimen, the Cross – sectional area A = 200 mm
2

This is according to tables Determine the limit of proportion try stream & the modulus of
elasticity for the material.
Ans: 30 MN /m
2
, 70.5 GN /m
2

solution:

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31. Visual Inspection
(NDT)
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32. Visual Inspection (VT)
Visual is the most common inspection method
Basic principle: – illuminate the test specimen with light –
examine the specimen with the eye.
VT reveals spatter, excessive buildup, incomplete slag
removal, cracks, heat distortion, undercutting, & poor
penetration
Simple, easy to apply, quickly carried out and usually low
in cost.
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33. Visual Inspection Equipment
Magnifying Glass
The eye can not focus sharply on objects closer than
approximately 250 mm.
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34. Visual Inspection Equipment
Magnifying Mirror
Fillet gauges / Weld gauge
Fillet gauges measure
The “Legs“ of the weld
Convexity
(weld rounded outward)
Concavity
(weld rounded inward)
Flatness


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35. VT:
Uses of Weld Gauge
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36. Visual Inspection Equipment
Microscope
Bore-scope – endoscopes or endoprobes
Endoscope:
https://www.youtube.com/watch?v=9pv5Eg1PwLE
End probe:
https://www.youtube.com/watch?v=4OVWq6wG3Ic
Flexible Fiber Optic Borescope – working lengths are
normally 60 to 365 cm with diameters from 3 to 12.5 mm
Video Image-scope

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37. Borescope
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38. Endoscope
An endoscope can consist of:
a rigid or flexible tube.
a light delivery system to illuminate the object under
inspection. The light source is normally outside the object
and the light is typically directed via an optical
fiber system.
a lens system transmitting the image from the objective
lens to the viewer.
an eyepiece. Modern instruments may be videoscopes,
with no eyepiece, a camera transmits image to a screen
for image capture.

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39. 8-1
components with stated hourly operating limitations is
normally accomplished during the calendar inspection
falling nearest the hourly limitation.
In some instances, a flight hour limitation is established
to limit the number of hours that may be flown during
the calendar interval.
Aircraft operating under the flight hour system are
inspected when a specified number of flight hours are
accumulated. Components with stated hourly operating
limitations are normally replaced during the inspection
that falls nearest the hourly limitation.
Basic Inspection
Techniques/Practices
Before starting an inspection, be certain all plates,
access doors, fairings, and cowling have been opened
or removed and the structure cleaned. When opening
inspection plates and cowling and before cleaning the
area, take note of any oil or other evidence of fluid
leakage.
Preparation
In order to conduct a thorough inspection, a great
deal of paperwork and/or reference information must
be accessed and studied before actually proceeding
to the aircraft to conduct the inspection. The aircraft
logbooks must be reviewed to provide background
information and a maintenance history of the particular
aircraft. The appropriate checklist or checklists must
be utilized to ensure that no items will be forgotten or
overlooked during the inspection. Also, many addi-
tional publications must be available, either in hard
copy or in electronic format to assist in the inspections.
These additional publications may include information
provided by the aircraft and engine manufacturers,
appliance manufacturers, parts venders, and the Federal
Aviation Administration (FAA).
Inspections are visual examinations and manual checks
to determine the condition of an aircraft or component.
An aircraft inspection can range from a casual walk-
around to a detailed inspection involving complete
disassembly and the use of complex inspection aids.
An inspection system consists of several processes,
including reports made by mechanics or the pilot or
crew flying an aircraft and regularly scheduled inspec-
tions of an aircraft. An inspection system is designed
to maintain an aircraft in the best possible condition.
Thorough and repeated inspections must be considered
the backbone of a good maintenance program. Irregu-
lar and haphazard inspection will invariably result in
gradual and certain deterioration of an aircraft. The
time spent in repairing an abused aircraft often totals far
more than any time saved in hurrying through routine
inspections and maintenance.
It has been proven that regularly scheduled inspections
and preventive maintenance assure airworthiness.
Operating failures and malfunctions of equipment are
appreciably reduced if excessive wear or minor defects
are detected and corrected early. The importance of
inspections and the proper use of records concerning
these inspections cannot be overemphasized.
Airframe and engine inspections may range from
preflight inspections to detailed inspections. The time
intervals for the inspection periods vary with the models
of aircraft involved and the types of operations being
conducted. The airframe and engine manufacturer’s
instructions should be consulted when establishing
inspection intervals.
Aircraft may be inspected using flight hours as a basis
for scheduling, or on a calendar inspection system.
Under the calendar inspection system, the appropriate
inspection is performed on the expiration of a speci-
fied number of calendar weeks. The calendar inspec-
tion system is an efficient system from a maintenance
management standpoint. Scheduled replacement of
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40. 8-2
Aircraft Logs
“Aircraft logs,” as used in this handbook, is an inclu-
sive term which applies to the aircraft logbook and all
supplemental records concerned with the aircraft. They
may come in a variety of formats. For a small aircraft,
the log may indeed be a small 5" × 8" logbook. For
larger aircraft, the logbooks are often larger, in the form
of a three-ring binder. Aircraft that have been in service
for a long time are likely to have several logbooks.
The aircraft logbook is the record in which all data con-
cerning the aircraft is recorded. Information gathered
in this log is used to determine the aircraft condition,
date of inspections, time on airframe, engines and
propellers. It reflects a history of all significant events
occurring to the aircraft, its components, and acces-
sories, and provides a place for indicating compliance
with FAA airworthiness directives or manufactur-
ers’ service bulletins. The more comprehensive the
logbook, the easier it is to understand the aircraft’s
maintenance history.
When the inspections are completed, appropriate
entries must be made in the aircraft logbook certifying
that the aircraft is in an airworthy condition and may
be returned to service. When making logbook entries,
exercise special care to ensure that the entry can be
clearly understood by anyone having a need to read it
in the future. Also, if making a hand-written entry, use
good penmanship and write legibly. To some degree,
the organization, comprehensiveness, and appearance
of the aircraft logbooks have an impact on the value of
the aircraft. High quality logbooks can mean a higher
value for the aircraft.
Checklists
Always use a checklist when performing an inspection.
The checklist may be of your own design, one provided
by the manufacturer of the equipment being inspected,
or one obtained from some other source. The checklist
should include the following:
1. Fuselage and hull group.
a. Fabric and skin—for deterioration,
distortion, other evidence of failure, and
defective or insecure attachment of fittings.
b. Systems and components—for proper
installation, apparent defects, and satisfactory
operation.
c. Envelope gas bags, ballast tanks, and related
parts—for condition.
2. Cabin and cockpit group.
a. Generally—for cleanliness and loose
equipment that should be secured.
b. Seats and safety belts—for condition and
security.
c. Windows and windshields—for deterioration
and breakage.
d. Instruments—for condition, mounting,
marking, and (where practicable) for proper
operation.
e. Flight and engine controls—for proper
installation and operation.
f. Batteries—for proper installation and charge.
g. All systems—for proper installation, general
condition, apparent defects, and security of
attachment.
3. Engine and nacelle group.
a. Engine section—for visual evidence of
excessive oil, fuel, or hydraulic leaks, and
sources of such leaks.
b. Studs and nuts—for proper torquing and
obvious defects.
c. Internal engine—for cylinder compression
and for metal particles or foreign matter on
screens and sump drain plugs. If cylinder
compression is weak, check for improper
internal condition and improper internal
tolerances.
d. Engine mount—for cracks, looseness of
mounting, and looseness of engine to mount.
e. Flexible vibration dampeners—for condition
and deterioration.
f. Engine controls—for defects, proper travel,
and proper safetying.
g. Lines, hoses, and clamps—for leaks,
condition, and looseness.
h. Exhaust stacks—for cracks, defects, and
proper attachment.
i. Accessories—for apparent defects in security
of mounting.
j. All systems—for proper installation, general
condition defects, and secure attachment.
k. Cowling—for cracks and defects.
l. Ground runup and functional check—check
all powerplant controls and systems for
correct response, all instruments for proper
operation and indication.
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41. 8-3
c. Anti-icing devices—for proper operation and
obvious defects.
d. Control mechanisms—for proper operation,
secure mounting, and travel.
8. Communication and navigation group.
a. Radio and electronic equipment—for proper
installation and secure mounting.
b. Wiring and conduits—for proper routing,
secure mounting, and obvious defects.
c. Bonding and shielding—for proper
installation and condition.
d. Antennas—for condition, secure mounting,
and proper operation.
9. Miscellaneous.
a. Emergency and first aid equipment—for
general condition and proper stowage.
b. Parachutes, life rafts, flares, and so forth—
inspect in accordance with the manufacturer’s
recommendations.
c. Autopilot system—for general condition,
security of attachment, and proper operation.
Publications
Aeronautical publications are the sources of informa-
tion for guiding aviation mechanics in the operation
and maintenance of aircraft and related equipment.
The proper use of these publications will greatly aid
in the efficient operation and maintenance of all air-
craft. These include manufacturers’ service bulletins,
manuals, and catalogs; FAA regulations; airworthiness
directives; advisory circulars; and aircraft, engine and
propeller specifications.
Manufacturers’ Service Bulletins/Instructions
Service bulletins or service instructions are two of sev-
eral types of publications issued by airframe, engine,
and component manufacturers.
The bulletins may include: (1) purpose for issuing
the publication, (2) name of the applicable airframe,
engine, or component, (3) detailed instructions for
service, adjustment, modification or inspection, and
source of parts, if required and (4) estimated number
of manhours required to accomplish the job.
Maintenance Manual
The manufacturer’s aircraft maintenance manual
contains complete instructions for maintenance of all
systems and components installed in the aircraft. It
contains information for the mechanic who normally
4. Landing gear group.
a. All units—for condition and security of
attachment.
b. Shock absorbing devices—for proper oleo
fluid level.
c. Linkage, trusses, and members—for undue or
excessive wear, fatigue, and distortion.
d. Retracting and locking mechanism—for
proper operation.
e. Hydraulic lines—for leakage.
f. Electrical system—for chafing and proper
operation of switches.
g. Wheels—for cracks, defects, and condition of
bearings.
h. Tires—for wear and cuts.
i. Brakes—for proper adjustment.
j. Floats and skis—for security of attachment
and obvious defects.
5. Wing and center section.
a. All components—for condition and security.
b. Fabric and skin—for deterioration, distortion,
other evidence of failure, and security of
attachment.
c. Internal structure (spars, ribs compression
members)—for cracks, bends, and security.
d. Movable surfaces—for damage or obvious
defects, unsatisfactory fabric or skin
attachment and proper travel.
e. Control mechanism—for freedom of
movement, alignment, and security.
f. Control cables—for proper tension, fraying,
wear and proper routing through fairleads and
pulleys.
6. Empennage group.
a. Fixed surfaces—for damage or obvious
defects, loose fasteners, and security of
attachment.
b. Movable control surfaces—for damage or
obvious defects, loose fasteners, loose fabric,
or skin distortion.
c. Fabric or skin—for abrasion, tears, cuts or
defects, distortion, and deterioration.
7. Propeller group.
a. Propeller assembly—for cracks, nicks, bends,
and oil leakage.
b. Bolts—for proper torquing and safetying.
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42. 8-4
works on components, assemblies, and systems while
they are installed in the aircraft, but not for the over-
haul mechanic. A typical aircraft maintenance manual
contains:
• A description of the systems (i.e., electrical,
hydraulic, fuel, control)
• Lubrication instructions setting forth the frequency
and the lubricants and fluids which are to be used
in the various systems,
• Pressures and electrical loads applicable to the
various systems,
• Tolerances and adjustments necessary to proper
functioning of the airplane,
• Methods of leveling, raising, and towing,
• Methods of balancing control surfaces,
• Identification of primary and secondary structures,
• Frequency and extent of inspections necessary to
the proper operation of the airplane,
• Special repair methods applicable to the airplane,
• Special inspection techniques requiring x-ray,
ultrasonic, or magnetic particle inspection, and
• A list of special tools.
Overhaul Manual
The manufacturer’s overhaul manual contains brief
descriptive information and detailed step by step
instructions covering work normally performed on a
unit that has been removed from the aircraft. Simple,
inexpensive items, such as switches and relays on
which overhaul is uneconomical, are not covered in
the overhaul manual.
Structural Repair Manual
This manual contains the manufacturer’s information
and specific instructions for repairing primary and sec-
ondary structures. Typical skin, frame, rib, and stringer
repairs are covered in this manual. Also included are
material and fastener substitutions and special repair
techniques.
Illustrated Parts Catalog
This catalog presents component breakdowns of struc-
ture and equipment in disassembly sequence. Also
included are exploded views or cutaway illustrations
for all parts and equipment manufactured by the aircraft
manufacturer.
Code of Federal Regulations (CFRs)
The CFRs were established by law to provide for the
safe and orderly conduct of flight operations and to
prescribe airmen privileges and limitations. A knowl-
edge of the CFRs is necessary during the performance
of maintenance, since all work done on aircraft must
comply with CFR provisions.
Airworthiness Directives
A primary safety function of the FAA is to require
correction of unsafe conditions found in an aircraft,
aircraft engine, propeller, or appliance when such con-
ditions exist and are likely to exist or develop in other
products of the same design. The unsafe condition may
exist because of a design defect, maintenance, or other
causes. Title 14 of the Code of Federal Regulations
(14 CFR) part 39, Airworthiness Directives, defines
the authority and responsibility of the Administra-
tor for requiring the necessary corrective action. The
Airworthiness Directives (ADs) are published to notify
aircraft owners and other interested persons of unsafe
conditions and to prescribe the conditions under which
the product may continue to be operated.
Airworthiness Directives are Federal Aviation Regu-
lations and must be complied with unless specific
exemption is granted.
Airworthiness Directives may be divided into two
categories: (1) those of an emergency nature requiring
immediate compliance upon receipt and (2) those of a
less urgent nature requiring compliance within a rela-
tively longer period of time. Also, ADs may be a one-
time compliance item or a recurring item that requires
future inspection on an hourly basis (accrued flight time
since last compliance) or a calendar time basis.
The contents of ADs include the aircraft, engine, pro-
peller, or appliance model and serial numbers affected.
Also included are the compliance time or period, a
description of the difficulty experienced, and the nec-
essary corrective action.
Type Certificate Data Sheets
The type certificate data sheet (TCDS) describes the
type design and sets forth the limitations prescribed
by the applicable CFR part. It also includes any other
limitations and information found necessary for type
certification of a particular model aircraft.
Type certificate data sheets are numbered in the upper
right-hand corner of each page. This number is the
same as the type certificate number. The name of the
type certificate holder, together with all of the approved
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43. 8-5
models, appears immediately below the type certificate
number. The issue date completes this group. This
information is contained within a bordered text box
to set it off.
The data sheet is separated into one or more sections.
Each section is identified by a Roman numeral followed
by the model designation of the aircraft to which the
section pertains. The category or categories in which
the aircraft can be certificated are shown in parenthe-
ses following the model number. Also included is the
approval date shown on the type certificate.
The data sheet contains information regarding:
1. Model designation of all engines for which the
aircraft manufacturer obtained approval for use
with this model aircraft.
2. Minimum fuel grade to be used.
3. Maximum continuous and takeoff ratings of the
approved engines, including manifold pressure
(when used), rpm, and horsepower (hp).
4. Name of the manufacturer and model designation for
each propeller for which the aircraft manufacturer
obtained approval will be shown together with
the propeller limits and any operating restrictions
peculiar to the propeller or propeller engine
combination.
5. Airspeed limits in both mph and knots.
6. Center of gravity range for the extreme loading
conditions of the aircraft is given in inches from
the datum. The range may also be stated in percent
of MAC (Mean Aerodynamic Chord) for transport
category aircraft.
7. Empty weight center of gravity (CG) range (when
established) will be given as fore and aft limits in
inches from the datum. If no range exists, the word
“none” will be shown following the heading on the
data sheet.
8. Location of the datum.
9. Means provided for leveling the aircraft.
10. All pertinent maximum weights.
11. Number of seats and their moment arms.
12. Oil and fuel capacity.
13. Control surface movements.
14. Required equipment.
15. Additional or special equipment found necessary
for certification.
16. Information concerning required placards.
It is not within the scope of this handbook to list all
the items that can be shown on the type certificate data
sheets. Those items listed above serve only to acquaint
aviation mechanics with the type of information gener-
ally included on the data sheets. Type certificate data
sheets may be many pages in length. Figure 8-1 shows
a typical TCDS.
When conducting a required or routine inspection, it is
necessary to ensure that the aircraft and all the major
items on it are as defined in the type certificate data
sheets. This is called a conformity check, and verifies
that the aircraft conforms to the specifications of the
aircraft as it was originally certified. Sometimes altera-
tions are made that are not specified or authorized in the
TCDS. When that condition exists, a supplemental type
certificate (STC) will be issued. STCs are considered a
part of the permanent records of an aircraft, and should
be maintained as part of that aircraft’s logs.
Routine/Required Inspections
For the purpose of determining their overall condition,
14 CFR provides for the inspection of all civil aircraft
at specific intervals, depending generally upon the type
of operations in which they are engaged. The pilot in
command of a civil aircraft is responsible for determin-
ing whether that aircraft is in condition for safe flight.
Therefore, the aircraft must be inspected before each
flight. More detailed inspections must be conducted by
aviation maintenance technicians at least once each 12
calendar months, while inspection is required for others
after each 100 hours of flight. In other instances, an
aircraft may be inspected in accordance with a system
set up to provide for total inspection of the aircraft over
a calendar or flight time period.
To determine the specific inspection requirements
and rules for the performance of inspections, refer to
the CFR, which prescribes the requirements for the
inspection and maintenance of aircraft in various types
of operations.
Preflight/Postflight Inspections
Pilots are required to follow a checklist contained
within the Pilot’s Operating Handbook (POH) when
operating aircraft. The first section of a checklist
includes a section entitled Preflight Inspection. The
preflight inspection checklist includes a “walk-around”
section listing items that the pilot is to visually check
for general condition as he or she walks around the
airplane. Also, the pilot must ensure that fuel, oil and
other items required for flight are at the proper levels
(Continued on page 8-12)
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Figure 8-1. Type certificate data sheet.
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45. 8-7
Figure 8-1. Type certificate data sheet. (continued)
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46. 8-8
Figure 8-1. Type certificate data sheet. (continued)
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47. 8-9
Figure 8-1. Type certificate data sheet. (continued)
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Figure 8-1. Type certificate data sheet. (continued)
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Figure 8-1. Type certificate data sheet. (continued)
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50. 8-12
and not contaminated. Additionally, it is the pilot’s
responsibility to review the airworthiness certificate,
maintenance records, and other required paperwork to
verify that the aircraft is indeed airworthy. After each
flight, it is recommended that the pilot or mechanic
conduct a postflight inspection to detect any problems
that might require repair or servicing before the next
flight.
Annual/100-Hour Inspections
Title 14 of the Code of Federal Regulations (14 CFR)
part 91 discusses the basic requirements for annual
and 100-hour inspections. With some exceptions, all
aircraft must have a complete inspection annually.
Aircraft that are used for commercial purposes and are
likely to be used more frequently than noncommercial
aircraft must have this complete inspection every 100
hours. The scope and detail of items to be included in
annual and 100-hour inspections is included as appen-
dix D of 14 CFR part 43 and shown as Figure 8-2.
A properly written checklist, such as the one shown
earlier in this chapter, will include all the items of
appendix D. Although the scope and detail of annual
and 100-hour inspections is identical, there are two
significant differences. One difference involves persons
authorized to conduct them. A certified airframe and
powerplant maintenance technician can conduct a 100-
hour inspection, whereas an annual inspection must
be conducted by a certified airframe and powerplant
maintenance technician with inspection authorization
(IA). The other difference involves authorized over-
flight of the maximum 100 hours before inspection.
An aircraft may be flown up to 10 hours beyond the
100-hour limit if necessary to fly to a destination where
the inspection is to be conducted.
Progressive Inspections
Because the scope and detail of an annual inspection
is very extensive and could keep an aircraft out of
service for a considerable length of time, alternative
(a) Each person performing an annual or 100-hour
inspection shall, before that inspection, remove
or open all necessary inspection plates, access
doors, fairing, and cowling. He shall thoroughly
clean the aircraft and aircraft engine.
(b) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) the
following components of the fuselage and hull
group:
(1) Fabric and skin—for deterioration,
distortion, other evidence of failure, and
defective or insecure attachment of fittings.
(2) Systems and components—for improper
installation, apparent defects, and
unsatisfactory operation.
(3) Envelope, gas bags, ballast tanks, and
related parts—for poor condition.
(c) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) the
following components of the cabin and cockpit
group: (1) Generally—for uncleanliness and loose
equipment that might foul the controls.
(2) Seats and safety belts—for poor condition
and apparent defects.
Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft)
To Be Included in Annual and 100-Hour Inspections
Figure 8-2. Scope and detail of annual and 100-hour inspections.
(3) Windows and windshields—for
deterioration and breakage.
(4) Instruments—for poor condition,
mounting, marking, and (where practicable)
improper operation.
(5) Flight and engine controls—for improper
installation and improper operation.
(6) Batteries—for improper installation and
improper charge.
(7) All systems—for improper installation,
poor general condition, apparent and
obvious defects, and insecurity of
attachment.
(d) Each person performing an annual or 100-hour
inspection shall inspect (where applicable)
components of the engine and nacelle group as
follows: (1) Engine section—for visual evidence of
excessive oil, fuel, or hydraulic leaks, and
sources of such leaks.
(2) Studs and nuts—for improper torquing
and obvious defects.
(3) Internal engine—for cylinder compression
and for metal particles or foreign matter on
screens and sump drain plugs. If there is
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51. 8-13
weak cylinder compression, for improper
internal condition and improper internal
tolerances.
(4) Engine mount—for cracks, looseness
of mounting, and looseness of engine to
mount.
(5) Flexible vibration dampeners—for poor
condition and deterioration.
(6) Engine controls—for defects, improper
travel, and improper safetying.
(7) Lines, hoses, and clamps—for leaks,
improper condition, and looseness.
(8) Exhaust stacks—for cracks, defects, and
improper attachment.
(9) Accessories—for apparent defects in
security of mounting.
(10) All systems—for improper installation,
poor general condition, defects, and
insecure attachment.
(11) Cowling—for cracks and defects.
(e) Each person performing an annual or 100-hour
inspection shall inspect (where applicable)
the following components of the landing gear
group:
(1) All units—for poor condition and
insecurity of attachment.
(2) Shock absorbing devices—for improper
oleo fluid level.
(3) Linkages, trusses, and members—for
undue or excessive wear fatigue, and
distortion.
(4) Retracting and locking mechanism—for
improper operation.
(5) Hydraulic lines—for leakage.
(6) Electrical system—for chafing and
improper operation of switches.
(7) Wheels—for cracks, defects, and
condition of bearings.
(8) Tires—for wear and cuts.
(9) Brakes—for improper adjustment.
(10) Floats and skis—for insecure attachment
and obvious or apparent defects.
Figure 8-2. Scope and detail of annual and 100-hour inspections. (continued)
(f) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) all
components of the wing and center section
assembly for poor general condition, fabric or
skin deterioration, distortion, evidence of failure,
and insecurity of attachment.
(g) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) all
components and systems that make up the com-
plete empennage assembly for poor general
condition, fabric or skin deterioration, distortion,
evidence of failure, insecure attachment, improper
component installation, and improper component
operation.
(h) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) the
following components of the propeller group:
(1) Propeller assembly—for cracks, nicks,
binds, and oil leakage.
(2) Bolts—for improper torquing and lack of
safetying.
(3) Anti-icing devices—for improper
operations and obvious defects.
(4) Control mechanisms—for improper
operation, insecure mounting, and restricted
travel.
(i) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) the
following components of the radio group:
(1) Radio and electronic equipment—for
improper installation and insecure
mounting.
(2) Wiring and conduits—for improper
routing, insecure mounting, and obvious
defects.
(3) Bonding and shielding—for improper
installation and poor condition.
(4) Antenna including trailing antenna—for
poor condition, insecure mounting, and
improper operation.
(j) Each person performing an annual or 100-hour
inspection shall inspect (where applicable) each
installed miscellaneous item that is not otherwise
covered by this listing for improper installation
and improper operation.
Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft)
To Be Included in Annual and 100-Hour Inspections (continued)
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52. 8-14
inspection programs designed to minimize down time
may be utilized. A progressive inspection program
allows an aircraft to be inspected progressively. The
scope and detail of an annual inspection is essentially
divided into segments or phases (typically four to
six). Completion of all the phases completes a cycle
that satisfies the requirements of an annual inspection.
The advantage of such a program is that any required
segment may be completed overnight and thus enable
the aircraft to fly daily without missing any revenue
earning potential. Progressive inspection programs
include routine items such as engine oil changes and
detailed items such as flight control cable inspection.
Routine items are accomplished each time the aircraft
comes in for a phase inspection and detailed items
focus on detailed inspection of specific areas. Detailed
inspections are typically done once each cycle. A cycle
must be completed within 12 months. If all required
phases are not completed within 12 months, the remain-
ing phase inspections must be conducted before the
end of the 12th month from when the first phase was
completed.
Each registered owner or operator of an aircraft desiring
to use a progressive inspection program must submit
a written request to the FAA Flight Standards District
Office (FSDO) having jurisdiction over the area in
which the applicant is located. Title 14 of the Code
of Federal Regulations (14 CFR) part 91, §91.409(d)
Figure 8-3. 14 CFR §91.409(d) Progressive inspection.
(d) Progressive inspection. Each registered owner or
operator of an aircraft desiring to use a progressive
inspection program must submit a written request
to the FAA Flight Standards district office having
jurisdiction over the area in which the applicant
is located, and shall provide—
(1) A certificated mechanic holding an
inspection authorization, a certificated
airframe repair station, or the manufacturer
of the aircraft to supervise or conduct the
progressive inspection;
(2) A current inspection procedures manual
available and readily understandable to pilot
and maintenance personnel containing, in
detail—
(i) An explanation of the progressive
inspection, including the continuity of
inspection responsibility, the making of
reports, and the keeping of records and
technical reference material;
(ii) An inspection schedule, specifying
the intervals in hours or days when
routine and detailed inspections will be
performed and including instructions
for exceeding an inspection interval by
not more than 10 hours while en route
and for changing an inspection interval
because of service experience;
(iii) Sample routine and detailed inspection
forms and instructions for their use; and
(iv) Sample reports and records and
instructions for their use;
(3) Enough housing and equipment for
necessary disassembly and proper inspection
of the aircraft; and
(4) Appropriate current technical information
for the aircraft.
The frequency and detail of the progressive inspection
shall provide for the complete inspection of the aircraft
within each 12 calendar months and be consistent
with the manufacturer's recommendations, field
service experience, and the kind of operation in which
the aircraft is engaged. The progressive inspection
schedule must ensure that the aircraft, at all times,
will be airworthy and will conform to all applicable
FAA aircraft specifications, type certificate data sheets,
airworthiness directives, and other approved data. If
the progressive inspection is discontinued, the owner
or operator shall immediately notify the local FAA
Flight Standards district office, in writing, of the
discontinuance. After the discontinuance, the first
annual inspection under §91.409(a)(1) is due within
12 calendar months after the last complete inspection
of the aircraft under the progressive inspection. The
100-hour inspection under §91.409(b) is due within
100 hours after that complete inspection. A complete
inspection of the aircraft, for the purpose of determining
when the annual and 100-hour inspections are due,
requires a detailed inspection of the aircraft and all
its components in accordance with the progressive
inspection. A routine inspection of the aircraft and
a detailed inspection of several components is not
considered to be a complete inspection.
§91.409   Inspections.
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53. 8-15
establishes procedures to be followed for progressive
inspections and is shown in Figure 8-3.
Continuous Inspections
Continuous inspection programs are similar to pro-
gressive inspection programs, except that they apply
to large or turbine-powered aircraft and are therefore
more complicated.
Like progressive inspection programs, they require
approval by the FAA Administrator. The approval may
be sought based upon the type of operation and the
CFR parts under which the aircraft will be operated.
The maintenance program for commercially operated
aircraft must be detailed in the approved operations
specifications (OpSpecs) of the commercial certificate
holder.
Airlines utilize a continuous maintenance program
that includes both routine and detailed inspections.
However, the detailed inspections may include differ-
ent levels of detail. Often referred to as “checks,” the
A-check, B-check, C-check, and D-checks involve
increasing levels of detail. A-checks are the least com-
prehensive and occur frequently. D-checks, on the other
hand, are extremely comprehensive, involving major
disassembly, removal, overhaul, and inspection of
systems and components. They might occur only three
to six times during the service life of an aircraft.
Altimeter and Transponder Inspections
Aircraft that are operated in controlled airspace under
instrument flight rules (IFR) must have each altimeter
and static system tested in accordance with procedures
described in 14 CFR part 43, appendix E, within the
preceding 24 calendar months. Aircraft having an air
traffic control (ATC) transponder must also have each
transponder checked within the preceding 24 months.
All these checks must be conducted by appropriately
certified individuals.
ATA iSpec 2200
In an effort to standardize the format for the way in
which maintenance information is presented in aircraft
maintenance manuals, the Air Transport Association
of America (ATA) issued specifications for Manufac-
turers Technical Data. The original specification was
called ATA Spec 100. Over the years, Spec 100 has
been continuously revised and updated. Eventually,
ATA Spec 2100 was developed for electronic docu-
mentation. These two specifications evolved into one
document called ATA iSpec 2200. As a result of this
standardization, maintenance technicians can always
find information regarding a particular system in
the same section of an aircraft maintenance manual,
regardless of manufacturer. For example, if you are
seeking information about the electrical system on
any aircraft, you will always find that information in
section (chapter) 24.
The ATA Specification 100 has the aircraft divided
into systems, such as air conditioning, which covers
the basic air conditioning system (ATA 21). Number-
ing in each major system provides an arrangement for
breaking the system down into several subsystems. Late
model aircraft, both over and under the 12,500 pound
designation, have their parts manuals and maintenance
manuals arranged according to the ATA coded system.
The following abbreviated table of ATA System, Subsys-
tem, and Titles is included for familiarization purposes.
ATA Specification 100­ Systems
Sys. Sub. Title
21 AIR CONDITIONING
21 00 General
21 10 Compression
21 20 Distribution
21 30 Pressurization Control
21 40 Heating
21 50 Cooling
21 60 Temperature Control
21 70 Moisture/Air Contaminate Control
The remainder of this list shows the systems and title
with subsystems deleted in the interest of brevity. Con-
sult specific aircraft maintenance manuals for a com-
plete description of the subsystems used in them.
22 AUTO FLIGHT
23 COMMUNICATIONS
24 ELECTRICAL POWER
25 EQUIPMENT/FURNISHINGS
26 FIRE PROTECTION
27 FLIGHT CONTROLS
28 FUEL
29 HYDRAULIC POWER
30 ICE AND RAIN PROTECTION
31 INDICATING/RECORDING SYSTEMS
32 LANDING GEAR
33 LIGHTS
34 NAVIGATION
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35 OXYGEN
36 PNEUMATIC
37 VACUUM/PRESSURE
38 WATER/WASTE
39 ELECTRICAL/ELECTRONIC PANELS AND
MULTIPURPOSE COMPONENTS
49 AIRBORNE AUXILIARY POWER
51 STRUCTURES
52 DOORS
53 FUSELAGE
54 NACELLES/PYLONS
55 STABILIZERS
56 WINDOWS
57 WINGS
61 PROPELLERS
65 ROTORS
71 POWERPLANT
72 (T) TURBINE/TURBOPROP
72 (R) ENGINE RECIPROCATING
73 ENGINE FUEL AND CONTROL
74 IGNITION
75 BLEED AIR
76 ENGINE CONTROLS
77 ENGINE INDICATING
78 ENGINE EXHAUST
79 ENGINE OIL
80 STARTING
81 TURBINES (RECIPROCATING ENG)
82 WATER INJECTION
83 REMOTE GEAR BOXES (ENG DR)
Keep in mind that not all aircraft will have all these
systems installed. Small and simple aircraft have far
fewer systems than larger more complex aircraft.
Special Inspections
During the service life of an aircraft, occasions may
arise when something out of the ordinary care and
use of an aircraft might happen that could possibly
affect its airworthiness. When these situations are
encountered, special inspection procedures should
be followed to determine if damage to the aircraft
structure has occurred. The procedures outlined on the
following pages are general in nature and are intended
to acquaint the aviation mechanic with the areas which
should be inspected. As such, they are not all inclusive.
When performing any of these special inspections,
always follow the detailed procedures in the aircraft
maintenance manual. In situations where the manual
does not adequately address the situation, seek advice
from other maintenance technicians who are highly
experienced with them.
Hard or Overweight Landing Inspection
The structural stress induced by a landing depends not
only upon the gross weight at the time but also upon the
severity of impact. However, because of the difficulty
in estimating vertical velocity at the time of contact,
it is hard to judge whether or not a landing has been
sufficiently severe to cause structural damage. For this
reason, a special inspection should be performed after
a landing is made at a weight known to exceed the
design landing weight or after a rough landing, even
though the latter may have occurred when the aircraft did
not exceed the design landing weight.
Wrinkled wing skin is the most easily detected sign of
an excessive load having been imposed during a land-
ing. Another indication which can be detected easily is
fuel leakage along riveted seams. Other possible loca-
tions of damage are spar webs, bulkheads, nacelle skin
and attachments, firewall skin, and wing and fuselage
stringers. If none of these areas show adverse effects,
it is reasonable to assume that no serious damage has
occurred. If damage is detected, a more extensive
inspection and alignment check may be necessary.
Severe Turbulence Inspection/Over “G”
When an aircraft encounters a gust condition, the
airload on the wings exceeds the normal wingload
supporting the aircraft weight. The gust tends to
accelerate the aircraft while its inertia acts to resist
this change. If the combination of gust velocity and
airspeed is too severe, the induced stress can cause
structural damage.
A special inspection should be performed after a flight
through severe turbulence. Emphasis should be placed
upon inspecting the upper and lower wing surfaces
for excessive buckles or wrinkles with permanent set.
Where wrinkles have occurred, remove a few rivets
and examine the rivet shanks to determine if the rivets
have sheared or were highly loaded in shear.
Through the inspection doors and other accessible
openings, inspect all spar webs from the fuselage to
the tip. Check for buckling, wrinkles, and sheared
attachments. Inspect for buckling in the area around
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55. 8-17
the nacelles and in the nacelle skin, particularly at the
wing leading edge.
Check for fuel leaks. Any sizeable fuel leak is an indi-
cation that an area may have received overloads which
have broken the sealant and opened the seams.
If the landing gear was lowered during a period of
severe turbulence, inspect the surrounding surfaces
carefully for loose rivets, cracks, or buckling. The
interior of the wheel well may give further indications
of excessive gust conditions. Inspect the top and bottom
fuselage skin. An excessive bending moment may have
left wrinkles of a diagonal nature in these areas.
Inspect the surface of the empennage for wrinkles,
buckling, or sheared attachments. Also, inspect the
area of attachment of the empennage to the fuselage.
The above inspections cover the critical areas. If exces-
sive damage is noted in any of the areas mentioned,
the inspection should be continued until all damage
is detected.
Lightning Strike
Although lightning strikes to aircraft are extremely
rare, if a strike has occurred, the aircraft must be care-
fully inspected to determine the extent of any damage
that might have occurred. When lightning strikes
an aircraft, the electrical current must be conducted
through the structure and be allowed to discharge or
dissipate at controlled locations. These controlled
locations are primarily the aircraft’s static discharge
wicks, or on more sophisticated aircraft, null field dis-
chargers. When surges of high voltage electricity pass
through good electrical conductors, such as aluminum
or steel, damage is likely to be minimal or nonexistent.
When surges of high voltage electricity pass through
non-metallic structures, such as a fiberglass radome,
engine cowl or fairing, glass or plastic window, or a
composite structure that does not have built-in electri-
cal bonding, burning and more serious damage to the
structure could occur. Visual inspection of the structure
is required. Look for evidence of degradation, burning
or erosion of the composite resin at all affected struc-
tures, electrical bonding straps, static discharge wicks
and null field dischargers.
Fire Damage
Inspection of aircraft structures that have been sub-
jected to fire or intense heat can be relatively simple
if visible damage is present. Visible damage requires
repair or replacement. If there is no visible damage,
the structural integrity of an aircraft may still have
been compromised. Since most structural metallic
components of an aircraft have undergone some sort
of heat treatment process during manufacture, an
exposure to high heat not encountered during normal
operations could severely degrade the design strength
of the structure. The strength and airworthiness of an
aluminum structure that passes a visual inspection but
is still suspect can be further determined by use of a
conductivity tester. This is a device that uses eddy cur-
rent and is discussed later in this chapter. Since strength
of metals is related to hardness, possible damage to
steel structures might be determined by use of a hard-
ness tester such as a Rockwell C hardness tester.
Flood Damage
Like aircraft damaged by fire, aircraft damaged by
water can range from minor to severe, depending
on the level of the flood water, whether it was fresh
or salt water and the elapsed time between the flood
occurrence and when repairs were initiated. Any parts
that were totally submerged should be completely
disassembled, thoroughly cleaned, dried and treated
with a corrosion inhibitor. Many parts might have
to be replaced, particularly interior carpeting, seats,
side panels, and instruments. Since water serves as an
electrolyte that promotes corrosion, all traces of water
and salt must be removed before the aircraft can again
be considered airworthy.
Seaplanes
Because they operate in an environment that acceler-
ates corrosion, seaplanes must be carefully inspected
for corrosion and conditions that promote corrosion.
Inspect bilge areas for waste hydraulic fluids, water,
dirt, drill chips, and other debris. Additionally, since
seaplanes often encounter excessive stress from the
pounding of rough water at high speeds, inspect for
loose rivets and other fasteners; stretched, bent or
cracked skins; damage to the float attach fitting; and
general wear and tear on the entire structure.
Aerial Application Aircraft
Two primary factors that make inspecting these aircraft
different from other aircraft are the corrosive nature of
some of the chemicals used and the typical flight pro-
file. Damaging effects of corrosion may be detected in
a much shorter period of time than normal use aircraft.
Chemicals may soften the fabric or loosen the fabric
tapes of fabric covered aircraft. Metal aircraft may need
to have the paint stripped, cleaned, and repainted and
corrosion treated annually. Leading edges of wings and
other areas may require protective coatings or tapes.
Hardware may require more frequent replacement.
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56. 8-18
During peak use, these aircraft may fly up to 50 cycles
(takeoffs and landings) or more in a day, most likely
from an unimproved or grass runway. This can greatly
accelerate the failure of normal fatigue items. Landing
gear and related items require frequent inspections.
Because these aircraft operate almost continuously at
very low altitudes, air filters tend to become obstructed
more rapidly.
Special Flight Permits
For an aircraft that does not currently meet airworthi-
ness requirements because of an overdue inspection,
damage, expired replacement times for time-limited
parts or other reasons, but is capable of safe flight,
a special flight permit may be issued. Special flight
permits, often referred to as ferry permits, are issued
for the following purposes:
• Flying the aircraft to a base where repairs,
alterations, or maintenance are to be performed,
or to a point of storage.
• Delivering or exporting the aircraft.
• Production flight testing new production
aircraft.
• Evacuating aircraft from areas of impending
danger.
• Conducting customer demonstration flights in
new production aircraft that have satisfactorily
completed production flight tests.
Additional information about special flight permits
may be found in 14 CFR part 21. Application forms for
special flight permits may be requested from the nearest
FAA Flight Standards District Office (FSDO).
Nondestructive Inspection/Testing
The preceding information in this chapter provided
general information regarding aircraft inspection. The
remainder of this chapter deals with several methods
often used on specific components or areas on an air-
craft when carrying out the more specific inspections.
They are referred to as nondestructive inspection (NDI)
or nondestructive testing (NDT). The objective of
NDI and NDT is to determine the airworthiness of a
component without damaging it, which would render
it unairworthy. Some of these methods are simple,
requiring little additional expertise, while others are
highly sophisticated and require that the technician be
highly trained and specially certified.
Additional information on NDI may be found by
referring to chapter 5 of FAA Advisory Circular (AC)
43.13-1B, Acceptable Methods, Techniques, and Prac-
tices—Aircraft Inspection and Repair. Information
regarding training, qualifications, and certification of
NDI personnel may be found in FAA Advisory Circular
(AC) 65-31A, Training, Qualification and Certification
of Non-destructive Inspection (NDI) Personnel.
General Techniques
Before conducting NDI, it is necessary to follow
preparatory steps in accordance with procedures spe-
cific to that type of inspection. Generally, the parts or
areas must be thoroughly cleaned. Some parts must
be removed from the aircraft or engine. Others might
need to have any paint or protective coating stripped. A
complete knowledge of the equipment and procedures
is essential and if required, calibration and inspection
of the equipment must be current.
Visual Inspection
Visual inspection can be enhanced by looking at the
suspect area with a bright light, a magnifying glass,
and a mirror (when required). Some defects might
be so obvious that further inspection methods are not
required. The lack of visible defects does not neces-
sarily mean further inspection is unnecessary. Some
defects may lie beneath the surface or may be so small
that the human eye, even with the assistance of a mag-
nifying glass, cannot detect them.
Borescope
Inspection by use of a borescope is essentially a visual
inspection. A borescope is a device that enables the
inspector to see inside areas that could not otherwise be
inspected without disassembly. An example of an area
that can be inspected with a borescope is the inside of
a reciprocating engine cylinder. The borescope can be
inserted into an open spark plug hole to detect damaged
pistons, cylinder walls, or valves. Another example
would be the hot section of a turbine engine to which
access could be gained through the hole of a removed
igniter or removed access plugs specifically installed
for inspection purposes.
Borescopes are available in two basic configurations.
The simpler of the two is a rigid type of small diameter
telescope with a tiny mirror at the end that enables the
user to see around corners. The other type uses fiber
optics that enables greater flexibility. Many borescopes
provide images that can be displayed on a computer or
video monitor for better interpretation of what is being
viewed and to record images for future reference. Most
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57. 8-19
borescopes also include a light to illuminate the area
being viewed.
Liquid Penetrant Inspection
Penetrant inspection is a nondestructive test for defects
open to the surface in parts made of any nonporous
material. It is used with equal success on such metals
as aluminum, magnesium, brass, copper, cast iron,
stainless steel, and titanium. It may also be used on
ceramics, plastics, molded rubber, and glass.
Penetrant inspection will detect such defects as surface
cracks or porosity. These defects may be caused by
fatigue cracks, shrinkage cracks, shrinkage poros-
ity, cold shuts, grinding and heat treat cracks, seams,
forging laps, and bursts. Penetrant inspection will also
indicate a lack of bond between joined metals.
The main disadvantage of penetrant inspection is that
the defect must be open to the surface in order to let
the penetrant get into the defect. For this reason, if the
part in question is made of material which is magnetic,
the use of magnetic particle inspection is generally
recommended.
Penetrant inspection uses a penetrating liquid that
enters a surface opening and remains there, making
it clearly visible to the inspector. It calls for visual
examination of the part after it has been processed,
increasing the visibility of the defect so that it can
be detected. Visibility of the penetrating material is
increased by the addition of one of two types of dye,
visible or fluorescent.
The visible penetrant kit consists of dye penetrant,
dye remover emulsifier, and developer. The fluores-
cent penetrant inspection kit contains a black light
assembly, as well as spray cans of penetrant, cleaner,
and developer. The light assembly consists of a power
transformer, a flexible power cable, and a hand-held
lamp. Due to its size, the lamp may be used in almost
any position or location.
Briefly, the steps for performing a penetrant inspec-
tion are:
1. Thorough cleaning of the metal surface.
2. Applying penetrant.
3. Removing penetrant with remover emulsifier or
cleaner.
4. Drying the part.
5. Applying the developer.
6. Inspecting and interpreting results.
Interpretation of Results
The success and reliability of a penetrant inspection
depends upon the thoroughness with which the part
was prepared. Several basic principles applying to
penetrant inspection are: 1. The penetrant must enter the defect in order to form
an indication. It is important to allow sufficient
time so the penetrant can fill the defect. The defect
must be clean and free of contaminating materials
so that the penetrant is free to enter.
2. If all penetrant is washed out of a defect, an
indication cannot be formed. During the washing
or rinsing operation, prior to development, it is
possible that the penetrant will be removed from
within the defect, as well as from the surface.
3. Clean cracks are usually easy to detect. Surface
openings that are uncontaminated, regardless of
how fine, are seldom difficult to detect with the
penetrant inspection.
4. The smaller the defect, the longer the penetrating
time. Fine crack-like apertures require a longer
penetrating time than defects such as pores.
5. When the part to be inspected is made of a material
susceptible to magnetism, it should be inspected
by a magnetic particle inspection method if the
equipment is available.
6. Visible penetrant-type developer, when applied to
the surface of a part, will dry to a smooth, even,
white coating. As the developer dries, bright red
indications will appear where there are surface
defects. If no red indications appear, there are no
surface defects.
7. When conducting the fluorescent penetrant-type
inspection, the defects will show up (under black
light) as a brilliant yellow-green color and the
sound areas will appear deep blue-violet.
8. It is possible to examine an indication of a defect
and to determine its cause as well as its extent.
Such an appraisal can be made if something is
known about the manufacturing processes to which
the part has been subjected.
The size of the indication, or accumulation of pen-
etrant, will show the extent of the defect and the bril-
liance will be a measure of its depth. Deep cracks will
hold more penetrant and will be broader and more bril-
liant. Very fine openings can hold only small amounts
of penetrants and will appear as fine lines. Figure 8-4
shows some of the types of defects that can be located
using dry penetrant.
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58. 8-20
False Indications
With the penetrant inspection, there are no false indi-
cations in the sense that they occur in the magnetic
particle inspection. There are, however, two condi-
tions which may create accumulations of penetrant
that are sometimes confused with true surface cracks
and discontinuities.
The first condition involves indications caused by poor
washing. If all the surface penetrant is not removed in
the washing or rinsing operation following the pen-
etrant dwell time, the unremoved penetrant will be vis-
ible. Evidences of incomplete washing are usually easy
to identify since the penetrant is in broad areas rather
than in the sharp patterns found with true indications.
When accumulations of unwashed penetrant are found
on a part, the part should be completely reprocessed.
Degreasing is recommended for removal of all traces
of the penetrant.
False indications may also be created where parts
press fit to each other. If a wheel is press fit onto a
shaft, penetrant will show an indication at the fit line.
This is perfectly normal since the two parts are not
meant to be welded together. Indications of this type
are easy to identify since they are regular in form and
shape.
Eddy Current Inspection
Electromagnetic analysis is a term which describes the
broad spectrum of electronic test methods involving the
intersection of magnetic fields and circulatory currents.
The most widely used technique is the eddy current.
Eddy currents are composed of free electrons under the
influence of an induced electromagnetic field which
are made to “drift” through metal.
Eddy current is used in aircraft maintenance to inspect
jet engine turbine shafts and vanes, wing skins, wheels,
bolt holes, and spark plug bores for cracks, heat or
frame damage. Eddy current may also be used in repair
of aluminum aircraft damaged by fire or excessive heat.
Different meter readings will be seen when the same
metal is in different hardness states. Readings in the
affected area are compared with identical materials in
known unaffected areas for comparison. A difference
in readings indicates a difference in the hardness state
of the affected area. In aircraft manufacturing plants,
eddy current is used to inspect castings, stampings,
machine parts, forgings, and extrusions. Figure 8-5
shows a technician performing an eddy current inspec-
tion on an aluminum wheel half.
Basic Principles
When an alternating current is passed through a coil,
it develops a magnetic field around the coil, which in
turn induces a voltage of opposite polarity in the coil
and opposes the flow of original current. If this coil
is placed in such a way that the magnetic field passes
Pits of porosity Tight crack or partially welded lap Crack or similar opening
Figure 8-4. Types of defects.
Figure 8-5. Eddy current inspection of wheel half.
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59. 8-21
ing surface and subsurface corrosion, pots and heat
treat condition.
Ultrasonic Inspection
Ultrasonic detection equipment makes it possible to
locate defects in all types of materials. Minute cracks,
checks, and voids too small to be seen by x-ray can
be located by ultrasonic inspection. An ultrasonic test
instrument requires access to only one surface of the
material to be inspected and can be used with either
straight line or angle beam testing techniques.
Two basic methods are used for ultrasonic inspection.
The first of these methods is immersion testing. In this
method of inspection, the part under examination and
the search unit are totally immersed in a liquid couplant,
which may be water or any other suitable fluid.
The second method is called contact testing, which is
readily adapted to field use and is the method discussed
in this chapter. In this method, the part under exami-
nation and the search unit are coupled with a viscous
material, liquid or a paste, which wets both the face of
the search unit and the material under examination.
There are three basic ultrasonic inspection meth-
ods: (1) pulse echo; (2) through transmission; and
(3) resonance.
through an electrically conducting specimen, eddy
currents will be induced into the specimen. The eddy
currents create their own field which varies the original
field’s opposition to the flow of original current. The
specimen’s susceptibility to eddy currents determines
the current flow through the coil. [Figure 8-6]
The magnitude and phase of this counter field is depen-
dent primarily upon the resistance and permeability of
the specimen under consideration, and which enables
us to make a qualitative determination of various
physical properties of the test material. The interaction
of the eddy current field with the original field results
is a power change that can be measured by utilizing
electronic circuitry similar to a Wheatstone bridge.
The specimen is either placed in or passed through the
field of an electromagnetic induction coil, and its effect
on the impedance of the coil or on the voltage output of
one or more test coils is observed. The process, which
involves electric fields made to explore a test piece for
various conditions, involves the transmission of energy
through the specimen much like the transmission of
x-rays, heat, or ultrasound.
Eddy current inspection can frequently be performed
without removing the surface coatings such as primer,
paint, and anodized films. It can be effective in detect-
Oscillator Amplifier
Probe
Meter
Sample part
Figure 8-6. Eddy current inspection circuit.
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Pulse Echo
Flaws are detected by measuring the amplitude of sig-
nals reflected and the time required for these signals to
travel between specific surfaces and the discontinuity.
[Figure 8-7]
The time base, which is triggered simultaneously with
each transmission pulse, causes a spot to sweep across
the screen of the cathode ray tube (CRT). The spot
sweeps from left to right across the face of the scope
50 to 5,000 times per second, or higher if required for
high speed automated scanning. Due to the speed of
the cycle of transmitting and receiving, the picture on
the oscilloscope appears to be stationary.
A few microseconds after the sweep is initiated, the
rate generator electrically excites the pulser, and the
pulser in turn emits an electrical pulse. The transducer
converts this pulse into a short train of ultrasonic
sound waves. If the interfaces of the transducer and
the specimen are properly oriented, the ultrasound
will be reflected back to the transducer when it reaches
the internal flaw and the opposite surface of the speci-
men. The time interval between the transmission of
the initial impulse and the reception of the signals
from within the specimen are measured by the timing
circuits. The reflected pulse received by the transducer
is amplified, then transmitted to and displayed on the
instrument screen. The pulse is displayed in the same
relationship to the front and back pulses as the flaw is
in relation to the front and back surfaces of the speci-
men. [Figure 8-8]
Pulse-echo instruments may also be used to detect flaws
not directly underneath the probe by use of the angle-
beam testing method. Angle beam testing differs from
straight beam testing only in the manner in which the
ultrasonic waves pass through the material being tested.
As shown in Figure 8-9, the beam is projected into the
material at an acute angle to the surface by means of
a crystal cut at an angle and mounted in plastic. The
beam or a portion thereof reflects successively from
the surfaces of the material or any other discontinuity,
including the edge of the piece. In straight beam test-
ing, the horizontal distance on the screen between the
initial pulse and the first back reflection represents the
thickness of the piece; while in angle beam testing, this
distance represents the width of the material between
the searching unit and the opposite edge of the piece.
Through Transmission
Through transmission inspection uses two transducers,
one to generate the pulse and another placed on the
opposite surface to receive it. A disruption in the sound
path will indicate a flaw and be displayed on the instru-
ment screen. Through transmission is less sensitive to
small defects than the pulse-echo method. Resonance
This system differs from the pulse method in that the
frequency of transmission may be continuously varied.
The resonance method is used principally for thickness
measurements when the two sides of the material being
tested are smooth and parallel and the backside is inac-
cessible. The point at which the frequency matches
the resonance point of the material being tested is the
thickness determining factor.
RF pulser
Rate generator
Timing circuit
Specimen
Flaw
1
2
3
Transducer
Amplifier
Cathode ray oscilloscope
1 2 3
Figure 8-7. Block diagram of basic pulse-echo system.
Flaw
Cathode ray tube
F
T
Transducer
Flaw
Speciman
B
T‘ F’ B’
Figure 8-8. Pulse-echo display in relationship
to flaw detection.
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61. 8-23
It is necessary that the frequency of the ultrasonic
waves corresponding to a particular dial setting be
accurately known. Checks should be made with standard
test blocks to guard against possible drift of frequency.
If the frequency of an ultrasonic wave is such that its
wavelength is twice the thickness of a specimen (funda-
mental frequency), then the reflected wave will arrive
back at the transducer in the same phase as the original
transmission so that strengthening of the signal will
occur. This results from constructive interference or a
resonance and is shown as a high amplitude value on
the indicating screen. If the frequency is increased such
that three times the wavelength equals four times the
thickness, the reflected signal will return completely
out of phase with the transmitted signal and cancella-
tion will occur. Further increase of the frequency causes
the wavelength to be equal to the thickness again and
gives a reflected signal in phase with the transmitted
signal and a resonance once more.
By starting at the fundamental frequency and gradually
increasing the frequency, the successive cancellations
and resonances can be noted and the readings used
to check the fundamental frequency reading. [Figure
8-10]
In some instruments, the oscillator circuit contains a
motor driven capacitor which changes the frequency
of the oscillator. [Figure 8-11] In other instruments, the
frequency is changed by electronic means.
The change in frequency is synchronized with the
horizontal sweep of a CRT. The horizontal axis thus
represents a frequency range. If the frequency range
contains resonances, the circuitry is arranged to pres-
ent these vertically. Calibrated transparent scales are
then placed in front of the tube, and the thickness can
be read directly. The instruments normally operate
Coaxial cable
Material
Quartz crystal Defect
Figure 8-9. Pulse-echo angle beam testing.
F = F
1 (Fundamental frequency)
Transducer
incident wave Reflective wave
Reflecting
surface
Material
under test
T =
T = W F = 2F
1
(2nd Harmonic)
T = 1
1
⁄
2W F = 3F
1
(3rd Harmonic)
T = 2W F = 4F
1
(4th Harmonic)
Wavelength
2
A
B
C
D
Figure 8-10. Conditions of ultrasonic resonance
in a metal plate.
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62. 8-24
between 0.25 millicycle (mc) and 10 mc in four or
five bands.
The resonance thickness instrument can be used to test
the thickness of such metals as steel, cast iron, brass,
nickel, copper, silver, lead, aluminum, and magnesium.
In addition, areas of corrosion or wear on tanks, tubing,
airplane wing skins, and other structures or products
can be located and evaluated.
Direct reading dial-operated units are available that
measure thickness between 0.025 inch and 3 inches
with an accuracy of better than ±1 percent.
Ultrasonic inspection requires a skilled operator who is
familiar with the equipment being used as well as the
inspection method to be used for the many different
parts being tested. [Figure 8-12]
Figure 8-12. Ultrasonic inspection of a composite structure.
Acoustic Emission Inspection
Acoustic emission is an NDI technique that involves
the placing of acoustic emission sensors at various
locations on an aircraft structure and then applying
a load or stress. The materials emit sound and stress
waves that take the form of ultrasonic pulses. Cracks
and areas of corrosion in the stressed airframe structure
emit sound waves which are registered by the sensors.
These acoustic emission bursts can be used to locate
flaws and to evaluate their rate of growth as a func-
tion of applied stress. Acoustic emission testing has
an advantage over other NDI methods in that it can
detect and locate all of the activated flaws in a struc-
ture in one test. Because of the complexity of aircraft
structures, application of acoustic emission testing to
aircraft has required a new level of sophistication in
testing technique and data interpretation.
Magnetic Particle Inspection
Magnetic particle inspection is a method of detecting
invisible cracks and other defects in ferromagnetic
materials such as iron and steel. It is not applicable to
nonmagnetic materials.
In rapidly rotating, reciprocating, vibrating, and other
highly stressed aircraft parts, small defects often
develop to the point that they cause complete failure
of the part. Magnetic particle inspection has proven
extremely reliable for the rapid detection of such
defects located on or near the surface. With this method
of inspection, the location of the defect is indicated and
the approximate size and shape are outlined.
The inspection process consists of magnetizing the
part and then applying ferromagnetic particles to the
surface area to be inspected. The ferromagnetic par-
ticles (indicating medium) may be held in suspension
in a liquid that is flushed over the part; the part may
be immersed in the suspension liquid; or the particles,
in dry powder form, may be dusted over the surface
of the part. The wet process is more commonly used
in the inspection of aircraft parts.
If a discontinuity is present, the magnetic lines of force
will be disturbed and opposite poles will exist on either
side of the discontinuity. The magnetized particles
thus form a pattern in the magnetic field between the
opposite poles. This pattern, known as an “indica-
tion,” assumes the approximate shape of the surface
projection of the discontinuity. A discontinuity may
be defined as an interruption in the normal physical
structure or configuration of a part, such as a crack,
forging lap, seam, inclusion, porosity, and the like.
CRT
Pulse
amplifier
Horizontal
time-base
generator
Contacts
Transducer
Material
Motor
Tuning
capacitor
H. F.
Oscillator
Figure 8-11. Block diagram of resonance thickness
measuring system.
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63. 8-25
A discontinuity may or may not affect the usefulness
of a part.
Development of Indications
When a discontinuity in a magnetized material is open
to the surface, and a magnetic substance (indicating
medium) is available on the surface, the flux leak-
age at the discontinuity tends to form the indicating
medium into a path of higher permeability. (Perme-
ability is a term used to refer to the ease with which a
magnetic flux can be established in a given magnetic
circuit.) Because of the magnetism in the part and the
adherence of the magnetic particles to each other, the
indication remains on the surface of the part in the form
of an approximate outline of the discontinuity that is
immediately below it.
The same action takes place when the discontinuity is
not open to the surface, but since the amount of flux
leakage is less, fewer particles are held in place and a
fainter and less sharply defined indication is obtained.
If the discontinuity is very far below the surface, there
may be no flux leakage and no indication on the sur-
face. The flux leakage at a transverse discontinuity is
shown in Figure 8-13. The flux leakage at a longitudinal
discontinuity is shown in Figure 8-14.
Types of Discontinuities Disclosed
The following types of discontinuities are normally
detected by the magnetic particle test: cracks, laps,
seams, cold shuts, inclusions, splits, tears, pipes, and
voids. All of these may affect the reliability of parts
in service.
Cracks, splits, bursts, tears, seams, voids, and pipes
are formed by an actual parting or rupture of the solid
metal. Cold shuts and laps are folds that have been
formed in the metal, interrupting its continuity.
Inclusions are foreign material formed by impurities in
the metal during the metal processing stages. They may
consist, for example, of bits of furnace lining picked up
during the melting of the basic metal or of other foreign
constituents. Inclusions interrupt the continuity of the
metal because they prevent the joining or welding of
adjacent faces of the metal.
Preparation of Parts for Testing
Grease, oil, and dirt must be cleaned from all parts
before they are tested. Cleaning is very important
since any grease or other foreign material present can
produce nonrelevant indications due to magnetic par-
ticles adhering to the foreign material as the suspension
drains from the part.
Grease or foreign material in sufficient amount over a
discontinuity may also prevent the formation of a pat-
tern at the discontinuity. It is not advisable to depend
upon the magnetic particle suspension to clean the part.
Cleaning by suspension is not thorough and any foreign
materials so removed from the part will contaminate the
suspension, thereby reducing its effectiveness.
In the dry procedure, thorough cleaning is absolutely
necessary. Grease or other foreign material will hold
the magnetic powder, resulting in nonrelevant indica-
tions and making it impossible to distribute the indicat-
ing medium evenly over the part’s surface.
All small openings and oil holes leading to internal
passages or cavities should be plugged with paraffin
or other suitable nonabrasive material.
Coatings of cadmium, copper, tin, and zinc do not
interfere with the satisfactory performance of magnetic
particle inspection, unless the coatings are unusually
heavy or the discontinuities to be detected are unusu-
ally small.
Chromium and nickel plating generally will not inter-
fere with indications of cracks open to the surface
of the base metal but will prevent indications of fine
discontinuities, such as inclusions.
Figure 8-13. Flux leakage at transverse discontinuity.Figure 8-14. Flux leakage at longitudinal discontinuity.
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64. 8-26
In longitudinal magnetization, the magnetic field is
produced in a direction parallel to the long axis of
the part. This is accomplished by placing the part in
a solenoid excited by electric current. The metal part
then becomes the core of an electromagnet and is mag-
netized by induction from the magnetic field created
in the solenoid.
In longitudinal magnetization of long parts, the solenoid
must be moved along the part in order to magnetize it.
[Figure 8-17] This is necessary to ensure adequate field
strength throughout the entire length of the part.
Solenoids produce effective magnetization for approxi-
mately 12 inches from each end of the coil, thus accom-
modating parts or sections approximately 30 inches
in length. Longitudinal magnetization equivalent to
that obtained by a solenoid may be accomplished by
wrapping a flexible electrical conductor around the
part. Although this method is not as convenient, it has
Because it is more strongly magnetic, nickel plating
is more effective than chromium plating in preventing
the formation of indications.
Effect of Flux Direction
To locate a defect in a part, it is essential that the mag-
netic lines of force pass approximately perpendicular to
the defect. It is therefore necessary to induce magnetic
flux in more than one direction since defects are likely
to exist at any angle to the major axis of the part. This
requires two separate magnetizing operations, referred
to as circular magnetization and longitudinal magne-
tization. The effect of flux direction is illustrated in
Figure 8-15.
Circular magnetization is the induction of a magnetic
field consisting of concentric circles of force about and
within the part which is achieved by passing electric
current through the part. This type of magnetization
will locate defects running approximately parallel to
the axis of the part. Figure 8-16 illustrates circular
magnetization of a camshaft.
Figure 8-15. Effect of flux direction on strength of indication.
Figure 8-16. Circular magnetization of a camshaft.
Figure 8-17. Longitudinal magnetization of crankshaft
(solenoid method).
Longitudinal magnetization
A
Attraction of particles at defects
B
Circular magnetization
Attraction of particles at defects
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65. 8-27
an advantage in that the coils conform more closely
to the shape of the part, producing a somewhat more
uniform magnetization.
The flexible coil method is also useful for large or
irregularly shaped parts for which standard solenoids
are not available.
Effect of Flux Density
The effectiveness of the magnetic particle inspection
also depends on the flux density or field strength at
the surface of the part when the indicating medium
is applied. As the flux density in the part is increased,
the sensitivity of the test increases because of the
greater flux leakages at discontinuities and the resulting
improved formation of magnetic particle patterns.
Excessively high flux densities may form nonrelevant
indications; for example, patterns of the grain flow
in the material. These indications will interfere with
the detection of patterns resulting from significant
discontinuities. It is therefore necessary to use a field
strength high enough to reveal all possible harmful
discontinuities but not strong enough to produce con-
fusing nonrelevant indications. Magnetizing Methods
When a part is magnetized, the field strength in the part
increases to a maximum for the particular magnetiz-
ing force and remains at this maximum as long as the
magnetizing force is maintained.
When the magnetizing force is removed, the field
strength decreases to a lower residual value depending
on the magnetic properties of the material and the shape
of the part. These magnetic characteristics determine
whether the continuous or residual method is used in
magnetizing the part.
In the continuous inspection method, the part is mag-
netized and the indicating medium applied while the
magnetizing force is maintained. The available flux
density in the part is thus at a maximum. The maximum
value of flux depends directly upon the magnetizing
force and the permeability of the material of which
the part is made.
The continuous method may be used in practically all
circular and longitudinal magnetization procedures.
The continuous procedure provides greater sensitivity
than the residual procedure, particularly in locating
subsurface discontinuities. The highly critical nature
of aircraft parts and assemblies and the necessity for
subsurface inspection in many applications have resulted
in the continuous method being more widely used.
Inasmuch as the continuous procedure will reveal
more nonsignificant discontinuities than the residual
procedure, careful and intelligent interpretation and
evaluation of discontinuities revealed by this procedure
are necessary.
The residual inspection procedure involves magne-
tization of the part and application of the indicating
medium after the magnetizing force has been removed.
This procedure relies on the residual or permanent
magnetism in the part and is more practical than the
continuous procedure when magnetization is accom-
plished by flexible coils wrapped around the part.
In general, the residual procedure is used only with steels
which have been heat treated for stressed applications. Identification of Indications
The correct evaluation of the character of indications
is extremely important but is sometimes difficult to
make from observation of the indications alone. The
principal distinguishing features of indications are
shape, buildup, width, and sharpness of outline. These
characteristics are more valuable in distinguishing
between types of discontinuities than in determining
their severity. Careful observation of the character of
the magnetic particle pattern should always be included
in the complete evaluation of the significance of an
indicated discontinuity.
The most readily distinguished indications are those
produced by cracks open to the surface. These discon-
tinuities include fatigue cracks, heat treat cracks, shrink
cracks in welds and castings, and grinding cracks. An
example of a fatigue crack is shown in Figure 8-18. Magnaglo Inspection
Magnaglo inspection is similar to the preceding method
but differs in that a fluorescent particle solution is
used and the inspection is made under black light.
Efficiency of inspection is increased by the neon-like
glow of defects allowing smaller flaw indications to
be seen. This is an excellent method for use on gears,
threaded parts, and aircraft engine components. The
reddish brown liquid spray or bath that is used consists
of Magnaglo paste mixed with a light oil at the ratio of
0.10 to 0.25 ounce of paste per gallon of oil.
After inspection, the part must be demagnetized and
rinsed with a cleaning solvent.
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66. 8-28
Magnetizing Equipment
Fixed (Nonportable) General Purpose Unit
A fixed general purpose unit is shown in Figure 8-19.
This unit provides direct current for wet continuous
or residual magnetization procedures. Circular or
longitudinal magnetization may be used and it may
be powered with rectified alternating current (ac), as
well as direct current (dc). The contact heads provide
the electrical terminals for circular magnetization.
One head is fixed in position with its contact plate
mounted on a shaft surrounded by a pressure spring, so
that the plate may be moved longitudinally. The plate
is maintained in the extended position by the spring
until pressure transmitted through the work from the
movable head forces it back.
The motor driven movable head slides horizontally in
longitudinal guides and is controlled by a switch. The
spring allows sufficient overrun of the motor driven
head to avoid jamming it and also provides pressure on
the ends of the work to ensure good electrical contact.
Figure 8-18. Fatigue crack in a landing gear.
Main gear outer cylinder
Fatigue crack
Torsion link lugs
Figure 8-19. Fixed general-purpose magnetizing unit.
Movable headAmmeterPressure spring
Contact plate
Solenoid
Nozzle
Contact plate
Fixed head
Movable head switch
Push button
Pump switch
Rheostat
Short-circuiting
switch
Solenoid switch
Circulating
pump
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67. 8-29
A plunger operated switch in the fixed head cuts out
the forward motion circuit of the movable head motor
when the spring has been properly compressed.
In some units the movable head is hand operated, and
the contact plate is sometimes arranged for operation
by an air ram. Both contact plates are fitted with vari-
ous fixtures for supporting the work.
The magnetizing circuit is closed by depressing a
pushbutton on the front of the unit. It is set to open
automatically, usually after about one-half second.
The strength of the magnetizing current may be set
manually to the desired value by means of the rheostat
or increased to the capacity of the unit by the rheostat
short circuiting switch. The current utilized is indicated
on the ammeter.
Longitudinal magnetization is produced by the sole-
noid, which moves in the same guide rail as the mov-
able head and is connected in the electrical circuit by
means of a switch.
The suspension liquid is contained in a sump tank and
is agitated and circulated by a pump. The suspension
is applied to the work through a nozzle. The suspen-
sion drains from the work through a nonmetallic grill
into a collecting pan that leads back to the sump. The
circulating pump is operated by a pushbutton switch.
Portable General Purpose Unit
It is often necessary to perform the magnetic particle
inspection at locations where fixed general purpose
equipment is not available or to perform an inspection
on members of aircraft structures without removing
them from the aircraft. It is particularly useful for
inspecting landing gear and engine mounts suspected
of having developed cracks in service. Portable units
supply both alternating current and direct current
magnetization.
This unit is only a source of magnetizing and demag-
netizing current and does not provide a means for
supporting the work or applying the suspension. It
operates on 200 volt, 60 cycle, alternating current and
contains a rectifier for producing direct current when
required. [Figure 8-20]
The magnetizing current is supplied through the flex-
ible cables. The cable terminals may be fitted with
prods, as shown in the illustration, or with contact
clamps. Circular magnetization may be developed by
using either the prods or clamps.
Longitudinal magnetization is developed by wrapping
the cable around the part.
The strength of the magnetizing current is controlled
by an eight point tap switch, and the time duration for
which it is applied is regulated by an automatic cutoff
similar to that used in the fixed general purpose unit.
This portable unit also serves as a demagnetizer and
supplies high amperage low voltage alternating current
for this purpose. For demagnetization, the alternat-
ing current is passed through the part and gradually
reduced by means of a current reducer.
In testing large structures with flat surfaces where cur-
rent must be passed through the part, it is sometimes
impossible to use contact clamps. In such cases, contact
prods are used.
Prods can be used with the fixed general purpose unit as
well as the portable unit. The part or assembly being tested
may be held or secured above the standard unit and the
suspension hosed onto the area; excess suspension drains
into the tank. The dry procedure may also be used.
Prods should be held firmly against the surface being
tested. There is a tendency for a high amperage cur-
rent to cause burning at contact areas, but with proper
care, such burning is usually slight. For applications
where prod magnetization is acceptable, slight burning
is normally acceptable.
Figure 8-20. Portable general purpose unit.
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68. 8-30
Indicating Mediums
The various types of indicating mediums available for
magnetic particle inspection may be divided into two
general material types: wet and dry. The basic require-
ment for any indicating medium is that it produce
acceptable indications of discontinuities in parts.
The contrast provided by a particular indicating
medium on the background or part surface is particu-
larly important. The colors most extensively used are
black and red for the wet procedure and black, red, and
gray for the dry procedure.
For acceptable operation, the indicating medium must
be of high permeability and low retentivity. High per-
meability ensures that a minimum of magnetic energy
will be required to attract the material to flux leakage
caused by discontinuities. Low retentivity ensures
that the mobility of the magnetic particles will not be
hindered by the particles themselves becoming mag-
netized and attracting one another.
Demagnetizing
The permanent magnetism remaining after inspection
must be removed by a demagnetization operation if
the part is to be returned to service. Parts of operat-
ing mechanisms must be demagnetized to prevent
magnetized parts from attracting filings, grindings, or
chips inadvertently left in the system, or steel particles
resulting from operational wear.
An accumulation of such particles on a magnetized
part may cause scoring of bearings or other working
parts. Parts of the airframe must be demagnetized so
they will not affect instruments.
Demagnetization between successive magnetizing
operations is not normally required unless experience
indicates that omission of this operation results in
decreased effectiveness for a particular application.
Demagnetization may be accomplished in a number
of different ways. A convenient procedure for aircraft
parts involves subjecting the part to a magnetizing
force that is continually reversing in direction and, at
the same time, gradually decreasing in strength. As
the decreasing magnetizing force is applied first in one
direction and then the other, the magnetization of the
part also decreases.
Standard Demagnetizing Practice
The simplest procedure for developing a reversing
and gradually decreasing magnetizing force in a part
involves the use of a solenoid coil energized by alter-
nating current. As the part is moved away from the
alternating field of the solenoid, the magnetism in the
part gradually decreases.
A demagnetizer whose size approximates that of the
work should be used. For maximum effectiveness,
small parts should be held as close to the inner wall
of the coil as possible.
Parts that do not readily lose their magnetism should
be passed slowly in and out of the demagnetizer sev-
eral times and, at the same time, tumbled or rotated
in various directions. Allowing a part to remain in the
demagnetizer with the current on accomplishes very
little practical demagnetization.
The effective operation in the demagnetizing procedure
is that of slowly moving the part out of the coil and
away from the magnetizing field strength. As the part
is withdrawn, it should be kept directly opposite the
opening until it is 1 or 2 feet from the demagnetizer.
The demagnetizing current should not be cut off until
the part is 1 or 2 feet from the opening as the part may
be remagnetized if current is removed too soon.
Another procedure used with portable units is to pass
alternating current through the part being demagne-
tized, while gradually reducing the current to zero.
Radiographic
X and gamma radiations, because of their unique ability
to penetrate material and disclose discontinuities, have
been applied to the radiographic (x-ray) inspection of
metal fabrications and nonmetallic products.
The penetrating radiation is projected through the part
to be inspected and produces an invisible or latent
image in the film. When processed, the film becomes
a radiograph or shadow picture of the object. This
inspection medium and portable unit provides a fast
and reliable means for checking the integrity of air-
frame structures and engines. [Figure 8-21]
Radiation
Void
Specimen
Film
Black
area
After processing
Black
area
White
area
White
area
Gray
area
Figure 8-21. Radiograph.
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69. 8-31
Radiographic Inspection
Radiographic inspection techniques are used to locate
defects or flaws in airframe structures or engines with
little or no disassembly. This is in marked contrast to
other types of nondestructive testing which usually
require removal, disassembly, and stripping of paint
from the suspected part before it can be inspected. Due
to the radiation risks associated with x-ray, extensive
training is required to become a qualified radiographer.
Only qualified radiographers are allowed to operate
the x-ray units.
Three major steps in the x-ray process discussed in
subsequent paragraphs are: (1) exposure to radiation,
including preparation, (2) processing of film, and (3)
interpretation of the radiograph.
Preparation and Exposure
The factors of radiographic exposure are so interde-
pendent that it is necessary to consider all factors for
any particular radiographic exposure. These factors
include but are not limited to the following:
• Material thickness and density
• Shape and size of the object
• Type of defect to be detected
• Characteristics of x-ray machine used
• The exposure distance
• The exposure angle
• Film characteristics
• Types of intensifying screen, if used
Knowledge of the x-ray unit’s capabilities should
form a background for the other exposure factors.
In addition to the unit rating in kilovoltage, the size,
portability, ease of manipulation, and exposure particu-
lars of the available equipment should be thoroughly
understood.
Previous experience on similar objects is also very
helpful in the determination of the overall exposure
techniques. A log or record of previous exposures will
provide specific data as a guide for future radiographs. Film Processing
After exposure to x-rays, the latent image on the film is
made permanently visible by processing it successively
through a developer chemical solution, an acid bath,
and a fixing bath, followed by a clear water wash.
Radiographic Interpretation
From the standpoint of quality assurance, radiographic
interpretation is the most important phase of radiog-
raphy. It is during this phase that an error in judgment
can produce disastrous consequences. The efforts of the
whole radiographic process are centered in this phase;
the part or structure is either accepted or rejected.
Conditions of unsoundness or other defects which are
overlooked, not understood, or improperly interpreted
can destroy the purpose and efforts of radiography and
can jeopardize the structural integrity of an entire air-
craft. A particular danger is the false sense of security
imparted by the acceptance of a part or structure based
on improper interpretation.
As a first impression, radiographic interpretation may
seem simple, but a closer analysis of the problem soon
dispels this impression. The subject of interpretation
is so varied and complex that it cannot be covered
adequately in this type of document. Instead, this
chapter gives only a brief review of basic requirements
for radiographic interpretation, including some descrip-
tions of common defects.
Experience has shown that, whenever possible, radio-
graphic interpretation should be conducted close to the
radiographic operation. When viewing radiographs, it
is helpful to have access to the material being tested.
The radiograph can thus be compared directly with
the material being tested, and indications due to such
things as surface condition or thickness variations can
be immediately determined.
The following paragraphs present several factors which
must be considered when analyzing a radiograph.
There are three basic categories of flaws: voids, inclu-
sions, and dimensional irregularities. The last category,
dimensional irregularities, is not pertinent to these
discussions because its prime factor is one of degree,
and radiography is not exact. Voids and inclusions may
appear on the radiograph in a variety of forms ranging
from a two-dimensional plane to a three-dimensional
sphere. A crack, tear, or cold shut will most nearly
resemble a two-dimensional plane, whereas a cavity
will look like a three-dimensional sphere. Other types
of flaws, such as shrink, oxide inclusions, porosity,
and so forth, will fall somewhere between these two
extremes of form.
It is important to analyze the geometry of a flaw, espe-
cially for items such as the sharpness of terminal points.
For example, in a crack-like flaw the terminal points
appear much sharper in a sphere-like flaw, such as a
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70. 8-32
gas cavity. Also, material strength may be adversely
affected by flaw shape. A flaw having sharp points
could establish a source of localized stress concentra-
tion. Spherical flaws affect material strength to a far
lesser degree than do sharp pointed flaws. Specifica-
tions and reference standards usually stipulate that
sharp pointed flaws, such as cracks, cold shuts, and so
forth, are cause for rejection.
Material strength is also affected by flaw size. A metal-
lic component of a given area is designed to carry a
certain load plus a safety factor. Reducing this area by
including a large flaw weakens the part and reduces the
safety factor. Some flaws are often permitted in com-
ponents because of these safety factors; in this case,
the interpreter must determine the degree of tolerance
or imperfection specified by the design engineer. Both
flaw size and flaw shape should be considered carefully,
since small flaws with sharp points can be just as bad as
large flaws with no sharp points.
Another important consideration in flaw analysis is
flaw location. Metallic components are subjected to
numerous and varied forces during their effective ser-
vice life. Generally, the distribution of these forces is
not equal in the component or part, and certain critical
areas may be rather highly stressed. The interpreter
must pay special attention to these areas. Another
aspect of flaw location is that certain types of discon-
tinuities close to one another may potentially serve as
a source of stress concentrations creating a situation
that should be closely scrutinized.
An inclusion is a type of flaw which contains entrapped
material. Such flaws may be of greater or lesser den-
sity than the item being radiographed. The foregoing
discussions on flaw shape, size, and location apply
equally to inclusions and to voids. In addition, a flaw
containing foreign material could become a source of
corrosion.
Radiation Hazards
Radiation from x-ray units and radioisotope sources is
destructive to living tissue. It is universally recognized
that in the use of such equipment, adequate protection
must be provided. Personnel must keep outside the
primary x-ray beam at all times.
Radiation produces changes in all matter through which
it passes. This is also true of living tissue. When radia-
tion strikes the molecules of the body, the effect may be
no more than to dislodge a few electrons, but an excess
of these changes could cause irreparable harm. When a
complex organism is exposed to radiation, the degree
of damage, if any, depends on which of its body cells
have been changed.
Vital organs in the center of the body that are penetrated
by radiation are likely to be harmed the most. The skin
usually absorbs most of the radiation and reacts earli-
est to radiation.
If the whole body is exposed to a very large dose of
radiation, death could result. In general, the type and
severity of the pathological effects of radiation depend
on the amount of radiation received at one time and
the percentage of the total body exposed. Smaller
doses of radiation could cause blood and intestinal
disorders in a short period of time. The more delayed
effects are leukemia and other cancers. Skin damage
and loss of hair are also possible results of exposure
to radiation.
Inspection of Composites
Composite structures should be inspected for delamina-
tion, which is separation of the various plies, debonding
of the skin from the core, and evidence of moisture and
corrosion. Previously discussed methods including
ultrasonic, acoustic emission, and radiographic inspec-
tions may be used as recommended by the aircraft
manufacturer. The simplest method used in testing
composite structures is the tap test.
Tap Testing
Tap testing, also referred to as the ring test or coin
test, is widely used as a quick evaluation of any acces-
sible surface to detect the presence of delamination or
debonding. The testing procedure consists of lightly
tapping the surface with a light hammer (maximum
weight of 2 ounces), a coin or other suitable device.
The acoustic response or “ring” is compared to that
of a known good area. A “flat” or “dead” response
indicates an area of concern. Tap testing is limited to
finding defects in relatively thin skins, less than 0.080"
thick. On honeycomb structures, both sides need to be
tested. Tap testing on only one side would not detect
debonding on the opposite side.
Electrical Conductivity
Composite structures are not inherently electrically
conductive. Some aircraft, because of their relatively
low speed and type of use, are not affected by electrical
issues. Manufacturers of other aircraft, such as high-
speed high-performance jets, are required to utilize
various methods of incorporating aluminum into their
structures to make them conductive. The aluminum
is imbedded within the plies of the lay-ups either as a
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71. 8-33
thin wire mesh, screen, foil, or spray. When damaged
sections of the structure are repaired, care must be taken
to ensure that the conductive path be restored. Not only
is it necessary to include the conductive material in the
repair, but the continuity of the electrical path from
the original conductive material to the replacement
conductor and back to the original must be maintained.
Electrical conductivity may be checked by use of an
ohmmeter. Specific manufacturer’s instructions must
be carefully followed.
Inspection of Welds
A discussion of welds in this chapter will be confined
to judging the quality of completed welds by visual
means. Although the appearance of the completed
weld is not a positive indication of quality, it provides
a good clue about the care used in making it.
A properly designed joint weld is stronger than the
base metal which it joins. The characteristics of a
properly welded joint are discussed in the following
paragraphs.
A good weld is uniform in width; the ripples are even
and well feathered into the base metal, which shows no
burn due to overheating. [Figure 8-22] The weld has
good penetration and is free of gas pockets, porosity, or
inclusions. The edges of the bead illustrated in Figure
8-22 (B) are not in a straight line, yet the weld is good
since penetration is excellent.
Penetration is the depth of fusion in a weld. Thorough
fusion is the most important characteristic contributing
to a sound weld. Penetration is affected by the thickness
of the material to be joined, the size of the filler rod, and
how it is added. In a butt weld, the penetration should
be 100 percent of the thickness of the base metal. On
a fillet weld, the penetration requirements are 25 to 50
percent of the thickness of the base metal. The width
and depth of bead for a butt weld and fillet weld are
shown in Figure 8-23.
To assist further in determining the quality of a welded
joint, several examples of incorrect welds are discussed
in the following paragraphs.
The weld shown in Figure 8-24 (A) was made too
rapidly. The long and pointed appearance of the
A B
Figure 8-22. Examples of good welds.
Leg 2 to 3 T
25 to 50% T
Throat 1 
1
∕
3 to 1 
1
∕
2 T
Bead width
3 to 5 T
100%
Penetration
A B
Reinforcement
 
1
∕
4     
to 
1
∕
2     
T
T 
Approx. 
1
∕
2   
T
Figure 8-23. (A) Butt weld and (B) fillet weld, showing width and depth of bead.
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72. 8-34
ripples was caused by an excessive amount of heat or
an oxidizing flame. If the weld were cross-sectioned,
it would probably disclose gas pockets, porosity, and
slag inclusions.
Figure 8-24 (B) illustrates a weld that has improper
penetration and cold laps caused by insufficient heat.
It appears rough and irregular, and its edges are not
feathered into the base metal.
A
C D
B
Figure 8-24. Examples of poor welds.
The puddle has a tendency to boil during the welding
operation if an excessive amount of acetylene is used.
This often leaves slight bumps along the center and
craters at the finish of the weld. Cross-checks are appar-
ent if the body of the weld is sound. If the weld were
cross-sectioned, pockets and porosity would be visible.
Such a condition is shown in Figure 8-24 (C).
A bad weld with irregular edges and considerable varia-
tion in the depth of penetration is shown in D of Figure
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Liquid Penetrant Testing
Liquid penetrant testing is one of the oldest and simplists NDT methods where its
earliest versions (using kerosene and oil mixture) dates back to the 19
th
century. This
method is used to reveal surface discontinuities by bleedout of a colored or fluorescent
dye from the flaw. The technique is based on the ability
of a liquid to be drawn into a "clean" surface
discontinuity by capillary action. After a period of time
called the "dwell time", excess surface penetrant is
removed and a developer applied. This acts as a blotter
that draws the penetrant from the discontinuity to reveal
its presence.
The advantage that a liquid penetrant inspection offers over an
unaided visual inspection is that it makes defects easier to see for the
inspector where that is done in two ways:
 It produces a flaw indication that is much larger and easier for
the eye to detect than the flaw itself. Many flaws are so small
or narrow that they are undetectable by the unaided eye (a
person with a perfect vision can not resolve features smaller
than 0.08 mm).
 It improves the detectability of a flaw due to the high level of
contrast between the indication and the background which helps
to make the indication more easily seen (such as a red indication
on a white background for visable penetrant or a penetrant that
glows under ultraviolate light for flourecent penetrant).

Liquid penetrant testing is one of the most widely used NDT methods. Its popularity
can be attributed to two main factors: its relative ease of use and its flexibility. It can
be used to inspect almost any material provided that its surface is not extremely rough
or porous. Materials that are commonly inspected using this method include; metals,
glass, many ceramic materials, rubber and plastics.
However, liquid penetrant testing can only be used to inspect for flaws that break the
surface of the sample (such as surface cracks, porosity, laps, seams, lack of fusion, etc.).

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Steps of Liquid Penetrant Testing
The exact procedure for liquid penetrant testing can vary from case to case depending
on several factors such as the penetrant system being used, the size and material of
the component being inspected, the type of discontinuities being expected in the
component and the condition and environment under which the inspection is
performed. However, the general steps can be summarized as follows:
1. Surface Preparation: One of the most critical steps of a liquid penetrant testing is
the surface preparation. The surface must be free of oil, grease, water, or other
contaminants that may prevent penetrant from entering flaws. The sample may
also require etching if mechanical operations such as machining, sanding, or grit
blasting have been performed. These and other mechanical operations can
smear metal over the flaw opening and prevent the penetrant from entering.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried,
the penetrant material is applied by spraying, brushing, or immersing the part in
a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to
allow as much penetrant as possible to be drawn from or to seep into a defect.
Penetrant dwell time is the total time that the penetrant is in contact with the
part surface. Dwell times are usually recommended by the penetrant producers
or required by the specification being
followed. The times vary depending on
the application, penetrant materials
used, the material, the form of the
material being inspected, and the type
of discontinuity being inspected for.
Minimum dwell times typically range
from five to 60 minutes. Generally,
there is no harm in using a longer penetrant dwell time as long as the penetrant
is not allowed to dry. The ideal dwell time is often determined by
experimentation and may be very specific to a particular application.

4. Excess Penetrant Removal: This is the most delicate part of the inspection
procedure because the excess penetrant must be removed from the surface of
the sample while removing as little penetrant as possible from defects.
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Depending on the penetrant system
used, this step may involve cleaning
with a solvent, direct rinsing with water,
or first treating the part with an
emulsifier and then rinsing with water.

5. Developer Application: A thin layer of developer is then applied to the sample to
draw penetrant trapped in flaws back to the surface where it will be visible.
Developers come in a variety of forms that may be applied by dusting (dry
powders), dipping, or spraying (wet developers).

6. Indication Development: The developer
is allowed to stand on the part surface
for a period of time sufficient to permit
the extraction of the trapped penetrant
out of any surface flaws. This
development time is usually a minimum
of 10 minutes. Significantly longer
times may be necessary for tight cracks.

7. Inspection: Inspection is then performed under appropriate lighting to detect
indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part
surface to remove the developer from the parts that were found to be
acceptable.

Advantages and Disadvantages
The primary advantages and disadvantages when compared to other NDT methods
are:
Advantages
 High sensitivity (small discontinuities can be detected).
 Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic,
and conductive and nonconductive materials may be inspected).
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 Rapid inspection of large areas and volumes.
 Suitable for parts with complex shapes.
 Indications are produced directly on the surface of the part and constitute a visual
representation of the flaw.
 Portable (materials are available in aerosol spray cans)
 Low cost (materials and associated equipment are relatively inexpensive)

Disadvantages
 Only surface breaking defects can be detected.
 Only materials with a relatively nonporous surface can be inspected.
 Pre-cleaning is critical since contaminants can mask defects.
 Metal smearing from machining, grinding, and grit or vapor blasting must be
removed.
 The inspector must have direct access to the surface being inspected.
 Surface finish and roughness can affect inspection sensitivity.
 Multiple process operations must be performed and controlled.
 Post cleaning of acceptable parts or materials is required.
 Chemical handling and proper disposal is required.

Penetrants
Penetrants are carefully formulated to produce the level of sensitivity desired by the
inspector. The penetrant must possess a number of important characteristics:
- spread easily over the surface of the material being inspected to provide
complete and even coverage.
- be drawn into surface breaking defects by capillary action.
- remain in the defect but remove easily from the surface of the part.
- remain fluid so it can be drawn back to the surface of the part through the
drying and developing steps.
- be highly visible or fluoresce brightly to produce easy to see indications.
- not be harmful to the material being tested or the inspector.
Penetrant materials are not designed to perform the same. Penetrant manufactures
have developed different formulations to address a variety of inspection applications.
Some applications call for the detection of the smallest defects possible while in other
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applications, the rejectable defect size may be larger. The penetrants that are used to
detect the smallest defect will also produce the largest amount of irrelevant
indications.
Standard specifications classify penetrant materials according to their physical
characteristics and their performance.
 Penetrant materials come in two basic types:
Type 1 - Fluorescent Penetrants: they contain a dye or several dyes that fluoresce
when exposed to ultraviolet radiation.
Type 2 - Visible Penetrants: they contain a red dye that provides high contrast
against the white developer background.
Fluorescent penetrant systems are more sensitive than visible penetrant systems
because the eye is drawn to the glow of the fluorescing indication. However,
visible penetrants do not require a darkened area and an ultraviolet light in order
to make an inspection.
 Penetrants are then classified by the method used to remove the excess penetrant
from the part. The four methods are:
Method A - Water Washable: penetrants can be removed from the part by rinsing
with water alone. These penetrants contain an emulsifying agent (detergent) that
makes it possible to wash the penetrant from the part surface with water alone.
Water washable penetrants are sometimes referred to as self-emulsifying
systems.
Method B - Post-Emulsifiable, Lipophilic: the penetrant is oil soluble and interacts
with the oil-based emulsifier to make removal possible.
Method C - Solvent Removable: they require the use of a solvent to remove the
penetrant from the part.
Method D - Post-Emulsifiable, Hydrophilic: they use an emulsifier that is a water
soluble detergent which lifts the excess penetrant from the surface of the part
with a water wash.

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 Penetrants are then classified based on the strength or detectability of the
indication that is produced for a number of very small and tight fatigue cracks. The
five sensitivity levels are:
Level ½ - Ultra Low Sensitivity
Level 1 - Low Sensitivity
Level 2 - Medium Sensitivity
Level 3 - High Sensitivity
Level 4 - Ultra-High Sensitivity
The procedure for classifying penetrants into one of the five sensitivity levels uses
specimens with small surface fatigue cracks. The brightness of the indication
produced is measured using a photometer.

Developers
The role of the developer is to pull the trapped penetrant material out of defects and
spread it out on the surface of the part so it can be seen by an inspector. Developers
used with visible penetrants create a white background so there is a greater degree of
contrast between the indication and the surrounding background. On the other hand,
developers used with fluorescent penetrants both reflect and refract the incident
ultraviolet light, allowing more of it to interact with the penetrant, causing more
efficient fluorescence.
According to standards, developers are classified based on the method that the
developer is applied (as a dry powder, or dissolved or suspended in a liquid carrier). The
six standard forms of developers are:
Form a - Dry Powder
Form b - Water Soluble
Form c - Water Suspendable
Form d - Nonaqueous Type 1: Fluorescent (Solvent Based)
Form e - Nonaqueous Type 2: Visible Dye (Solvent Based)
Form f - Special Applications
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Dry Powder
Dry powder developers are generally considered to be the least sensitive but they are
inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can
be applied to a thoroughly dry surface in a number of ways; by dipping parts in a
container of developer, by using a puffer to dust parts with the developer, or placing
parts in a dust cabinet where the developer is blown around. Since the powder only
sticks to areas of indications since they are wet, powder developers are seldom used
for visible inspections.
Water Soluble
As the name implies, water soluble developers consist of a group of chemicals that are
dissolved in water and form a developer layer when the water is evaporated away. The
best method for applying water soluble developers is by spraying it on the part. The
part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is
sometimes used but these methods are less desirable. Drying is achieved by placing
the wet but well drained part in a recirculating, warm air dryer with the temperature
21°C. Properly developed parts will have an even, pale white coating over the entire
surface.
Water Suspendable
Water suspendable developers consist of insoluble developer particles suspended in
water. Water suspendable developers require frequent stirring or agitation to keep the
particles from settling out of suspension. Water suspendable developers are applied to
parts in the same manner as water soluble developers then the parts are dried using
warm air.
Nonaqueous
Nonaqueous developers suspend the developer in a volatile solvent and are typically
applied with a spray gun. Nonaqueous developers are commonly distributed in aerosol
spray cans for portability. The solvent tends to pull penetrant from the indications by
solvent action. Since the solvent is highly volatile, forced drying is not required.
Special Applications
Plastic or lacquer developers are special developers that are primarily used when a
permanent record of the inspection is required.

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Preparation of Part
One of the most critical steps in the penetrant inspection process is preparing the part
for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be
removed to ensure that defects are open to the surface of the part. If the parts have
been machined, sanded, or blasted prior to the penetrant inspection, it is possible that
a thin layer of metal may have smeared across the surface and closed off defects. Also,
some cleaning operations, such as steam cleaning, can cause metal smearing in softer
materials. This layer of metal smearing must be removed before inspection.

Penetrant Application and Dwell Time
The penetrant material can be applied in a number of different
ways, including spraying, brushing, or immersing the parts in a
penetrant bath. Once the part is covered in penetrant it must be
allowed to dwell so the penetrant has time to enter any defect that
is present.
There are basically two dwell mode options:
- Immersion-dwell: keeping the part immersed in the penetrant
during the dwell period.
- Drain-dwell: letting the part drain during the dwell period
(this method gives better sensitivity).
Penetrant Dwell Time
Penetrant dwell time is the total time that the penetrant is in contact with the part
surface. The dwell time is important because it allows the penetrant the time
necessary to seep or be drawn into a defect. Dwell times are usually recommended by
the penetrant producers or required by the specification being followed. The time
required to fill a flaw depends on a number of variables which include:
 The surface tension of the penetrant.
 The contact angle of the penetrant.
 The dynamic shear viscosity of the penetrant.
 The atmospheric pressure at the flaw opening.
 The capillary pressure at the flaw opening.
 The pressure of the gas trapped in the flaw by the penetrant.
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 The radius of the flaw or the distance between the flaw walls.
 The density or specific gravity of the penetrant.
 Microstructural properties of the penetrant.
The ideal dwell time is often determined by experimentation and is often very specific
to a particular application. For example, the table shows the dwell time requirements
for steel parts according to some of the commonly used specifications.


Penetrant Removal Process
The penetrant removal procedure must effectively remove the penetrant from the
surface of the part without removing an appreciable amount of entrapped penetrant
from the discontinuity. If the removal process extracts penetrant from the flaw, the
flaw indication will be reduced by a proportional amount. If the penetrant is not
effectively removed from the part surface, the contrast between the indication and the
background will be reduced.
Removal Method
As mentioned previously, penetrant systems are classified into four types according to
the method used for excess penetrant removal.
- Method A: Water-Washable
- Method B: Post-Emulsifiable, Lipophilic
- Method C: Solvent Removable
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- Method D: Post-Emulsifiable, Hydrophilic
Method C, Solvent Removable, is used primarily for inspecting small localized areas.
This method requires hand wiping the surface with a cloth moistened with the solvent
remover, and is, therefore, too labor intensive for most production situations.
Method A, Water-Washable, is the most economical to apply of the different methods
and it is easy to use. Water-washable or self-emulsifiable penetrants contain an
emulsifier as an integral part of the formulation. The excess penetrant may be
removed from the object surface with a simple water rinse.
When removal of the penetrant from the defect due to over-washing of the part is a
concern, a post-emulsifiable penetrant system can be used. The post-emulsifiable
methods are generally only used when very high sensitivity is needed. Post-
emulsifiable penetrants require a separate emulsifier to breakdown the penetrant and
make it water washable. The part is usually immersed in the emulsifier but hydrophilic
emulsifiers may also be sprayed on the object. Brushing the emulsifier on to the part is
not recommended either because the bristles of the brush may force emulsifier into
discontinuities, causing the entrapped penetrant to be removed. The emulsifier is
allowed sufficient time to react with the penetrant on the surface of the part but not
given time to make its way into defects to react with the trapped penetrant.
Controlling the reaction time is of essential importance when using a post-emulsifiable
system. If the emulsification time is too short, an excessive amount of penetrant will
be left on the surface, leading to high background levels. If the emulsification time is
too long, the emulsifier will react with the penetrant entrapped in discontinuities,
making it possible to deplete the amount needed to form an indication.
The hydrophilic post-emulsifiable method (Method D) is more sensitive than the
lipophilic post-emulsifiable method (Method B). The major advantage of hydrophilic
emulsifiers is that they are less sensitive to variation in the contact and removal time.
When using an emulsifiable penetrant is used, the penetrant inspection process
includes the following steps (extra steps are underlined): 1. pre-clean part, 2. apply
penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. apply
hydrophilic emulsifier and allow contact for specified time, 5. rinse to remove excess
penetrant, 6. dry part, 7. apply developer and allow part to develop, and 8. inspect.
Rinse Method and Time for Water-Washable Penetrants
The method used to rinse the excess penetrant from the object surface and the time of
the rinse should be controlled so as to prevent over-washing. It is generally
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recommended that a coarse spray rinse or an air-agitated, immersion wash tank be
used. When a spray is being used, it should be directed at a 45° angle to the part
surface so as to not force water directly into any discontinuities that may be present.
The spray or immersion time should be kept to a minimum through frequent
inspections of the remaining background level.
Hand Wiping of Solvent Removable Penetrants
When a solvent removable penetrant is used, care must also be taken to carefully
remove the penetrant from the part surface while removing as little as possible from
the flaw. The first step in this cleaning procedure is to dry wipe the surface of the part
in one direction using a white, lint-free, cotton rag. One dry pass in one direction is all
that should be used to remove as much penetrant as possible. Next, the surface should
be wiped with one pass in one direction with a rag moistened with cleaner. One dry
pass followed by one damp pass is all that is recommended. Additional wiping may
sometimes be necessary; but keep in mind that with every additional wipe, some of
the entrapped penetrant will be removed and inspection sensitivity will be reduced.

Use and Selection of a Developer
The use of developer is almost always recommended. The output from a fluorescent
penetrant is improved significantly when a suitable powder developer is used. Also, the
use of developer can have a dramatic effect on the probability of detection of an
inspection.
Nonaqueous developers are generally recognized as the most sensitive when properly
applied. However, if the thickness of the coating becomes too great, defects can be
masked. The relative sensitivities of developers and application techniques as ranked
in Volume II of the Nondestructive Testing Handbook are shown in the table below.
Ranking
1
2
3
4
5
6
7
8
9
10
Developer Form
Nonaqueous, Wet Solvent
Plastic Film
Water-Soluble
Water-Suspendable
Water-Soluble
Water-Suspendable
Dry
Dry
Dry
Dry
Method of Application
Spray
Spray
Spray
Spray
Immersion
Immersion
Dust Cloud (Electrostatic)
Fluidized Bed
Dust Cloud (Air Agitation)
Immersion (Dip)
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The following table lists the main advantages and disadvantages of the various
developer types.
Developer Advantages Disadvantages
Dry Indications tend to remain
brighter and more distinct
over time
Easily to apply
Does not form contrast
background so cannot be
used with visible systems
Difficult to assure entire part
surface has been coated
Soluble Ease of coating entire part
White coating for good
contrast can be produced
which work well for both
visible and fluorescent
systems
Coating is translucent and
provides poor contrast (not
recommended for visual
systems)
Indications for water
washable systems are dim
and blurred
Suspendable Ease of coating entire part
Indications are bright and
sharp
White coating for good
contrast can be produced
which work well for both
visible and fluorescent
systems
Indications weaken and
become diffused after time
Nonaqueous Very portable
Easy to apply to readily
accessible surfaces
White coating for good
contrast can be produced
which work well for both
visible and fluorescent
systems
Indications show-up rapidly
and are well defined
Provides highest sensitivity
Difficult to apply evenly to
all surfaces
More difficult to clean part
after inspection

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Quality & Process Control
Quality control of the penetrant inspection process is essential to get good and
consistent results. Since several steps and materials are involved in the inspection
process, there are quality control procedures for each of them.

Temperature Control
The temperature of the penetrant materials and the part being inspected can have an
effect on the results. Temperatures from 27 to 49°C are reported in the literature to
produce optimal results. Many specifications allow testing in the range of 4 to 52°C.
Raising the temperature beyond this level will significantly raise the speed of
evaporation of penetrants causing them to dry out quickly.
Since the surface tension of most materials decrease as the temperature increases,
raising the temperature of the penetrant will increase the wetting of the surface and
the capillary forces. Of course, the opposite is also true, so lowering the temperature
will have a negative effect on the flow characteristics.

Penetrant Quality Control
The quality of a penetrant inspection is highly dependent on the quality of the
penetrant materials used. Only products meeting the requirements of an industry
specification, such as AMS 2644, should be used. Deterioration of new penetrants
primarily results from aging and contamination. Virtually all organic dyes deteriorate
over time, resulting in a loss of color or fluorescent response, but deterioration can be
slowed with proper storage. When possible, keep the materials in a closed container
and protect from freezing and exposure to high heat.
Contamination can occur during storage and use. Of course, open tank systems are
much more susceptible to contamination than are spray systems. Regular checks must
be performed to ensure that the material performance has not degraded. When the
penetrant is first received from the manufacturer, a sample of the fresh solution
should be collected and stored as a standard for future comparison. The standard
specimen should be stored in a sealed, opaque glass or metal container. Penetrants
that are in-use should be compared regularly to the standard specimen to detect any
changes in properties or performance.

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Dwell Quality Control
Dwell times are usually recommended by the penetrant producer or required by the
specification being followed. The only real quality control required in the dwell step of
the process is to ensure that a minimum dwell time is reached. There is no harm in
allowing a penetrant to dwell longer than the minimum time as long as the penetrant
is not allowed to dry on the part.

Emulsifier Path Quality Control
Quality control of the emulsifier bath is important and it should be performed per the
requirements of the applicable specification.
Lipophilic Emulsifiers
Lipophilic emulsifiers mix with penetrants but when the concentration of penetrant
contamination in the emulsifier becomes too great, the mixture will not function
effectively as a remover. Standards require that lipophilic emulsifiers be capable of
20% penetrant contamination without a reduction in performance. When the cleaning
action of the emulsifier becomes less than that of new material, it should be replaced.
Hydrophilic Emulsifiers
Hydrophilic emulsifiers have less tolerance for penetrant contamination. The penetrant
tolerance varies with emulsifier concentration and the type of contaminating
penetrant. In some cases, as little as 1% (by volume) penetrant contamination can
seriously affect the performance of an emulsifier.
Emulsifier Concentration and Contact Time
The optimal emulsifier contact time is dependent on a number of variables that include
the emulsifier used, the emulsifier concentration, the surface roughness of the part
being inspected, and other factors. Usually some experimentation is required to select
the proper emulsifier contact time.

Wash Quality Control
The wash temperature, pressure and time are three parameters that are typically
controlled in penetrant inspection process specification. A coarse spray or an
immersion wash tank with air agitation is often used. When the spray method is used,
the water pressure is usually limited to 276 kPa. The temperature range of the water is
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usually specified as a wide range (e.g., 10 to 38°C). The wash time should only be as
long as necessary to decrease the background to an acceptable level. Frequent visual
checks of the part should be made to determine when the part has been adequately
rinsed.

Drying Process Quality Control
The temperature used to dry parts after the application of an aqueous wet developer
or prior to the application of a dry powder or a nonaqueous wet developer, must be
controlled to prevent drying in the penetrant in the flaw. To prevent harming the
penetrant material, drying temperature should be kept to less than 71°C. Also, the
drying time should be limited to the minimum length necessary to thoroughly dry the
component being inspected.

Developer Quality Control
The function of the developer is very important in a penetrant inspection. In order to
accomplish its functions, a developer must adhere to the part surface and result in a
uniform, highly porous layer with many paths for the penetrant to be moved due to
capillary action. Developers are either applied wet or dry, but the desired end result is
always a uniform, highly porous, surface layer. Since the quality control requirements
for each of the developer types is slightly different, they will be covered individually.
Dry Powder Developer
A dry powder developer should be checked daily to ensure that it is fluffy and not
caked. It should be similar to fresh powdered sugar and not granulated like powdered
soap. It should also be relatively free from specks of fluorescent penetrant material
from previous inspection. This check is performed by spreading a sample of the
developer out and examining it under UV light.
When using the developer, a light coat is applied by immersing the test component or
dusting the surface. After the development time, excessive powder can be removed by
gently blowing on the surface with air not exceeding 35 kPa.
Wet Soluble/Suspendable Developer
Wet soluble developer must be completely dissolved in the water and wet
suspendable developer must be thoroughly mixed prior to application. The
concentration of powder in the carrier solution must be controlled in these developers.
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The concentration should be checked at least weekly using a hydrometer to make sure
it meets the manufacturer's specification. To check for contamination, the solution
should be examined weekly using both white light and UV light. Some specifications
require that a clean aluminum panel be dipped in the developer, dried, and examined
for indications of contamination by fluorescent penetrant materials.
These developers are applied by spraying, flowing or immersing the component. They
should never be applied with a brush. Care should be taken to avoid a heavy
accumulation of the developer solution in crevices and recesses.
Solvent Suspendable
Solvent suspendable developers are typically supplied in sealed aerosol spray cans.
Since the developer solution is in a sealed vessel, direct check of the solution is not
possible. However, the way that the developer is dispensed must be monitored. The
spray developer should produce a fine, even coating on the surface of the part. Make
sure the can is well shaken and apply a thin coating to a test article. If the spray
produces spatters or an uneven coating, the can should be discarded.
When applying a solvent suspendable developer, it is up to the inspector to control the
thickness of the coating. With a visible penetrant system, the developer coating must
be thick enough to provide a white contrasting background but not heavy enough to
mask indications. When using a fluorescent penetrant system, a very light coating
should be used. The developer should be applied under white light and should appear
evenly transparent.
Development Time
Parts should be allowed to develop for a minimum of 10 minutes and no more than 2
hours before inspecting.

Lighting Quality Control
Proper lighting is of great importance when visually inspecting a surface for a
penetrant indication. Obviously, the lighting requirements are different for an
inspection conducted using a visible dye penetrant than they are for an inspection
conducted using a fluorescent dye penetrant.
Lighting for Visible Dye Penetrant Inspections
When using a visible penetrant, the intensity of the white light is of principal
importance. Inspections can be conducted using natural lighting or artificial lighting.
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However, since natural daylight changes from time to time, the use of artificial lighting
is recommended to get better uniformity. Artificial lighting should be white whenever
possible (halogen lamps are most commonly used). The light intensity is required to be
100 foot-candles at the surface being inspected.
Lighting for Fluorescent Penetrant Inspections
Fluorescent penetrant dyes are excited by UV light of 365nm wavelength and emit
visible light somewhere in the green-yellow range between 520 and 580nm. The
source of ultraviolet light is often a mercury arc lamp with a filter. The lamps emit
many wavelengths and a filter is used to remove all but the UV and a small amount of
visible light between 310 and 410nm. Visible light of wavelengths above 410nm
interferes with contrast, and UV emissions below 310nm include some hazardous
wavelengths.
Standards and procedures require verification of filter condition and light intensity. The
black light filter should be clean and the light should never be used with a cracked
filter. Most UV light must be warmed up prior to use and should be on for at least 15
minutes before beginning an inspection. Since fluorescent brightness is linear with
respect to ultraviolet excitation, a change in the intensity of the light (from age or
damage) and a change in the distance of the light source from the surface being
inspected will have a direct impact on the inspection. For UV lights used in component
evaluations, the normally accepted intensity is 1000 µW/cm
2
at 38cm distance from
the filter face. The required check should be performed when a new bulb is installed,
at startup of the inspection cycle, if a change in intensity is noticed, or every eight
hours of continuous use.
When performing a fluorescent penetrant inspection, it is important to keep white
light to a minimum as it will significantly reduce the inspector’s ability to detect
fluorescent indications. Light levels of less than 2 foot-candles are required by most
procedures. When checking black light intensity a reading of the white light produced
by the black light may be required to verify white light is being removed by the filter.
Light Measurement
Light intensity measurements are made using a radiometer (an instrument that
transfers light energy into an electrical current). Some radiometers have the ability to
measure both black and white light, while others require a separate sensor for each
measurement. Whichever type is used, the sensing area should be clean and free of
any materials that could reduce or obstruct light reaching the sensor. Radiometers are
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relatively unstable instruments and readings often change considerable over time.
Therefore, they should be calibrated at least every six months.

System Performance Check
A system performance check is typically required daily, at the reactivation of a system
after maintenance or repairs, or any time the system is suspected of being out of
control. System performance checks involve processing a test specimen with known
defects to determine if the process will reveal discontinuities of the size required. The
specimen must be processed following the same procedure used to process production
parts. The ideal specimen is a production item that has natural defects of the minimum
acceptable size. As with penetrant inspections in general, results are directly
dependent on the skill of the operator and, therefore, each
operator should process a test specimen.
There are some universal test specimens that can be used if a
standard part is not available. The most commonly used test
specimen is the TAM or PSM panel which is used for
fluorescent penetrant systems. These panels are usually made
of stainless steel that has been chrome plated on one half and
surfaced finished on the other half to produce the desired
roughness. The chrome plated section is impacted from the back
side to produce a starburst set of cracks in the chrome. There are
five impacted areas with a range of different crack sizes
corresponding to the five levels of sensitivity.
Care of system performance check specimens is critical. Specimens
should be handled carefully to avoid damage. They should be
cleaned thoroughly between uses and storage in a solvent is
generally recommended. Before processing a specimen, it should
be inspected under UV light to make sure that it is clean and not
already producing an indication.

Nature of the Defect
The nature of the defect can have a large effect on sensitivity of a liquid penetrant
inspection. Sensitivity is defined as the smallest defect that can be detected with a high
degree of reliability. Typically, the crack length at the sample surface is used to define
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size of the defect. However, the crack length alone does not determine whether a flaw
will be seen or go undetected. The volume of the defect is likely to be the more
important feature. The flaw must be of sufficient volume so that enough penetrant will
bleed back out to a size that is detectable by the eye or that will satisfy the
dimensional thresholds of fluorescence. The figure shows an example of fluorescent
penetrant inspection probability of detection (POD) curve as a function of crack length.


In general, penetrant testing is more effective at finding:
 Small round defects than small linear defects.
 Deeper flaws than shallow flaws.
 Flaws with a narrow opening at the surface than wide open flaws.
 Flaws on smooth surfaces than on rough surfaces.
 Flaws with rough fracture surfaces than smooth fracture surfaces.
 Flaws under tensile or no loading than flaws under compression loading.


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Health and Safety Precautions
When proper health and safety precautions are followed, liquid penetrant inspection
operations can be completed without harm to inspection personnel. However, there is
a number of health and safety related issues that need to be taken in consideration.
The most common of those are discussed here.
Chemical Safety
Whenever chemicals must be handled, certain precautions must be taken. Before
working with a chemical of any kind, it is highly recommended that the material safety
data sheets (MSDS) be reviewed so that proper chemical safety and hygiene practices
can be followed. Some of the penetrant materials are flammable and, therefore,
should be used and stored in small quantities. They should only be used in a well
ventilated area and ignition sources avoided. Eye protection should always be worn to
prevent contact of the chemicals with the eyes. Gloves and other protective clothing
should be worn to limit contact with the chemicals.
Ultraviolet Light Safety
Ultraviolet (UV) light has wavelengths ranging from 180 to 400 nanometers. These
wavelengths place UV light in the invisible part of the electromagnetic spectrum
between visible light and X-rays. The most familiar source of UV radiation is the sun
and is necessary in small doses for certain chemical processes to occur in the body.
However, too much exposure can be harmful to the skin and eyes. The greatest threat
with UV light exposure is that the individual is generally unaware that the damage is
occurring. There is usually no pain associated with the injury until several hours after
the exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter
which is well below the 365 nm wavelength, where penetrants are designed to
fluoresce. Therefore, UV lamps sold for use in penetrant testing are almost always
filtered to remove the harmful UV wavelengths. The lamps produce radiation at the
harmful wavelengths so it is essential that they be used with the proper filter in place
and in good condition.
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93. MAGNETIC PARTICLE TESTING Library Study Material
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94. Introduction
•This module is intended to present
information on the widely used method of
magnetic particle inspection.
•Magnetic particle inspection can detect
both production discontinuities (seams,
laps, grinding cracks and quenching cracks)
and in-service damage (fatigue and overload
cracks).
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95. Outline
•Magnetism and Ferromagnetic Materials
•Introduction of Magnetic Particle Inspection
•Basic Procedure and Important
Considerations
1.Component pre-cleaning
2.Introduction of magnetic field
3.Application of magnetic media
4.Interpretation of magnetic particle indications
•Examples of MPI Indications Library Study Material
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96. Magnetic lines of force
around a bar magnet
Opposite poles attracting Similar poles repelling
Introduction to Magnetism
Magnetism is the ability of matter to
attract other matter to itself. Objects
that possess the property of
magnetism are said to be magnetic or
magnetized and magnetic lines of
force can be found in and around the
objects. A magnetic pole is a point
where the a magnetic line of force
exits or enters a material.
Magnetic field lines:
•Form complete loops.
•Do not cross.
•Follow the path of least
resistance.
•All have the same strength.
•Have a direction such that
they cause poles to attract
or repel. Library Study Material
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97. How Does Magnetic Particle
Inspection Work?
A ferromagnetic test specimen is magnetized with a
strong magnetic field created by a magnet or special
equipment. If the specimen has a discontinuity, the
discontinuity will interrupt the magnetic field
flowing through the specimen and a leakage field
will occur. Library Study Material
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98. How Does Magnetic Particle
Inspection Work? (Cont.)
Finely milled iron particles coated with a dye pigment
are applied to the test specimen. These particles are
attracted to leakage fields and will cluster to form an
indication directly over the discontinuity. This
indication can be visually detected under proper
lighting conditions. Library Study Material
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99. Basic Procedure
Basic steps involved:

1.Component pre-cleaning

2.Introduction of magnetic field

3.Application of magnetic media

4.Interpretation of magnetic particle indications
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100. Pre-cleaning
When inspecting a test part with the magnetic
particle method it is essential for the particles to
have an unimpeded path for migration to both
strong and weak leakage fields alike. The part’s
surface should be clean and dry before inspection.
Contaminants such as oil,
grease, or scale may not
only prevent particles from
being attracted to leakage
fields, they may also
interfere with interpretation
of indications. Library Study Material
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101. Introduction of the Magnetic Field
The required magnetic field can be introduced into a
component in a number of different ways.
1.Using a permanent magnet or an electromagnet that
contacts the test piece
2.Flowing an electrical current through the specimen
3.Flowing an electrical current through a coil of wire around
the part or through a central conductor running near the
part. Library Study Material
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102. Direction of the Magnetic Field

Two general types of magnetic fields (longitudinal
and circular) may be established within the specimen.
The type of magnetic field established is determined
by the method used to magnetize the specimen.
•A longitudinal magnetic field has
magnetic lines of force that run
parallel to the long axis of the
part.
•A circular magnetic field has
magnetic lines of force that run
circumferentially around the
perimeter of a part. Library Study Material
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103. Importance of Magnetic Field Direction
Being able to magnetize the part in two
directions is important because the best
detection of defects occurs when the lines of
magnetic force are established at right angles to
the longest dimension of the defect. This
orientation creates the largest disruption of the
magnetic field within the part and the greatest
flux leakage at the surface of the part. An
orientation of 45 to 90 degrees between the
magnetic field and the defect is necessary to
form an indication.

Since defects may
occur in various and
unknown directions,
each part is normally
magnetized in two
directions at right
angles to each other.
Flux Leakage
No Flux Leakage Library Study Material
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104. Question
? From the previous slide regarding the optimum
test sensitivity, which kinds of defect are easily
found in the images below?
Longitudinal (along the axis) Transverse (perpendicular the axis) Library Study Material
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105. Producing a Longitudinal Magnetic Field
Using a Coil
A longitudinal magnetic field
is usually established by
placing the part near the
inside or a coil’s annulus. This
produces magnetic lines of
force that are parallel to the
long axis of the test part.
Coil on Wet Horizontal Inspection Unit
Portable Coil Library Study Material
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106. Producing a Longitudinal Field Using
Permanent or Electromagnetic Magnets
Permanent magnets and
electromagnetic yokes
are also often used to
produce a longitudinal
magnetic field. The
magnetic lines of force
run from one pole to the
other, and the poles are
positioned such that any
flaws present run normal
to these lines of force. Library Study Material
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107. Circular Magnetic Fields
Circular magnetic fields are produced by
passing current through the part or by
placing the part in a strong circular
magnet field.
A headshot on a wet horizontal test unit
and the use of prods are several common
methods of injecting current in a part to
produce a circular magnetic field. Placing
parts on a central conductors carrying
high current is another way to produce the
field.
Magnetic Field
Electric
Current Library Study Material
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108. Application of Magnetic
Media (Wet Versus Dry)
MPI can be performed using either
dry particles, or particles
suspended in a liquid.

With the dry method, the particles
are lightly dusted on to the
surface. With the wet method, the
part is flooded with a solution
carrying the particles.
The dry method is more portable.
The wet method is generally more
sensitive since the liquid carrier
gives the magnetic particles
additional mobility. Library Study Material
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109. Dry Magnetic Particles
Magnetic particles come in a variety of colors. A
color that produces a high level of contrast against
the background should be used.


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110. Wet Magnetic Particles
Wet particles are typically supplied
as visible or fluorescent.
Visible particles are viewed under
normal white light and;
fluorescent particles are viewed
under black light. Library Study Material
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111. Interpretation of Indications
After applying the magnetic field, indications that
form must interpreted. This process requires that
the inspector distinguish between relevant and non-
relevant indications.
The following series of images depict
relevant indications produced from a
variety of components inspected
with the magnetic particle method. Library Study Material
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112. Crane Hook with
Service Induced Crack

Fluorescent, Wet Particle Method Library Study Material
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113. Gear with
Service Induced Crack

Fluorescent, Wet Particle Method Library Study Material
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114. Drive Shaft with
Heat Treatment Induced Cracks

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115. Splined Shaft with
Service Induced Cracks

Fluorescent, Wet Particle Method Library Study Material
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116. Threaded Shaft with
Service Induced Crack

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117. Large Bolt with
Service Induced Crack
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118. Crank Shaft with
Service Induced Crack Near Lube Hole
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119. Lack of Fusion in SMAW Weld
Visible, Dry Powder Method
Indication Library Study Material
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120. Toe Crack in SMAW Weld
Visible, Dry Powder Method Library Study Material
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121. Throat and Toe Cracks in
Partially Ground Weld
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122. Demagnetization
•Parts inspected by the magnetic particle method
may sometimes have an objectionable residual
magnetic field that may interfere with subsequent
manufacturing operations or service of the
component.
•Possible reasons for demagnetization include:
–May interfere with welding and/or machining
operations
–Can effect gauges that are sensitive to magnetic
fields if placed in close proximity.
–Abrasive particles may adhere to components
surface and cause and increase in wear to engines
components, gears, bearings etc. Library Study Material
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123. Demagnetization (Cont.)
•Demagnetization requires that the residual
magnetic field is reversed and reduced by the
inspector.
•This process will scramble the magnetic domains
and reduce the strength of the residual field to an
acceptable level.
Demagnetized Magnetized Library Study Material
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124. Advantages of
Magnetic Particle Inspection
•Can detect both surface and near sub-surface defects.
•Can inspect parts with irregular shapes easily.
•Pre-cleaning of components is not as critical as it is for
some other inspection methods. Most contaminants
within a flaw will not hinder flaw detectability.
•Fast method of inspection and indications are visible
directly on the specimen surface.
•Considered low cost compared to many other NDT
methods.
•Is a very portable inspection method especially when used
with battery powered equipment. Library Study Material
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125. Limitations of
Magnetic Particle Inspection
•Cannot inspect non-ferrous materials such as
aluminum, magnesium or most stainless steels.
•Inspection of large parts may require use of equipment
with special power requirements.
•Some parts may require removal of coating or plating
to achieve desired inspection sensitivity.
•Limited subsurface discontinuity detection capabilities.
Maximum depth sensitivity is approximately 0.6” (under ideal
conditions).
•Post cleaning, and post demagnetization is often
necessary.
•Alignment (angle) between magnetic flux and defect
is important. Library Study Material
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126. Glossary of Terms
•Black Light: ultraviolet light which is filtered to produce
a wavelength of approximately 365 nanometers.
Black light will cause certain materials to fluoresce.
•Central conductor: an electrically conductive bar
usually made of copper used to introduce a circular
magnetic field in to a test specimen.
•Coil: an electrical conductor such a copper wire or cable
that is wrapped in several or many loops that are brought
close to one another to form a strong longitudinal
magnetic field. Library Study Material
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127. Glossary of Terms
•Discontinuity: an interruption in the structure of the
material such as a crack.
•Ferromagnetic: a material such as iron, nickel and cobalt
or one of it’s alloys that is strongly attracted to a magnetic
field.
•Heads: electrical contact pads on a wet horizontal
magnetic particle inspection machine. The part to be
inspected is clamped and held in place between the heads
and shot of current is sent through the part from the
heads to create a circular magnetic field in the part.
•Leakage field: a disruption in the magnetic field. This
disruption must extend to the surface of the part for
particles to be attracted.
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128. Glossary of Terms
•Non-relevant indications: indications produced due to
some intended design feature of a specimen such
•a keyways, splines or press fits.
•Prods: two electrodes usually made of copper or
aluminum that are used to introduce current in to a test
part. This current in turn creates a circular magnetic field
where each prod touches the part. (Similar in principal to a
welding electrode and ground clamp).
•Relevant indications: indications produced from
something other than a design feature of a test specimen.
Cracks, stringers, or laps are examples of relevant
indications. Library Study Material
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129. Glossary of Terms
•Suspension: a bath created by mixing particles with
either oil or water.
•Yoke: a horseshoe magnet used to create a longitudinal
magnetic field.
•Yokes may be made from permanent magnets or electromagnets. Library Study Material
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130. Introduction to Non-Destructive Testing Techniques
Magnetic Particle Testing Page 1 of 34
Magnetic Particle Testing
Magnetic particle testing is one of the most widely utilized NDT methods
since it is fast and relatively easy to apply and part surface preparation is
not as critical as it is for some other methods. This mithod uses magnetic
fields and small magnetic particles (i.e.iron filings) to detect flaws in
components. The only requirement from an inspectability standpoint is
that the component being inspected must be made of a ferromagnetic
material (a materials that can be magnetized) such as iron, nickel, cobalt,
or some of their alloys.
The method is used to inspect a variety of product forms including
castings, forgings, and weldments. Many different industries use
magnetic particle inspection such as structural steel, automotive,
petrochemical, power generation, and aerospace industries. Underwater
inspection is another area where magnetic particle inspection may be
used to test items such as offshore structures and underwater pipelines.

Basic Principles
In theory, magnetic particle testing has a relatively simple concept. It can be
considered as a combination of two nondestructive testing methods: magnetic flux
leakage testing and visual testing. For the case of a bar
magnet, the magnetic field is in and around the magnet.
Any place that a magnetic line of force exits or enters the
magnet is called a “pole” (magnetic lines of force exit the
magnet from north pole and enter from the south pole).
When a bar magnet is broken in the center of its length, two complete bar magnets
with magnetic poles on each end of each piece will result. If the magnet is just cracked
but not broken completely in two, a north and south pole will form at each edge of the
crack. The magnetic field exits the north pole and reenters at the south pole. The
magnetic field spreads out when it encounters the
small air gap created by the crack because the air
cannot support as much magnetic field per unit
volume as the magnet can. When the field spreads
out, it appears to leak out of the material and, thus
is called a flux leakage field.
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131. Introduction to Non-Destructive Testing Techniques
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If iron particles are sprinkled on a cracked magnet, the particles will be attracted to
and cluster not only at the poles at the ends of the magnet, but also at the poles at the
edges of the crack. This cluster of particles is much easier to see than the actual crack
and this is the basis for magnetic particle inspection.
The first step in a magnetic particle testing is to magnetize the component that is to be
inspected. If any defects on or near the
surface are present, the defects will
create a leakage field. After the
component has been magnetized, iron
particles, either in a dry or wet
suspended form, are applied to the
surface of the magnetized part. The
particles will be attracted and cluster
at the flux leakage fields, thus forming
a visible indication that the inspector can detect.

Advantages and Disadvantages
The primary advantages and disadvantages when compared to other NDT methods
are:
Advantages
 High sensitivity (small discontinuities can be detected).
 Indications are produced directly on the surface of the part and constitute a visual
representation of the flaw.
 Minimal surface preparation (no need for paint removal)
 Portable (small portable equipment & materials available in spray cans)
 Low cost (materials and associated equipment are relatively inexpensive)
Disadvantages
 Only surface and near surface defects can be detected.
 Only applicable to ferromagnetic materials.
 Relatively small area can be inspected at a time.
 Only materials with a relatively nonporous surface can be inspected.
 The inspector must have direct access to the surface being inspected.

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132. Introduction to Non-Destructive Testing Techniques
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Magnetism
The concept of magnetism centers around the magnetic field and what is known as a
dipole. The term "magnetic field" simply describes a volume of space where there is a
change in energy within that volume. The location where a magnetic field exits or
enters a material is called a magnetic pole. Magnetic poles have never been detected
in isolation but always occur in pairs, hence the name dipole. Therefore, a dipole is an
object that has a magnetic pole on one end and a second, equal but opposite,
magnetic pole on the other. A bar magnet is a dipole with a north pole at one end and
south pole at the other.
The source of magnetism lies in the basic building block of all matter,
the atom. Atoms are composed of protons, neutrons and electrons.
The protons and neutrons are located in the atom's nucleus and the
electrons are in constant motion around the nucleus. Electrons carry
a negative electrical charge and produce a magnetic field as they
move through space. A magnetic field is produced whenever an
electrical charge is in motion. The strength of this field is called the
magnetic moment.
When an electric current flows through a conductor, the movement of electrons
through the conductor causes a magnetic field to form around the conductor. The
magnetic field can be detected using a compass. Since all matter is comprised of
atoms, all materials are affected in some way by a magnetic field; however, materials
do not react the same way to the magnetic field.

Reaction of Materials to Magnetic Field
When a material is placed within a magnetic field, the magnetic forces of the material's
electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction.
However, materials can react quite differently to the presence of an external magnetic
field. The magnetic moments associated with atoms have three origins: the electron
motion, the change in motion caused by an external magnetic field, and the spin of the
electrons.
In most atoms, electrons occur in pairs where these pairs spin in
opposite directions. The opposite spin directions of electron pairs
cause their magnetic fields to cancel each other. Therefore, no net
magnetic field exists. Alternately, materials with some unpaired
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electrons will have a net magnetic field and will react more to an external field.
According to their interaction with a magnetic field, materials can be classified as:
Diamagnetic materials which have a weak, negative susceptibility to magnetic
fields. Diamagnetic materials are slightly repelled by a magnetic field and the
material does not retain the magnetic properties when the external field is
removed. In diamagnetic materials all the electrons are paired so there is no
permanent net magnetic moment per atom. Most elements in the periodic table,
including copper, silver, and gold, are diamagnetic.
Paramagnetic materials which have a small, positive susceptibility to magnetic
fields. These materials are slightly attracted by a magnetic field and the material
does not retain the magnetic properties when the external field is removed.
Paramagnetic materials have some unpaired electrons. Examples of paramagnetic
materials include magnesium, molybdenum, and lithium.
Ferromagnetic materials have a large, positive susceptibility to an external
magnetic field. They exhibit a strong attraction to magnetic fields and are able to
retain their magnetic properties after the external field has been removed.
Ferromagnetic materials have some unpaired electrons so their atoms have a net
magnetic moment. They get their strong magnetic properties due to the presence
of magnetic domains. In these domains, large numbers of atom's moments are
aligned parallel so that the magnetic force within the domain is strong (this happens
during the solidification of the material where the atom moments are aligned within
each crystal ”i.e., grain” causing a strong magnetic force in one direction). When a
ferromagnetic material is in the
unmagnetized state, the domains are nearly
randomly organized (since the crystals are in
arbitrary directions) and the net magnetic
field for the part as a whole is zero. When a
magnetizing force is applied, the domains
become aligned to produce a strong
magnetic field within the part. Iron, nickel,
and cobalt are examples of ferromagnetic
materials. Components made of these
materials are commonly inspected using the
magnetic particle method.

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Magnetic Field Characteristics
Magnetic Field In and Around a Bar Magnet
The magnetic field surrounding a bar magnet can be seen in the magnetograph below.
A magnetograph can be created by placing a piece
of paper over a magnet and sprinkling the paper
with iron filings. The particles align themselves
with the lines of magnetic force produced by the
magnet. It can be seen in the magnetograph that
there are poles all along the length of the magnet
but that the poles are concentrated at the ends of
the magnet (the north and south poles).

Magnetic Fields in and around Horseshoe and Ring Magnets
Magnets come in a variety of shapes and one of the more common is
the horseshoe (U) magnet. The horseshoe magnet has north and
south poles just like a bar magnet but the magnet is curved so the
poles lie in the same plane. The magnetic lines of force flow from pole
to pole just like in the bar magnet. However, since the poles are
located closer together and a more direct path exists for the lines of
flux to travel between the poles, the magnetic field is concentrated
between the poles.

General Properties of Magnetic Lines of Force
Magnetic lines of force have a number of important properties, which include:
 They seek the path of least resistance between opposite
magnetic poles (in a single bar magnet shown, they attempt to
form closed loops from pole to pole).
 They never cross one another.
 They all have the same strength.
 Their density decreases with increasing distance from the poles.
 Their density decreases (they spread out) when they move from
an area of higher permeability to an area of lower permeability.
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 They are considered to have direction as if flowing, though no actual movement
occurs.
 They flow from the south pole to the north pole within a material and north pole
to south pole in air.

Electromagnetic Fields
Magnets are not the only source of magnetic fields. The flow of electric current
through a conductor generates a magnetic field. When electric current flows in a long
straight wire, a circular magnetic field is generated around the wire and the intensity of
this magnetic field is directly proportional to the amount of current
carried by the wire. The strength of the field is strongest next to
the wire and diminishes with distance. In most conductors, the
magnetic field exists only as long as the current is flowing.
However, in ferromagnetic materials the electric current will cause
some or all of the magnetic domains to align and a residual
magnetic field will remain.
Also, the direction of the magnetic field is dependent on the direction of the electrical
current in the wire. The direction of the magnetic field around a conductor can be
determined using a simple rule called the “right-hand clasp rule”. If a person grasps a
conductor in one's right hand with the thumb pointing in the direction of the current,
the fingers will circle the conductor in the direction of the magnetic field.
Note: remember that current flows from the positive terminal to the negative
terminal (electrons flow in the opposite direction).

Magnetic Field Produced by a Coil
When a current carrying wire is formed into
several loops to form a coil, the magnetic field
circling each loop combines with the fields from
the other loops to produce a concentrated field
through the center of the coil (the field flows along
the longitudinal axis and circles back around the
outside of the coil).
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When the coil loops are tightly wound, a uniform magnetic field is developed
throughout the length of the coil. The strength of the magnetic field increases not only
with increasing current but also with each loop that is added to the coil. A long,
straight coil of wire is called a solenoid and it can be used to generate a nearly uniform
magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a
coil is very useful in magnetizing ferromagnetic materials for inspection using the
magnetic particle testing method.

Quantifying Magnetic Properties
The various characteristics of magnetism can be measured and expressed
quantitatively. Different systems of units can be used for quantifying magnetic
properties. SI units will be used in this material. The advantage of using SI units is that
they are traceable back to an agreed set of four base units; meter, kilogram, second,
and Ampere.
 The unit for magnetic field strength H is ampere/meter
(A/m). A magnetic field strength of 1 A/m is produced
at the center of a single circular conductor with a 1
meter diameter carrying a steady current of 1 ampere.

 The number of magnetic lines of force cutting through a plane of a given area at
a right angle is known as the magnetic flux density, B. The flux density or
magnetic induction has the Tesla as its unit. One Tesla is equal to 1
Newton/(A/m). From these units, it can be seen that the flux density is a
measure of the force applied to a particle by the magnetic field.

 The total number of lines of magnetic force in a material is called magnetic flux,
ɸ. The strength of the flux is determined by the number of magnetic domains
that are aligned within a material. The total flux is simply the flux density applied
over an area. Flux carries the unit of a weber, which is simply a Tesla-meter
2
.

 The magnetization M is a measure of the extent to which an object is
magnetized. It is a measure of the magnetic dipole moment per unit volume of
the object. Magnetization carries the same units as a magnetic field A/m.


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Quantity SI Units
(Sommerfeld)
SI Units
(Kennelly)
CGS Units
(Gaussian)
Field
(Magnetization
Force)
H A/m A/m oersteds
Flux Density
(Magnetic
Induction)
B Tesla Tesla gauss
Flux ɸ Weber Weber maxwell
Magnetization M A/m - erg/Oe-cm
3


The Hysteresis Loop and Magnetic Properties
A great deal of information can be learned about the magnetic properties of a material
by studying its hysteresis loop. A hysteresis loop shows the relationship between the
induced magnetic flux density (B) and the magnetizing force (H). It is often referred to
as the B-H loop. An example hysteresis loop is shown below.

The loop is generated by measuring the magnetic flux of a ferromagnetic material
while the magnetizing force is changed. A ferromagnetic material that has never been
previously magnetized or has been thoroughly demagnetized will follow the dashed
line as H is increased. As the line demonstrates, the greater the amount of current
applied (H+), the stronger the magnetic field in the component (B+). At point "a"
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almost all of the magnetic domains are aligned and an additional increase in the
magnetizing force will produce very little increase in magnetic flux. The material has
reached the point of magnetic saturation. When H is reduced to zero, the curve will
move from point "a" to point "b". At this point, it can be seen that some magnetic flux
remains in the material even though the magnetizing force is zero. This is referred to as
the point of retentivity on the graph and indicates the level of residual magnetism in
the material (Some of the magnetic domains remain aligned but some have lost their
alignment). As the magnetizing force is reversed, the curve moves to point "c", where
the flux has been reduced to zero. This is called the point of coercivity on the curve
(the reversed magnetizing force has flipped enough of the domains so that the net flux
within the material is zero). The force required to remove the residual magnetism from
the material is called the coercive force or coercivity of the material.
As the magnetizing force is increased in the negative direction, the material will again
become magnetically saturated but in the opposite direction, point "d". Reducing H to
zero brings the curve to point "e". It will have a level of residual magnetism equal to
that achieved in the other direction. Increasing H back in the positive direction will
return B to zero. Notice that the curve did not return to the origin of the graph because
some force is required to remove the residual magnetism. The curve will take a
different path from point "f" back to the saturation point where it with complete the
loop.
From the hysteresis loop, a number of primary magnetic properties of a material can
be determined:
1. Retentivity - A measure of the residual flux density corresponding to the
saturation induction of a magnetic material. In other words, it is a material's
ability to retain a certain amount of residual magnetic field when the magnetizing
force is removed after achieving saturation (The value of B at point b on the
hysteresis curve).
2. Residual Magnetism or Residual Flux - The magnetic flux density that remains in a
material when the magnetizing force is zero. Note that residual magnetism and
retentivity are the same when the material has been magnetized to the saturation
point. However, the level of residual magnetism may be lower than the retentivity
value when the magnetizing force did not reach the saturation level.
3. Coercive Force - The amount of reverse magnetic field which must be applied to a
magnetic material to make the magnetic flux return to zero (The value of H at
point c on the hysteresis curve).
4. Permeability, µ - A property of a material that describes the ease with which a
magnetic flux is established in the material.
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5. Reluctance - Is the opposition that a ferromagnetic material shows to the
establishment of a magnetic field. Reluctance is analogous to the resistance in an
electrical circuit.

Permeability
As previously mentioned, permeability (µ) is a material property that describes the
ease with which a magnetic flux is established in a component. It is the ratio of the flux
density (B) created within a material to the magnetizing field (H) and is represented by
the following equation:
µ = B/H
This equation describes the slope of the curve at any
point on the hysteresis loop. The permeability value
given in letrature for materials is usually the
maximum permeability or the maximum relative
permeability. The maximum permeability is the point
where the slope of the B/H curve for the
unmagnetized material is the greatest. This point is
often taken as the point where a straight line from
the origin is tangent to the B/H curve.
The shape of the hysteresis loop tells a great deal about the material being
magnetized. The hysteresis curves of two different materials are shown in the graph.
 Relative to other materials, a material with a wider
hysteresis loop has:
- Lower Permeability
- Higher Retentivity
- Higher Coercivity
- Higher Reluctance
- Higher Residual Magnetism
 Relative to other materials, a material with a narrower
hysteresis loop has:
- Higher Permeability
- Lower Retentivity
- Lower Coercivity
- Lower Reluctance
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- Lower Residual Magnetism
In magnetic particle testing, the level of residual magnetism is important. Residual
magnetic fields are affected by the permeability, which can be related to the carbon
content and alloying of the material. A component with high carbon content will have
low permeability and will retain more magnetic flux than a material with low carbon
content.

Magnetic Field Orientation and Flaw Detectability
To properly inspect a component for cracks or other defects, it is important to
understand that the orientation of the crack relative to the magnetic lines of force
determinies if the crack can or cannot be detected. There are two general types of
magnetic fields that can be established within a component.
 A longitudinal magnetic field has magnetic lines of force
that run parallel to the long axis of the part. Longitudinal
magnetization of a component can be accomplished using
the longitudinal field set up by a coil or solenoid. It can also
be accomplished using permanent magnets or
electromagnets.
 A circular magnetic field has magnetic lines of force that
run circumferentially around the perimeter of a part. A
circular magnetic field is induced in an article by either
passing current through the component or by passing
current through a conductor surrounded by the
component.
The type of magnetic field established is determined by the method used to magnetize
the specimen. Being able to magnetize the part in two directions is important because
the best detection of defects occurs when the lines of magnetic force are established
at right angles to the longest dimension of the defect. This
orientation creates the largest disruption of the magnetic field
within the part and the greatest flux leakage at the surface of the
part. If the magnetic field is parallel to the defect, the field will
see little disruption and no flux leakage field will be produced.
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An orientation of 45 to 90 degrees between the magnetic field and the defect is
necessary to form an indication. Since defects may occur in various and unknown
directions, each part is normally magnetized in two directions at right angles to each
other. If the component shown is considered, it is known that passing current through
the part from end to end will establish a circular magnetic field that will be 90 degrees
to the direction of the current.
Therefore, defects that have a
significant dimension in the
direction of the current
(longitudinal defects) should be
detectable, while transverse-type
defects will not be detectable
with circular magnetization.

Magnetization of Ferromagnetic Materials
There are a variety of methods that can be used to establish a magnetic field in a
component for evaluation using magnetic particle inspection. It is common to classify
the magnetizing methods as either direct or indirect.
Magnetization Using Direct Induction (Direct Magnetization)
With direct magnetization, current is passed directly through the component. The flow
of current causes a circular magnetic field to form in and around the conductor. When
using the direct magnetization method, care must be taken to ensure that good
electrical contact is established and maintained between the test equipment and the
test component to avoid damage of the the component (due to arcing or overheating
at high resistance ponts).
There are several ways that direct magnetization is commonly accomplished.
- One way involves clamping the component between two
electrical contacts in a special piece of equipment.
Current is passed through the component and a circular
magnetic field is established in and around the
component. When the magnetizing current is stopped, a
residual magnetic field will remain within the component.
The strength of the induced magnetic field is proportional
to the amount of current passed through the component.
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- A second technique involves using clamps or prods, which are
attached or placed in contact with the component. Electrical
current flows through the component from contact to contact.
The current sets up a circular magnetic field around the path of
the current.
Magnetization Using Indirect Induction (Indirect Magnetization)
Indirect magnetization is accomplished by using a strong external magnetic field to
establish a magnetic field within the component. As with direct magnetization, there
are several ways that indirect magnetization can be accomplished.
- The use of permanent magnets is a low cost method of
establishing a magnetic field. However, their use is limited due
to lack of control of the field strength and the difficulty of
placing and removing strong permanent magnets from the
component.

- Electromagnets in the form of an adjustable
horseshoe magnet (called a yoke) eliminate
the problems associated with permanent
magnets and are used extensively in industry.
Electromagnets only exhibit a magnetic flux
when electric current is flowing around the
soft iron core. When the magnet is placed on
the component, a magnetic field is established
between the north and south poles of the
magnet.

- Another way of indirectly inducting a magnetic field in a material is
by using the magnetic field of a current carrying conductor. A
circular magnetic field can be established in cylindrical components
by using a central conductor. Typically, one or more cylindrical
components are hung from a solid copper bar running through the
inside diameter. Current is passed through the copper bar and the
resulting circular magnetic field establishes a magnetic field within
the test components.

- The use of coils and solenoids is a third method of indirect magnetization. When the
length of a component is several times larger than its diameter, a longitudinal
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magnetic field can be established in the
component. The component is placed
longitudinally in the concentrated
magnetic field that fills the center of a coil
or solenoid. This magnetization technique
is often referred to as a "coil shot".

Types of Magnetizing Current
As mentioned previously, electric current is often used to establish the magnetic field
in components during magnetic particle inspection. Alternating current (AC) and direct
current (DC) are the two basic types of current commonly used. The type of current
used can have an effect on the inspection results, so the types of currents commonly
used are briefly discussed here.
Direct Current
Direct current (DC) flows continuously in one direction at a constant voltage. A battery
is the most common source of direct current. The current is said to flow from the
positive to the negative terminal, though electrons flow in the opposite direction. DC is
very desirable when inspecting for subsurface defects because DC generates a
magnetic field that penetrates deeper into the material. In ferromagnetic materials,
the magnetic field produced by DC generally penetrates the entire cross-section of the
component.
Alternating Current
Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second.
Since AC is readily available in most facilities, it is convenient to make use of it for
magnetic particle inspection. However, when AC is used to induce a magnetic field in
ferromagnetic materials, the magnetic field will be limited to a thin layer at the surface
of the component. This phenomenon is known as the "skin effect" and it occurs
because the changing magnetic field generates eddy currents in the test object. The
eddy currents produce a magnetic field that opposes the primary field, thus reducing
the net magnetic flux below the surface. Therefore, it is recommended that AC be used
only when the inspection is limited to surface defects.


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Rectified Alternating Current
Clearly, the skin effect limits the use of AC since many inspection applications call for
the detection of subsurface defects. Luckily, AC can be converted to current that is very
much like DC through the process of rectification. With the use of rectifiers, the
reversing AC can be converted to a one directional current. The three commonly used
types of rectified current are described below.

Half Wave Rectified Alternating Current (HWAC)
When single phase alternating current is passed through a rectifier, current is allowed
to flow in only one direction. The reverse half of each cycle is blocked out so that a one
directional, pulsating current is produced. The current rises from zero to a maximum
and then returns to zero. No current flows during the time when the reverse cycle is
blocked out. The HWAC repeats at same rate as the unrectified current (50 or 60 Hz).
Since half of the current is blocked out, the amperage is half of the unaltered AC. This
type of current is often referred to as half wave DC or pulsating DC. The pulsation of
the HWAC helps in forming magnetic particle indications by vibrating the particles and
giving them added mobility where that is especially important when using dry
particles. HWAC is most often used to power electromagnetic yokes.
Full Wave Rectified Alternating Current (FWAC) (Single Phase)
Full wave rectification inverts the negative current to positive current rather than
blocking it out. This produces a pulsating DC with no interval between the pulses.
Filtering is usually performed to soften the sharp polarity switching in the rectified
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current. While particle mobility is not as good as half-wave AC due to the reduction in
pulsation, the depth of the subsurface magnetic field is improved.
Three Phase Full Wave Rectified Alternating Current
Three phase current is often used to power industrial equipment because it has more
favorable power transmission and line loading characteristics. This type of electrical
current is also highly desirable for magnetic particle testing because when it is rectified
and filtered, the resulting current very closely resembles direct current. Stationary
magnetic particle equipment wired with three phase AC will usually have the ability to
magnetize with AC or DC (three phase full wave rectified), providing the inspector with
the advantages of each current form.

Magnetic Fields Distribution and Intensity

Longitudinal Fields
When a long component is magnetized using a solenoid having a shorter length, only
the material within the solenoid and
about the same length on each side of
the solenoid will be strongly magnetized.
This occurs because the magnetizing
force diminishes with increasing distance
from the solenoid. Therefore, a long
component must be magnetized and
inspected at several locations along its
length for complete inspection coverage.

Circular Fields
When a circular magnetic field is forms in and around a conductor due to the passage
of electric current through it, the following can be said about the distribution and
intensity of the magnetic field:
- The field strength varies from zero at the center of the component to a maximum
at the surface.
- The field strength at the surface of the conductor decreases as the radius of the
conductor increases (when the current strength is held constant).
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- The field strength inside the conductor is dependent on the current strength,
magnetic permeability of the material, and if magnetic, the location on the B-H
curve.
- The field strength outside the conductor is directly proportional to the current
strength and it decreases with distance from the conductor.
The images below show the magnetic field strength graphed versus distance from the
center of the conductor when current passes through a solid circular conductor.
 In a nonmagnetic conductor carrying DC, the internal field strength rises from zero
at the center to a maximum value at the surface of the conductor.

 In a magnetic conductor carrying DC, the field strength within the conductor is
much greater than it is in the nonmagnetic conductor. This is due to the
permeability of the magnetic material. The external field is exactly the same for the
two materials provided the current level and conductor radius are the same.

 When the magnetic conductor is carrying AC, the internal magnetic field will be
concentrated in a thin layer near the surface of the conductor (skin effect). The
external field decreases with increasing distance from the surface same as with DC.


The magnetic field distribution in
and around a solid conductor of a
nonmagnetic material carrying
direct current.
The magnetic field distribution in
and around a solid conductor of a
magnetic material carrying direct
current.
The magnetic field distribution in
and around a solid conductor of a
magnetic material carrying
alternating current.

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In a hollow circular conductor there is no magnetic field in the void area. The magnetic
field is zero at the inner surface and rises until it reaches a maximum at the outer
surface.
 Same as with a solid conductor, when DC current is passed through a magnetic
conductor, the field strength within the conductor is much greater than in
nonmagnetic conductor due to the permeability of the magnetic material. The
external field strength decreases with distance from the surface of the conductor.
The external field is exactly the same for the two materials provided the current
level and conductor radius are the same.

 When AC current is passed through a hollow circular magnetic conductor, the skin
effect concentrates the magnetic field at the outside diameter of the component.




The magnetic field distribution in
and around a hollow conductor of a
nonmagnetic material carrying
direct current.
The magnetic field distribution in
and around a hollow conductor of
a magnetic material carrying direct
current.
The magnetic field distribution in
and around a hollow conductor of
a magnetic material carrying
alternating current.

As can be seen from these three field distribution images, the field strength at the
inside surface of hollow conductor is very low when a circular magnetic field is
established by direct magnetization. Therefore, the direct method of magnetization is
not recommended when inspecting the inside diameter wall of a hollow component
for shallow defects (if the defect has significant depth, it may be detectable using DC
since the field strength increases rapidly as one moves from the inner towards the outer
surface).
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 A much better method of magnetizing hollow
components for inspection of the ID and OD surfaces
is with the use of a central conductor. As can be seen
in the field distribution image, when current is passed
through a nonmagnetic central conductor (copper
bar), the magnetic field produced on the inside
diameter surface of a magnetic tube is much greater
and the field is still strong enough for defect detection
on the OD surface.


Demagnetization
After conducting a magnetic particle inspection, it is usually necessary to demagnetize
the component. Remanent magnetic fields can:
- affect machining by causing cuttings to cling to a component.
- interfere with electronic equipment such as a compass.
- create a condition known as "arc blow" in the welding process. Arc blow may
cause the weld arc to wonder or filler metal to be repelled from the weld.
- cause abrasive particles to cling to bearing or faying surfaces and increase wear.
Removal of a field may be accomplished in several ways. The most effective way to
demagnetize a material is by heating the material above its curie temperature (for
instance, the curie temperature for a low carbon steel is 770°C). When steel is heated
above its curie temperature then it is cooled back down, the the orientation of the
magnetic domains of the individual grains will become randomized again and thus the
component will contain no residual magnetic field. The material should also be placed
with it long axis in an east-west orientation to avoid any influence of the Earth's
magnetic field.
However, it is often inconvenient to heat a material above its curie temperature to
demagnetize it, so another method that returns the material to a nearly unmagnetized
state is commonly used.
Subjecting the component to a reversing and decreasing magnetic field will return the
dipoles to a nearly random orientation throughout the material. This can be
accomplished by pulling a component out and away from a coil with AC passing
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through it. With AC Yokes, demagnetization of local
areas may be accomplished by placing the yoke contacts
on the surface, moving them in circular patterns around
the area, and slowly withdrawing the yoke while the
current is applied. Also, many stationary magnetic
particle inspection units come with a demagnetization
feature that slowly reduces the AC in a coil in which the
component is placed.
A field meter is often used to verify that the residual flux
has been removed from a component. Industry standards usually require that the
magnetic flux be reduced to less than 3 Gauss (3x10
-4
Tesla) after completing a
magnetic particle inspection.

Measuring Magnetic Fields
When performing a magnetic particle inspection, it is very important to be able to
determine the direction and intensity of the magnetic field. The field intensity must be
high enough to cause an indication to form, but not too high to cause nonrelevant
indications to mask relevant indications. Also, after magnetic inspection it is often
needed to measure the level of residual magnetezm.
Since it is impractical to measure the actual field strength within the material, all the
devices measure the magnetic field that is outside of the material. The two devices
commonly used for quantitative measurement of magnetic fields n magnetic particle
inspection are the field indicator and the Hall-effect meter, which is also called a gauss
meter.
Field Indicators
Field indicators are small mechanical devices that utilize a soft iron
vane that is deflected by a magnetic field. The vane is attached to a
needle that rotates and moves the pointer for the scale. Field
indicators can be adjusted and calibrated so that quantitative
information can be obtained. However, the measurement range of
field indicators is usually small due to the mechanics of the device
(the one shown in the image has a range from plus 20 to minus 20
Gauss). This limited range makes them best suited for measuring
the residual magnetic field after demagnetization.
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Hall-Effect (Gauss/Tesla) Meter
A Hall-effect meter is an electronic device that provides a digital readout of the
magnetic field strength in Gauss or Tesla units. The meter uses a very small conductor
or semiconductor element at the tip of the probe. Electric
current is passed through the conductor. In a magnetic
field, a force is exerted on the moving electrons which
tends to push them to one side of the conductor. A buildup
of charge at the sides of the conductors will balance this
magnetic influence, producing a measurable voltage
between the two sides of the conductor. The probe is
placed in the magnetic field such that the magnetic lines of
force intersect the major dimensions of the sensing
element at a right angle.

Magnetization Equipment for Magnetic Particle Testing
To properly inspect a part for cracks or other defects, it is important to become
familiar with the different types of magnetic fields and the equipment used to
generate them. As discussed previously, one of the primary requirements for detecting
a defect in a ferromagnetic material is that the magnetic field induced in the part must
intercept the defect at a 45 to 90 degree angle. Flaws that are normal (90 degrees) to
the magnetic field will produce the strongest indications because they disrupt more of
the magnet flux. Therefore, for proper inspection of a component, it is important to be
able to establish a magnetic field in at least two directions.
A variety of equipment exists to establish the magnetic field for magnetic particle
testing. One way to classify equipment is based on its portability. Some equipment is
designed to be portable so that inspections can be made in the field and some is
designed to be stationary for ease of inspection in the laboratory or manufacturing
facility.
Portable Equipment
Permanent Magnets
Permanent magnets can be used for magnetic particle inspection as the source of
magnetism (bar magnets or horseshoe magnets). The use of industrial magnets is not
popular because they are very strong (they require significant strength to remove them
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from the surface, about 250 N for some magnets) and thus they
are difficult and sometimes dangerous to handle. However,
permanent magnets are sometimes used by divers for inspection
in underwater environments or other areas, such as explosive
environments, where electromagnets cannot be used. Permanent
magnets can also be made small enough to fit into tight areas
where electromagnets might not fit.

Electromagnetic Yokes
An electromagnetic yoke is a very common piece of equipment that is used to establish
a magnetic field. A switch is included in the electrical circuit so that the current and,
therefore, the magnetic field can be turned on
and off. They can be powered with AC from a wall
socket or by DC from a battery pack. This type of
magnet generates a very strong magnetic field in
a local area where the poles of the magnet touch
the part being inspected. Some yokes can lift
weights in excess of 40 pounds.

Prods
Prods are handheld electrodes that are pressed against the
surface of the component being inspected to make contact for
passing electrical current (AC or DC) through the metal. Prods
are typically made from copper and have an insulated handle to
help protect the operator. One of the prods has a trigger switch
so that the current can be quickly and easily turned on and off.
Sometimes the two prods are connected by any insulator, as
shown in the image, to facilitate one hand operation. This is
referred to as a dual prod and is commonly used for weld
inspections.
However, caution is required when using prods because electrical arcing can occur and
cause damage to the component if proper contact is not maintained between the
prods and the component surface. For this reason, the use of prods is not allowed
when inspecting aerospace and other critical components. To help prevent arcing, the
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prod tips should be inspected frequently to ensure that they are not oxidized, covered
with scale or other contaminant, or damaged.
Portable Coils and Conductive Cables
Coils and conductive cables are used to establish a
longitudinal magnetic field within a component. When a
preformed coil is used, the component is placed against
the inside surface on the coil. Coils typically have three
or five turns of a copper cable within the molded frame.
A foot switch is often used to energize the coil.
Also, flexible conductive cables can be wrapped around
a component to form a coil. The number of wraps is
determined by the magnetizing force needed and of
course, the length of the cable. Normally, the wraps are
kept as close together as possible. When using a coil or
cable wrapped into a coil, amperage is usually expressed
in ampere-turns. Ampere-turns is the amperage shown
on the amp meter times the number of turns in the coil.

Portable Power Supplies
Portable power supplies are used to provide the
necessary electricity to the prods, coils or cables.
Power supplies are commercially available in a
variety of sizes. Small power supplies generally
provide up to 1,500A of half-wave DC or AC. They
are small and light enough to be carried and operate
on either 120V or 240V electrical service.
When more power is necessary, mobile power
supplies can be used. These units come with wheels
so that they can be rolled where needed. These
units also operate on 120V or 240V electrical service
and can provide up to 6,000A of AC or half-wave DC.


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Stationery Equipment
Stationary magnetic particle inspection equipment
is designed for use in laboratory or production
environment. The most common stationary system
is the wet horizontal (bench) unit. Wet horizontal
units are designed to allow for batch inspections of
a variety of components. The units have head and
tail stocks (similar to a lathe) with electrical contact
that the part can be clamped between. A circular
magnetic field is produced with direct magnetization.
Most units also have a movable coil that can be moved
into place so the indirect magnetization can be used to
produce a longitudinal magnetic field. Most coils have
five turns and can be obtained in a variety of sizes. The
wet magnetic particle solution is collected and held in
a tank. A pump and hose system is used to apply the
particle solution to the components being inspected.
Some of the systems offer a variety of options in
electrical current used for magnetizing the component
(AC, half wave DC, or full wave DC). In some units, a
demagnetization feature is built in, which uses the coil and decaying AC.

Magnetic Field Indicators
Determining whether a magnetic field is of adequate strength and in the proper
direction is critical when performing magnetic particle testing. There is actually no
easy-to-apply method that permits an exact measurement of field intensity at a given
point within a material. Cutting a small slot or hole into the material and measuring the
leakage field that crosses the air gap with a Hall-effect meter is probably the best way
to get an estimate of the actual field strength within a part. However, since that is not
practical, there are a number of tools and methods that are used to determine the
presence and direction of the field surrounding a component.

Hall-Effect Meter (Gauss Meter)
As discussed earlier, a Gauss meter is commonly used to measure the tangential field
strength on the surface of the part. By placing the probe next to the surface, the meter
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measures the intensity of the field in the air adjacent to the component when a
magnetic field is applied. The advantages of this device are: it provides a quantitative
measure of the strength of magnetizing force tangential to the surface of a test piece,
it can be used for measurement of residual magnetic fields, and it can be used
repetitively. The main disadvantage is that such devices must be periodically
calibrated.

Quantitative Quality Indicator (QQI)
The Quantitative Quality Indicator (QQI) or Artificial Flaw Standard
is often the preferred method of assuring proper field direction
and adequate field strength (it is used with the wet method only).
The QQI is a thin strip (0.05 or 0.1 mm thick) of AISI 1005 steel
with a specific pattern, such as concentric circles or a plus sign,
etched on it. The QQI is placed directly on the surface, with the
itched side facing the surface, and it is usually fixed to the surface
using a tape then the component is then magnetized and particles
applied. When the field strength is adequate, the particles will
adhere over the engraved pattern and provide information about
the field direction.

Pie Gage
The pie gage is a disk of highly permeable material divided into four, six, or
eight sections by non-ferromagnetic material (such as copper). The divisions
serve as artificial defects that radiate out in different directions from the
center. The sections are furnace brazed and copper plated. The gage is
placed on the test piece copper side up and the test piece is magnetized.
After particles are applied and the excess removed, the indications provide
the inspector the orientation of the magnetic field. Pie gages are mainly
used on flat surfaces such as weldments or steel castings where dry powder
is used with a yoke or prods. The pie gage is not recommended for precision
parts with complex shapes, for wet-method applications, or for proving field
magnitude. The gage should be demagnetized between readings.

Slotted Strips
Slotted strips are pieces of highly permeable ferromagnetic material with slots of
different widths. These strips can be used with the wet or dry method. They are placed
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on the test object as it is inspected. The indications produced on the strips give the
inspector a general idea of the field strength in a particular area.

Magnetic Particles
As mentioned previously, the particles that are used for magnetic particle inspection
are a key ingredient as they form the indications that alert the inspector to the
presence of defects. Particles start out as tiny milled pieces of iron or iron oxide. A
pigment (somewhat like paint) is bonded to their surfaces to give the particles color.
The metal used for the particles has high magnetic permeability and low retentivity.
High magnetic permeability is important because it makes the particles attract easily to
small magnetic leakage fields from discontinuities, such as flaws. Low retentivity is
important because the particles themselves never become strongly magnetized so
they do not stick to each other or the surface of the part. Particles are available in a dry
mix or a wet solution.

Dry Magnetic Particles
Dry magnetic particles can typically be purchased in red, black, gray, yellow and several
other colors so that a high level of contrast between the particles and the part being
inspected can be achieved. The size of the magnetic particles is also very important.
Dry magnetic particle products are produced to include a range of particle sizes. The
fine particles have a diameter of about 50 µm while the course particles have a
diameter of 150 µm (fine particles are more than 20 times lighter than the coarse
particles). This makes fine particles more sensitive to the leakage fields from very small
discontinuities. However, dry testing particles cannot be made exclusively of the fine
particles where coarser particles are needed to bridge large discontinuities and to
reduce the powder's dusty nature. Additionally, small particles easily adhere to surface
contamination, such as remnant dirt or moisture, and get
trapped in surface roughness features. It should also be
recognized that finer particles will be more easily blown away by
the wind; therefore, windy conditions can reduce the sensitivity
of an inspection. Also, reclaiming the dry particles is not
recommended because the small particles are less likely to be
recaptured and the "once used" mix will result in less sensitive
inspections.
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The particle shape is also important. Long, slender particles tend align themselves
along the lines of magnetic force. However, if dry powder consists
only of elongated particles, the application process would be less
than desirable since long particles lack the ability to flow freely.
Therefore, a mix of rounded and elongated particles is used since
it results in a dry powder that flows well and maintains good
sensitivity. Most dry particle mixes have particles with L/D ratios
between one and two.

Wet Magnetic Particles
Magnetic particles are also supplied in a wet suspension such as water or oil. The wet
magnetic particle testing method is generally more sensitive than the dry because the
suspension provides the particles with more mobility and makes it possible for smaller
particles to be used (the particles are typically 10 µm and smaller) since dust and
adherence to surface contamination is reduced or eliminated. The wet method also
makes it easy to apply the particles uniformly to a relatively large area.
Wet method magnetic particles products differ from dry powder
products in a number of ways. One way is that both visible and
fluorescent particles are available. Most non-fluorescent particles are
ferromagnetic iron oxides, which are either black or brown in color.
Fluorescent particles are coated with pigments that fluoresce when
exposed to ultraviolet light. Particles that fluoresce green-yellow are
most common to take advantage of the peak color sensitivity of the
eye but other fluorescent colors are also available.
The carrier solutions can be water or oil-based. Water-based carriers
form quicker indications, are generally less expensive, present little or
no fire hazard, give off no petrochemical fumes, and are easier to
clean from the part. Water-based solutions are usually formulated
with a corrosion inhibitor to offer some corrosion protection.
However, oil-based carrier solutions offer superior corrosion and
hydrogen embrittlement protection to those materials that are prone
to attack by these mechanisms.
Also, both visible and fluorescent wet suspended particles are available in aerosol
spray cans for increased portability and ease of application.
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Dry Particle Inspection
In this magnetic particle testing technique, dry particles are dusted
onto the surface of the test object as the item is magnetized. Dry
particle inspection is well suited for the inspections conducted on
rough surfaces. When an electromagnetic yoke is used, the AC
current creates a pulsating magnetic field that provides mobility to
the powder.
Dry particle inspection is also used to detect shallow subsurface
cracks. Dry particles with half wave DC is the best approach when
inspecting for lack of root penetration in welds of thin materials.
Steps for performing dry particles inspection:
 Surface preparation - The surface should be relatively clean but this is not as
critical as it is with liquid penetrant inspection. The surface must be free of grease,
oil or other moisture that could keep particles from moving freely. A thin layer of
paint, rust or scale will reduce test sensitivity but can sometimes be left in place
with adequate results. Specifications often allow up to 0.076 mm of a
nonconductive coating (such as paint) or 0.025 mm of a ferromagnetic coating
(such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must
be removed.

o Some specifications require the surface to be coated with a thin layer of white
paint in order to improve the contrast difference between the background and
the particles (especially when gray color particles are used).

 Applying the magnetizing force - Use permanent magnets, an electromagnetic
yoke, prods, a coil or other means to establish the necessary magnetic flux.

 Applying dry magnetic particles - Dust on a light layer of magnetic particles.

 Blowing off excess powder - With the magnetizing force still applied, remove the
excess powder from the surface with a few gentle puffs of dry air. The force of the
air needs to be strong enough to remove the excess particles but not strong
enough to remove particles held by a magnetic flux leakage field.

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 Terminating the magnetizing force - If the magnetic flux is being generated with
an electromagnet or an electromagnetic field, the magnetizing force should be
terminated. If permanent magnets are being used, they can be left in place.

 Inspection for indications - Look for areas where the magnetic particles are
clustered.
Wet Suspension Inspection
Wet suspension magnetic particle inspection, more
commonly known as wet magnetic particle inspection,
involves applying the particles while they are suspended
in a liquid carrier. Wet magnetic particle inspection is
most commonly performed using a stationary, wet,
horizontal inspection unit but suspensions are also
available in spray cans for use with an electromagnetic
yoke.
A wet inspection has several advantages over a dry inspection. First, all of the surfaces
of the component can be quickly and easily covered with a relatively uniform layer of
particles. Second, the liquid carrier provides mobility to the particles for an extended
period of time, which allows enough particles to float to small leakage fields to form a
visible indication. Therefore, wet inspection is considered best for detecting very small
discontinuities on smooth surfaces. On rough surfaces, however, the particles (which
are much smaller in wet suspensions) can settle in the surface valleys and lose mobility,
rendering them less effective than dry powders under these conditions.
Steps for performing wet particle inspection:
 Surface preparation - Just as is required with dry particle inspections, the surface
should be relatively clean. The surface must be free of grease, oil and other
moisture that could prevent the suspension from wetting the surface and
preventing the particles from moving freely. A thin layer of paint, rust or scale will
reduce test sensitivity, but can sometimes be left in place with adequate results.
Specifications often allow up to 0.076 mm of a nonconductive coating (such as
paint) or 0.025 mm of a ferromagnetic coating (such as nickel) to be left on the
surface. Any loose dirt, paint, rust or scale must be removed.

o Some specifications require the surface to be coated with a thin layer of white
paint when inspecting using visible particles in order to improve the contrast
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difference between the background and the particles (especially when gray
color particles are used).

 Applying suspended magnetic particles - The suspension is gently sprayed or
flowed over the surface of the part. Usually, the stream of suspension is diverted
from the part just before the magnetizing field is applied.

 Applying the magnetizing force - The magnetizing force should be applied
immediately after applying the suspension of magnetic particles. When using a
wet horizontal inspection unit, the current is applied in two or three short busts
(1/2 second) which helps to improve particle mobility.

 Inspection for indications - Look for areas where the magnetic particles are
clustered. Surface discontinuities will produce a sharp indication. The indications
from subsurface flaws will be less defined and lose definition as depth increases.

Quality & Process Control

Particle Concentration and Condition
Particle Concentration
The concentration of particles in the suspension is a very important
parameter and it is checked after the suspension is prepared and
regularly monitored as part of the quality system checks. Standards
require concentration checks to be performed every eight hours or at
every shift change.
The standard process used to perform the check requires agitating the
carrier for a minimum of thirty minutes to ensure even particle
distribution. A sample is then taken in a pear-shaped 100 ml centrifuge
tube having a graduated stem (1.0 ml in 0.05 ml increments for
fluorescent particles, or 1.5 ml in 0.1 ml increments for visible particles).
The sample is then demagnetized so that the particles do not clump
together while settling. The sample must then remain undisturbed for a
period of time (60 minutes for a petroleum-based carrier or 30 minutes
for a water-based carrier). The volume of settled particles is then read.
Acceptable ranges are 0.1 to 0.4 ml for fluorescent particles and 1.2 to
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2.4 ml for visible particles. If the particle concentration is out of the acceptable range,
particles or the carrier must be added to bring the solution back in compliance with the
requirement.

Particle Condition
After the particles have settled, they should be examined for brightness and
agglomeration. Fluorescent particles should be evaluated under ultraviolet light and
visible particles under white light. The brightness of the particles should be evaluated
weekly by comparing the particles in the test solution to those in an unused reference
solution that was saved when the solution was first prepared. Additionally, the
particles should appear loose and not lumped together. If the brightness or the
agglomeration of the particles is noticeably different from the reference solution, the
bath should be replaced.

Suspension Contamination
The suspension solution should also be examined for contamination which may come
from inspected components (oils, greases, sand, or dirt) or from the environment
(dust). This examination is performed on the carrier and particles collected for
concentration testing. Differences in color, layering or banding within the settled
particles would indicate contamination. Some contamination is to be expected but if
the foreign matter exceeds 30 percent of the settled solids, the solution should be
replaced. The liquid carrier portion of the solution should also be inspected for
contamination. Oil in a water bath and water in a solvent bath are the primary
concerns.

Water Break Test
A daily water break check is required to evaluate the surface wetting performance of
water-based carriers. The water break check simply involves flooding a clean surface
similar to those being inspected and observing the surface film. If a continuous film
forms over the entire surface, sufficient wetting agent is present. If the film of
suspension breaks (water break) exposing the surface of the component, insufficient
wetting agent is present and the solution should be adjusted or replaced.

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Electrical System Checks
Changes in the performance of the electrical system of a magnetic particle inspection
unit can obviously have an effect on the sensitivity of an inspection. Therefore, the
electrical system must be checked when the equipment is new, when a malfunction is
suspected, or every six months. Listed below are the verification tests required by
active standards.
Ammeter Check
It is important that the ammeter provide consistent and correct readings. If the meter
is reading low, over magnetization will occur and possibly result in excessive
background "noise." If ammeter readings are high, flux density could be too low to
produce detectable indications. To verify ammeter accuracy, a calibrated ammeter is
connected in series with the output circuit and values are compared to the
equipment's ammeter values. Readings are taken at three output levels in the working
range. The equipment meter is not to deviate from the calibrated ammeter more than
±10 percent or 50 amperes, whichever is greater. If the meter is found to be outside
this range, the condition must be corrected.
Shot Timer Check
When a timer is used to control the shot duration, the timer must be calibrated.
Standards require the timer be calibrated to within ± 0.1 second. A certified timer
should be used to verify the equipment timer is within the required tolerances.

Magnetization Strength Check
Ensuring that the magnetization equipment provides sufficient magnetic field strength
is essential. Standard require the magnetization strength of electromagnetic yokes to
be checked prior to use each day. The magnetization strength is checked by lifting a
steel block of a standard weight using the yoke at the maximum pole spacing to be
used (10 lb weight for AC yokes or 40 lb weight for DC yokes).

Lighting
Magnetic particle inspection predominately relies on visual inspection to detect any
indications that form. Therefore, lighting is a very important element of the inspection
process. Obviously, the lighting requirements are different for an inspection conducted
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using visible particles than they are for an inspection conducted using fluorescent
particles.
Light Requirements When Using Visible Particles
Visible particles inspections can be conducted using natural lighting or artificial lighting.
However, since natural daylight changes from time to time, the use of artificial lighting
is recommended to get better uniformity. Artificial lighting should be white whenever
possible (halogen lamps are most commonly used). The light intensity is required to be
100 foot-candles (1076 lux) at the surface being inspected.

Light Requirements When Using Fluorescent Particles

Ultraviolet Lighting
When performing a magnetic particle inspection using fluorescent particles, the
condition of the ultraviolet light and the ambient white light must be monitored.
Standards and procedures require verification of lens condition and light intensity.
Black lights should never be used with a cracked filter as the output of white light and
harmful black light will be increased. Also, the cleanliness of the filter should also be
checked regularly. The filter should be checked visually and cleaned as necessary
before warming-up the light. Most UV light must be warmed up prior to use and
should be on for at least 15 minutes before beginning an inspection.
For UV lights used in component evaluations, the normally accepted intensity is 1000
µW/cm
2
at 38cm distance from the filter face. The required check should be
performed when a new bulb is installed, at startup of the inspection cycle, if a change
in intensity is noticed, or every eight hours of continuous use.

Ambient White Lighting
When performing a fluorescent magnetic particle inspection, it is important to keep
white light to a minimum as it will significantly reduce the inspector’s ability to detect
fluorescent indications. Light levels of less than 2 foot-candles (22 lux) are required by
most procedures. When checking black light intensity a reading of the white light
produced by the black light may be required to verify white light is being removed by
the filter.

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White Light for Indication Confirmation
While white light is held to a minimum in fluorescent inspections, procedures may
require that indications be evaluated under white light. The white light requirements
for this evaluation are the same as when performing an inspection with visible
particles. The minimum light intensity at the surface being inspected must be 100
foot-candles (1076 lux).

Light Measurement
Light intensity measurements are made using a radiometer (an instrument that
transfers light energy into an electrical current). Some radiometers have the ability to
measure both black and white light, while others require a separate sensor for each
measurement. Whichever type is used, the sensing area should be clean and free of
any materials that could reduce or obstruct light reaching the sensor. Radiometers are
relatively unstable instruments and readings often change considerable over time.
Therefore, they should be calibrated at least every six months.
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164. Chapter 6
Quality Control
of Electroless Nickel Deposits
Phillip Stapleton
Electroless nickel deposits are smooth, hard nickel-phosphorus alloy coatings
produced by a chemical reduction reaction. The properties of these coatings are
dependent upon the substrate, pretreatment, plating process, composition of
deposits, and post treatments. This interdependency of processes and materials
on the performance of the coating makes quality control and process control
key aspects of the manufacturing system.
In most applications, electroless nickel is used because of aggressive
corrosion
or wear conditions on the base material. In these applications, the
coating performance can be critical to the overall system, and the potential
loss
to society if the coating fails significant.
Changes in coating performance, likesurface finish and porosity, are affected
by the manufacturing process before plating and bath formulation, while
thickness, adhesion, and hardness are dependent on the plating process. Many
times a combination of effects will influence the performance of the coating.
Electroless nickel deposit properties can also vary significantly between
types
of solutions. This is a result of the electropotential in the application environment
and differences in the coating structure.
These chemical and physical changes in the deposit can be related to the
phosphorus content and the number of lattice defects in the structure.
By
design, some types of solutions have greater numbers of lattice defects than
others. Generally, as the lattice defects in the alloy increase, the deposit
becomes more brittle and harder with a reduction in the modulus and increase in
tensile strength.
The control of these properties is accomplished by maintaining the plating
solution within a narrow operating range and controlling trace elements such as
sulfur, antimony, cadmium, and bismuth, which are generally attributed with
having the greatest effect on the structure and composition of the deposit.
QUALITY METHOD
Developing a method of controlling the factors that affect quality requires an
overall scheme. This scheme must identify the quality level, analyze the process
system to identify critical conditions, modify the manufacturing process, and
169
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165. 170 ELECTROLESS PLATING
identify the new quality level. Many methods have been developed to describe
the relationship of quality to manufacturing process. One that works well for
metal finishing is based on the work of Taguchi (Fig.
6.1).
According to his method, the goal is to develop a more robust manufacturing
system that has no performance variation of the product, while experiencing a
higher level of
noise or variations within the processes.
The term
noise is used to describe process variations from a mean. These
variations can be from the wear of machines, operator training, contamination of
materials, as well as many other factors. The use of the term
inner noise refers to
variations that are being monitored and can be controlled, while
outer noise
refers to variations that are not being controlled.
In Fig.
6.1, the block “Off-line Quality Control” is generally provided by the
suppliers of processes, while “Transfer Design” and “On-line Quality Control”
are performed by the producing facility.
When these basic concepts are transferred to a shop floor system, several
familiar shop functions are revealed. The first item is the
specification, which
describes the requirements by acceptance and qualification tests that must be
completed to verify the quality of the coating. In addition to the specification, a
shop traveler is used to transfer the designs to the processor. This document
includes the sequence, conditions, times, and other critical information needed
to produceaquality part.
A phaseofthedesign includes thefrequencyoftesting
of process conditions, which allows the process engiiieer to determine the
Process Capability Index.
By following the shop traveler and collecting the process information, studies
on the coating performance can be related to process conditions, different
operators, base materials, and more. From these studies, the frequency of
testing can be modified, new, more robust conditions can be selected, and the
quality can be brought under control.
Fig. 6.1-Quallty control and process management.
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166. Quality Control of Elecfroless Nickel Deposits 171
Experience and the pursuit of these methodologies has provided the
information to build a series of cause and effect charts covering the electroless
nickel process (see Charts
6.1-6.4). These charts describe the relationship of a
coating failure to the processes.
Figure
6.2 shows the three areas that require control by operators: (a) process
analysis and history; (b) qualification tests; and (c) acceptance tests.
Process Analysis and Management
In order to maintain the quality of the electroless nickel deposit, the electroless
solution must be maintained at optimum chemical and physical condition.
Constant filtration of the plating solution is recommended. The solution
should be filtered more than
10 times the volume of the process every hour. At
this rate of filtration, the filter will be able to remove most of the solids and
prevent the formation of roughness on the parts.
An important aspect in the design
of the filter system is the pump dynamics
and how the system will operate with a high specific gravity and temperature.
The system should be sized for an operating temperature of 195" F and a specific
gravity of 1.310.
The maintenance of a relatively narrow pH range will insure a constant plating
rate and phosphorus content in the alloy.
A pH change of 0.1 points may cause a
variation of 0.1 mil/hr and affect the final product performance and cost. For
some solutions, a change in pH of 0.5 points will cause a
1 percent change in
phosphorus content and significantly affect the properties of the subsequent
deposit.
Maintenance
of the chemical composition of the plating solution in general,
and the nickel/hypophosphite ratio in particular, will ensure a uniform plating
rate and phosphorus content. Air agitation and low loading
will cause
oxidation of the available hypophosphite and may lead to
a lower phosphorus
content in the deposit and slower plating rates.
To overcome this, periodic
titrations for sodium hypophosphite and additions in the form of replenisher
should be made to the plating solution.
The concentration of trace metals within the plating solution will also affect
the deposit properties. There are several elements that will directly affect the
performance in all solutions. These include sulfur, iron, cadmium, bismuth,
antimony, mercury, lead, and zinc. There are several others that may affect the
deposit properties, and which are primarly controlled by atomic adsorption,
inductively coupled plasma, or poloragraphy. The presence of some
of these
materials may cause porosity in thin deposits and high stress. Some trace metals
increase the propensity to pitting in theenvironment by setting upactive cells on
the surface between nickel and the trace element. Excessive concentration of
trace elements in the solution may also produce a condition called
step plating,
in which edges are not plated or areas have low thicknesses.
The control of trace elements starts with keeping them out the solution. If they
are present, several techniques can be tried
to reduce their effect. Dilution is the
first choice, followed by dummying, and addition of secondary reducing agents.
Generally a combination of treatments will reduce the effect of the trace
elements and bring the solution back to a productive condition.
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167. 172 ELECTROLESS PLATING
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168. Quality Control of Electroless Nickel Deposits 173
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169. 174 ELECTROLESS PLATING
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170. Quality Control of Electroless Nickel Deposits 175
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171. 1 76 ELECTROLESS PLATING
flg. 6.2-Putting a quallty control rystem together.
Organic contamination within the solution will also cause deposit properties
to be compromised. The source of the organic materials may be from the part,
facilities and tank,
or masking. The problem is manifested by lack of adhesion,
porosity, and stress.
Some common sources of organic contamination include maskants, organic
materials, oils from within the substrate, plasticizers from hoses and liners,
airborne organics, drippage from overhead, and contaminated make-up water.
Silicates, though not organic in nature, can adversely contaminate the bath
through drag-in from the pretreatment cycle.
Activated carbon treatment
of the plating solution at operating temperature
can often help reduce or eliminate some types
of contaminants. If carbon
treatment is used on the solution to remove organic contaminants, care should
be taken to replace any desired organic control additives and that the carbon is
clean and is not a source of contamination.
Another type of contaminant is oxidizers. These materials, such as hydrogen
peroxide and nitric acid, will change the deposition potential within the solution
and cause black
or streaked deposits to be produced. With low levels of volatile
oxidizers, heating the solution may help. With higher levels, and with non-
volatile oxidizers, the solution must be discarded.
Other conditions that require control are agitation and loading. These two
factorsaffect thediffusion of nickel ions in the reduction reaction. High agitation
and either high or low loading can cause step plating and a low plating rate.
Generally, the plating solution will produce coatings when the velocity
of the
solution is less than
4 Wsec. This value will be dependent on the solution
chemistry and solution operating conditions. At extremely low loads, the
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172. Quality Control of Electroless Nickel Deposits 177
deposition potential may be low enough to prevent the reduction reaction from
proceeding. This problem may be observed on sharp edges, such as needles,
where the sharp point will not plate. In most cases, this problem can be corrected
by selection of solutions that can operate at higher velocities (7 to
10 Wsec).
Testing of Deposit Properties
The selection of requirements for the electroless nickel deposit are generally
made by the purchaser of the coating. These requirements are established in
specifications. Based on the sampling requirements, a part or specimen must be
tested to identify any variation in performance.
The choice of tests that will characterize performance variation specific to the
application of the part is important. Two types of tests are employed: acceptance
tests and qualification tests. These categories can be used to distinguish tests
that will be performed on an individual lot, and those that might be run on a
regular basis, such as weekly or monthly.
Acceptance tests include:
0 Thickness
0 Appearance
Tolerance
Adhesion
Porosity
Qualification tests include:
Corrosion resistance
Wear resistance
Alloy composition
Internal stress
Hydrogen embrittlement
Microhardness
The following list of requirements and test methods have been provided to
offer the user of electroless nickel coatings an overview of the options available.
The actual organization of requirements and test methods have been published
in MIL-C-26074C and ASTM 8733 and should be used to maintain a national
standard for ordering and performance.
TEST METHODS
Appearance
The coating surface shall have a uniform, metallic appearance without visible
defects such as blisters, pits, pimples, and cracks.
Imperfections that arise from surface conditions of the substrate and persist in
the coating shall not be cause for rejection. Also, discoloration that results from
heat treatment shall not be cause for rejection.
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173. I 7a ELECTROLESS PLATiNG
Thickness Measurement
The thickness shall be measured at any place on the significant surface
designated by the purchaser, and the measurement shall be made with an
accuracy of better than
5 percent by a method selected by the purchaser.
Examples
of common measuring devices are shown in Figs. 6.3 and 6.4.
Fig. 6.3-X-ray fluorescence device measures thickness by analyzing the mass per unit area of the
eieciroless nickel deposit according
lo ASTM 8568.
Fig. 6.4-Beta backscatter device also measures thickness by analyzing the mass per unit area
of the
electroless nickel deposit. There are some limitations
to this method, but the cost and availability of
instruments makes it an excellent choice for many applications. ASTM 8567 can be used to
standardize and peiform the measurements with this instrument.
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174. Qualify Control of Electroless Nickel Deposifs 179
Weigh, Plate, Weigh Method
Using a similar substrate material, weigh to the nearest milligram before and
after plating, ensuring that the part
is at the same temperature for each
measurement. Calculate the thickness from the increase in weight, surface area,
and density of the coating.
NOTE: The density of the coating will vary with the weight percentage of
phosphorus
in the coating. For a 9-percent-P alloy, the density is 8 glcm'.
Example-A coupon made of mild steel has a weight of 3198 mg with an area
of 19.736 cm' before plating. After plating with electroless nickel, the coupon
weighs 3583 mg. Calculation for thickness
is as follows:
3583 mg (after)
- 3198 mg (before)
T= + 19.736 cm-'
8.01 g/cm' x 1000 mg/g
T = 0.00244 cm x 10,000 pm/cm
T = 24.4 pm
Table 6.1
8ubstrate Densities
For Weigh, Plate, Weigh Method
Steel, mild 1020
Stainless steel 316
Aluminum 2024
Aluminum 6061
Copper
7.86
8.02
2.79
2.70
8.91
Metallographic Sectioning
Plate a specimen of similar composition and metallurgical condition to the
article being plated, or use a sample from the lot; mount and polish at 90" to the
surface. Using a Vernier Calibrated Microscope, examine the thickness of the
deposit and average over 10 readings.
NOTE: Accurate microscopic metallographic sectioning is very dependent on
the sample preparation. Backing springs are recommended
to reduce the
smearing effects
of the polishing step.
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175. 180 ELECTROLESS PLATING
Micrometer Method
Measure a part of the test coupon in a specific spot before and after plating,
usin'g a suitable micrometer. Ensure that the part is at the same temperature for
each measurement and that the surface measured is smooth.
Beta Backscatter Method
Thecoating thickness can be measured by the use of a beta backscatter device.
The use of beta backscatter is restricted to base metals that have an atomic
number
of less than 18 or greater than 40. The actual phosphorus content of the
coating shall be taken into consideration; consequently, the measuring device
shall be calibrated using specimens of the same substrate having the same
phosphorus content as the articles to be tested.
Magnetic Method
A magnetic thickness detector is applicable to magnetic substrates plated with
autocatalytic nickel deposits that contain more than
10 weight percent
phosphorus (non-magnetic) and that have not been heat treated. The instrument
must be calibrated with deposits plated in the same bath on steel and whose
thickness has been determined by the microscopic method.
X-ray Spectrometry
The coating thickness can be measured by X-ray spectrometry. This technique
will measure the mass per unit area of the coating applied over the substrate.
X-ray spectrometry equipment should be calibrated according to ASTM
8568
with standards of known phosphorus and thickness. This method is non-
destructive and will produce rapid and accurate results.
Coulometric Method
Measure the coating thickness in accordance with ASTM 8504. The solution to
be used shall be in accordance with manufacturer's recommendations. The
surface of the coating shall be cleaned prior to testing.
Standard thickness specimens shall be calibrated with deposits plated in the
same solution under the same conditions.
NOTE: This method is only recommended for deposits in the as-plated
condition. The phosphorus content
of the coating must be known in order to
calculate the thickness
of the deposit.
Adhesion Measurement
Bend Test
The sample specimen shall be bent through 180° over a minimum mandrel of 12
mm in diameter or four times the thickness of the specimen and examined at 4X
magnification. No detachment of the coating shall occur. Fine cracks in the
coating on the tension side of the bend are not an indication of poor adhesion.
Quench Test
Heat a plated article for 1 hr in an oven in accordance with Table 6.2 for the
appropriate basis metals within
210" C. Then quench in room temperature
water. The appearance
of blisters or peeling is evidence of inadequate adhesion.
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176. Ouality Control of Electroless Nickel Deposits
NOTE: This test procedure may have an adverse effect on the mechanical
properties
of the articles tested.
181
Table 6.2
Substrate Heat-Treatment
Temperatures
for Quench Test
Steel 300" c
Zinc 150" C
Copper
or copper alloy 250" c
Aluminum or aluminum alloy 250" C
Punch Test
Make several indentations (approximately 5 mm apart) in the coating, using a
spring-loaded center punch on which the point has been ground to a 2-mm
radius. Blistering or flaking indicates poor adhesion.
File Test
By agreement with the purchaser, a file may be applied to the coated article. A
non-significant area shall be filed at an angle of
45" to the coating, so that the
base rnetaVcoating interface is exposed.
No lifting of the coating shall be
observed.
Microhardness
ASTM 8578 shall be used for Knoop hardness with a test load of 100 g. The
instrument shall be verified on calibrated standard test blocks having a hardness
similar to that
of the deposit under test.
NOTE:
For thin (less than 25 pm) deposits using less than 100 g loads, the
standard commercial hardness tester produces varied
results. This is due to the
plastic deformation of the coating and the optical qualities of the instrument.
On thick
(75+ pm) deposits, a surface microhardness determination using
Conversion of microhardness (Knoop or Vickers)
to Rockwell scale is
ASTM
E384 is permissible.
inaccurate and therefore inappropriate (see ASTM
El40).
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177. 182 ELECTROLESS PLATING
Hydrogen Embrittlement
ASTM F519 shall be used once a month to evaluate the plating process for relief
of hydrogen embrittlement.
A minimum of three V-notch tensile specimens
made of
AIS1 4340 heat treated to a strength of 260 to 280 ksi shall be plated and
loaded at 75 percent or greater of their ultimate notch tensile strength and held
for 200 hours.
No evidence of fracture or cracks shall exist.
Alloy Determination
There are generally three phosphorus ranges for electroless nickel deposits.
Specific types of solution formulations will provide the selection of range of
phosphorus in the alloy. Specific confirmation of the alloy can be accomplished
by analyzing for nickel and phosphorus by one of the methods described below.
Most applications have been developed using a mid-range of
3 to 9.5 percent
phosphorus and are considered typical. Confirmation of the alloy for these
coatings can be by nickel
or phosphorus, while coatings of less than 3 percent P
or greater than 9.5 percent P should use both the nickel and phosphorus
analysis
to determine the alloy.
Preparation of Test Specimens
There are two general methods of preparing a foil specimen for this test. The
most efficient technique is to plate a stainless steel panel with a
25- to 50-pm-
thick deposit, cut the edges and peel the deposit off the panel.
Another way to produce an autocatalytic nickel phosphorus foil is to deposit a
25- to 50-pm-thick coating onto a masked aluminum panel. Then remove the
maskant and remove the aluminum by immersing in
10 percent sodium
hydroxide solution. When finished,
a foil will have been produced that is
acceptable
for analysis. Although better adhesion is obtained using a zincate
treatment, a coherent plate may be obtained by immersing clean aluminum foil
in the autocatalytic nickel solution.
Determination of Nickel Content-Dimethylglyoxime Method
Reagents:
1:l v/v concentrated nitric acid (specific gravity 1.42)
1 percent solution of dimethylglyoxime
Procedure:
Accurately weigh
0.1 g of autocatalytic nickel deposit and transfer to a 400 mL
beaker. Dissolve in
20 mL of 1:l nitric acid, boil to expel nitrous oxide fumes,
then cool and dilute to
150 mL with distilled water. Add approximately 1 g of
citric or tartaric acid to complex any ion that may be present and neutralize with
ammonium hydroxide to pH
8 to 9.
Heat gently to 60 to 70" C, and while stirring, add 30 mL of dimethylglyoxime
reagent. Allow to stand at
60 to 70" C for 1 hr, cool to below 20" C, and filter
through a clean sintered glass crucible of
No. 4 porosity. Wash the precipitate
well with distilled water, dry in an oven at
110" C for 1 hr, cool, and weigh the
precipitate as nickel dimethylglyoxime.
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178. Qualify Confrol of Electroless Nickel Deposifs 183
O/O Ni = (weight of precipitate x 0.2032 x 100)/sample weight
Determination
of Phosphorus Content
The percentage of phosphorus
is determined after dissolution of the deposit in
acid, either colorimetrically or volumetrically.
Reagents:
For dissolution and oxidation
40 percent v/v concentrated nitric acid (specific gravity 1.42)
2 percent sodium nitrate solution
Approximately 0.1
N potassium permanganate solution
For colorimetric phosphorus analysis
Molybdate vanadate reagent-Dissolve separately in hot water,
20 g
ammonium molybdate and 1 g ammonium vanadate, then mix the two solutions.
Add 200 mL of concentrated nitric acid (specific gravity
1.42) and dilute to 1 L.
For volumetric phosphorus analysis
Solution A: Dissolve 15 g of ammonium molybdate in 80 mL distilled water.
Solution
B: Dissolve 24 g ammonium nitrate in 60 mL distilled water. Add 33
Add
6 mL ammonium solution (specific gravity 0.880) and dilute to 100 mL.
mL concentrated nitric acid and dilute to 100 mL.
Procedure for dissolution and oxidation:
50 mL of 40 percent v/v nitric acid solution.
fumes.
approximately 0.1 N potassium permanganate solution.
1. Dissolve 0.1
9 to 0.21 g (weigh accurately) of autocatalytic nickel deposit in
2. Heat gently until the deposit
is fully dissolved. Then boil to remove brown
3. Dilute
to approximately 100 mL, bring to the boil, and add 20 mL of
4. Boil for
5 minutes.
5. Add 2 percent solution of sodium nitrite dropwise until the precipitated
6. Boil for 5 minutes, then cool to room temperature.
7. Dilute the solution in a volumetric flask to 250 mL and mix well.
At this stage, the phosphorus content may be estimated either colorimetrically
manganese dioxide is dissolved.
or volumetrically,
as described below.
Procedure for colorimetric analysis:
1. Transfer 10 mL of the solution from step
7 above to a 100-mL standard flask,
add
50 rnL distilled water, 25 mL molybdate-vanadate reagent, dilute to the mark
with water, and mix well.
2. Read the absorption at
420 nm after 5 minutes, using 1 cm glass cells with
water in the reference cell. Read off the concentration from a previously
prepared calibration curve.
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179. 184 ELECTROLESS PLATING
% P = mg P from graph/weight of sample (mg)
1. Dry potassium dihydrogen orthophosphate at 115O C for 1 hour.
2. Weigh out 0.4392 g. dissolve in water, and make up to 1 L of solution (1 mL
=
0.1 mg phosphorus).
3. Prepare a calibration curve by adding 25 mL of reagent to
2 mL, 4 rnL, 6 mL,
8 mL, and 10 rnL liquids of this standard solution in
100 rnL standard flasks and
diluting to the mark. Read the absorption of these solutions exactly as for the
estimated reading at 420 nrn, as described above. Plot a calibration curve of
absorption against rng phosphorus
in samples of 0.2 mg, 0.4 mg. etc.. up to 1 .O
rng when prepared as above.
Procedure for preparation of calibration curve:
Procedure for volumetric analysis:
1. Transfer 10 mL of the solution from step 7 above to a stoppered flask and
dilute to 100 mL with distilled water.
2. Warm to 40 to 50" C (do not exceed thls temperature) and slowly add 50 rnL
of ammonium molybdate reagent while stirring.
3. Stopper the flask.
4. Agitate the flask vigorously for 10 minutes.
5. Allow the flask to stand for 30 minutes and filter through a Whatrnan No.
542 filter paper.
6. Wash the flask and precipitate with
1 percent potassium nitrate until the
filtrate will not decolorize 1 mL of water containing 1 drop of 0.1N sodium
hydroxide and 1 drop of phenolphthalein. This will require about 100 mL of the
washing liquid.
7. Place the paper and precipitate in the original flask, add 50 mL water, and
shake well.
8. Add 10 mL of 0.1
N sodium hydroxide solution and shake well to dissolve
the precipitate.
9. Add phenolphthalein indicator and back-titrate with 0.1
N hydrochloric
acid. Let
"X rnL be the titration.
YO P = [25 x (10-X) x 0.01349]/weight of sample
Spectra Analysis
A suitable method using emission spectra produced by Inductively Coupled
Plasma (ICP) would be acceptable for analysis in nickel, phosphorus, and trace
elements.
The following lines have been found to have low interferences when using
argon ICP techniques. AA standards should be used for this analysis.
Phosphorus standards should be made weekly to ensure accuracy.
Ni 216.10 nm AI 202.55 nrn Cr 284.32 nm Pb 283.30 nm
P 215.40 nrn Cd 214.44 nm Cu 324.75 nm Sn 189.94 nrn
P
213.62 nm Co 238.34 nm Fe 238.20 nm Zn 206.20 nm
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180. Quality Confrol of Electroless Nickel Deposits 185
Porosity Measurement
The test to be applied shall be decided by the purchaser in agreement with the
plater.
Porosity Tests for Ferrous Substrates
Ferroxyl Test
The test solution is prepared by dissolving 25 g of potassium ferrocyanide and 15
g of sodium chloride in 1 L solution. The part is cleaned and immersed for 5 sec
in the test solution at 25" C, followed by water rinsing and air drying. Blue spots
visible
to the unaided eye will form at pore sites. Their allowable number should
be specified.
Alternately, strips of suitable paper (e.g., "wet strength" filter paper) are first
immersed in a warm (about 35" C) solution containing
50 g/L of sodium chloride
and 50 g/L of white gelatin and then allowed
to dry. Just before use, they are
immersed in a solution containing
50 g/L sodium chloride and 1 g/L of a
non-ionic wetting agent, and pressed firmly
to make satisfactory contrast onto
the cleaned nickel surface
to be tested and allowed to remain for 30 minutes.
If papers should become dry during the test, they should be moistened again,
in place, with the sodium chloride solution. The papers are then removed and
introduced at once into a solution containing
10 g/L potassium ferrocyanide to
produce sharply defined blue markings on the papers, wherever the basis metal
was exposed by discontinuities in the coating, leading
to attack by the sodium
chloride and transference of the ion components
to the paper. If necessary, the
same area may be retested.
Hot Water Test
Immerse the part to be tested in a beaker filled with aerated water at room
temperature. Apply heat
to the beaker at such a rate that the water begins to boil
in not less than 15 minutes, and not more than 20 minutes. Then remove the part
from the water and air dry. Examine the part for rust spots, which will indicate
pores.
Aerated water is prepared by bubbling clean compressed air through a
reservoir of distilled water by means of a glass diffusion disc for at least 24 hours.
The pH of the aerated water should be 6.7
k0.5. The test parts should be covered
by at least
30 +5 mm of aerated water.
Neutral Salt Spray
Testing in accordance with ASTM 8117 shall be conducted monthly on the
plating process. Coat a 4 x 6 x 0.20 AIS1 4130 steel panel with
0.0015 in. of
deposit. Wash edges and expose in salt spray chamber for a minimum of 240
hours. This panel shall have a rating of 9.5 or better in accordance with ASTM
8537.
NOTE: This test can be made more aggressive by reducing the plating thickness
and increasing the exposure time (e.g.,
0.0005 in. for 1000 hours).
page 186
Hot Chloride Porosity Test
Immerse a steel part in 50 percent reagent HCI for3 hoursat 83" C. After testing,
the acid shall
not be significantly discolored or the part will have exfoliation or
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181. 186 ELECTROLESS PLATING
blisters. Equipment needed to perform this test: a glass beaker, hot plate,
thermometer, and a timer.
The purposeof this test is to locate pore sites in thecoating when the porecell
potential
is greater than 150 mV. Rapid corrosion will occur, causing significant
failure of the coating and substrate. A polyethylene cover affixed to the beaker
with a large rubber band can be used
to reduce the vapor and evaporation of the
acid. This test should be performed under a fume hood to remove acid vapor.
Porosity Test for Aluminum Substrates
Wipe the specimens with a 10 percent solution of sodium hydroxide. After3 min,
rinse and apply a solution of sodium alizarin sulfonate (9,10-anthraquinon-l,2-
dihydroxy-3-sulfonate, sodium salt)
to the specimen. This solution is prepared
by dissolving
1.5 g methyl cellulose in 90 mL boiling water to which, after
cooling, a solution of
0.1 g sodium alizarin sulfonate dissolved in 5 mL ethanol is
added.
After
4 min, apply glacial acetic acid at ambient temperature until the violet
color disappears. Any red spots remaining indicate pores.
Porosity lest for Copper Substrates
Wipe the specimen with glacial acetic acid at ambient temperature. After 3 min,
apply
to the specimen surface a solution of potassium ferricyanide, prepared by
dissolving
1 g of potassium ferricyanideand 1.5 g of methyl cellulose in 90 mL of
boiling deionized water. After 2
to 3 min, the appearance of brown spots will
indicate pores.
Standard Method for Measuring Corrosion Rate
Scope
This method establishes the apparatus, specimen preparation, test procedure,
evaluation, and reporting of the corrosion rate of autocatalytic nickel deposits.
The results are determined by using linear polarization and potentiodynamic
techniques, and can be used
to rank the coatings in order of corrosion
resistance.
This test method is applicable
to electroless nickel deposits applied to
specimens of G5 specification and corroded in an artificial environment.
This test method is provided for use in an interlaboratory corrosion procedure
for evaluating electroless nickel deposits.
Applicable Documents
ASTM standards:
Measurements in Corrosion Testing
static and Potentiodynamic Anodic Polarization Measurements
G3 Standard Practice for Conventions Applicable
to Electrochemical
G5 Standard Practice for Standard Reference Method for Making Potentio-
G15 Definition of Terms Relating to Corrosion and Corrosion Testing
G59 Practice for Conducting Potentiodynamic Polarization Resistance
Measurements
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182. Quahty Control of Electroless Nickel Deposits 187
Summary of Method
The specimen (working electrode) is prepared by plating in an electroless nickel
solution a steel
G5 plug. The plug is then cleaned and mounted in an
electrochemical cell of
G5 design. The cell is charged with a test solution at
ambient temperature and purged with nitrogen. The cell is then pressurized with
the gas in question and heated to the desired temperature. Measurements are
then taken after
1000 sec to find the corrosion potential (E,,,,,). Polarization
measurements are then made to determine the instantaneous corrosion rate for
the applied pressure and temperature. The curves from various conditions and
alloys can then be compared and the relative corrosion resistance can be
established.
Significance and Use
The significance of these tests is that they can be used to rank electroless nickel
deposits in order of their corrosion resistance. These tests can then be used to
develop a comprehensive corrosion monitoring program to evaluate the
substrate pretreatment and electroless nickel deposit.
Results from these tests produce quantitative values as to the corrosive nature
of the environments and the protection afforded by specific electroless nickel
deposits under laboratory conditions.
Apparatus
The following required equipment is described in ASTM G5:
1. Potentiostat calibrated according to ASTM G5
2. Potential measuring device
3. Current measuring device
4. Saturated calomel electrode
5. Salt bridge probe
The specimen (working electrode) to be tested is plated with electroless nickel
and post-treated with the desired process. The specimen can be made from any
material tothefollowing physical dimensions: 1/2 in. (12.7 mm) long,
3.8 in. (9.5
rnrn) diameter, with a 3-84 tapped hole at one end. The entire surface shall be 16
rms or better.
NOTE: The specimens
used in this ASTM program are made from C7275 steel.
The polarization cell shall be made to the following ASTM
G5 specifications:
1. Carbon or platinum counter electrodes shall be used in the cell.
2. The salt bridge shall be adjustable and able to be located near the tip of the
3. The electrolytes shall be agitated constantly with a magnetic stirrer.
4. The cell shall be purged constantly with N, gas.
5. The cell shall be able to operate at 1 atmosphere.
6. The cell shall be able to operate at a temperature of 22" C.
7. The specimen holder shall be designed and maintained to form a tight
working electrode.
connection between the specimen and the teflon gasket of the holder.
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183. 188 ELECTROLESS PLATING
Reagents
Corroding Electrolyte
I is prepared by dissolving 12.5 g of reagent grade sodium
chloride (NaCI) into
900 mL of Type I reagent water.* Adjust pH to 7.8 20.1 with
sodium hydroxide. Fill to 1000 mL with Type
I reagent water.
Corroding Electrolyte
II is prepared by dissolving 40 g of reagent grade
sodium chloride (NaCI) into 1000 mL of Type
I reagent water.
Specimen Preparation
Prepare the specimen by the following steps:
1. Vapor degrease or solvent wash.
2. Caustic clean with appropriate cleaner for steel or aluminum substrate.
3. Rinse with Type I reagent water.
4. Wash with 25 percent sulfuric acid for 60 sec at 25" C.
5. Rinse with Type I reagent water.
6. Mount the specimen on the electrode holder. Care should be exercised not
Note mounting hole will expose substrate alloy to cleaning solution.
to contaminate or disturb the cleaned surface.
Test Parameters
Potentiodynamic polarization:
1. Initial potential -100 mV from
E,,,
2. Final potential -600 mV
3. Vertex potential +lo00 mV
4. Input anodic tafel constant (ATC) 0.145; cathodic tafel constant (CTC)
5. Scan rate 2.0 mV/sec
0.150
Linear polarization:
1. Initial potential
-30 mV €(,,,
2. Final potential 30 mV E,,,,
3. Anodic and cathodic tafel constant from potentiodynamic
4. Scan rate 0.2 mV/sec
Specimen physical parameters:
1. Area 4.285 cm'
2. Density 7.95 g/cm'
3. Equivalent weight 28.51 mols/electron
Wear Analysis
Abrasion Resistance
Abrasion resistance can be measured by a Taber Abrader using a CS-10 wheel.
Wear specimens should be dressed for the first 1000 cycles and then weighed.
TheCS-10 wheelsshould
beredressedfor50cyclesforeach 1OOOcyclesof wear
on thespecimen. Typical testsaretaken to 10,OOOcycleswith a rangeof 15 to30
'Type I reagent water is defined in ASTM D1193 as 16.67 M ohm-cm resistivity.
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184. Qualify Control of Electroless Nickel Deposifs
mg/l000 cycles for as-plated deposits, and 6 to 18 mg/l000 Cycles for
precipitation-hardened deposits. The test is not precise, but can be used
to
characterize the differences between coatings when several specimens are
tested.
189
Adhesive Wear Resistance
Adhesive wear resistance can be tested by several means. These include pin on
disc, block on ring, and pin on notch. Each of these types of adhesive wear tests
simulate slight differences in metal-to-metal wear. To evaluate the specific wear
for a particular application, the correct wear test method must be selected.
Conditions such as temperature, lubrication, load, vibration, wear scar, velocity,
and others will affect the results requiring careful study of the test methods
before a test can be selected.
The Alpha-LFW1 is a block-on-ring test that has been found
to provide wear
information on electroless nickel coatings. The Falex tester is a pin-on-notch
device that provides information on high load wear. Both of these tests can be
run dry
or lubricated, and can be used to characterize the differences between
coatings.
Stress Analysis
The intrinsic stress of the deposit can be determined using ASTM 8636. This test
uses a spiral contractometer, plating the spiral and measuring the amount of
movement in the helix while the spiral is plating. The amount
of intrinsic stress in
the deposit is determined by the rate
of swing in a needle attached to the spiral.
The movement is magnified by a factor of
IO, with readings being taken while the
process is operating. After the measurement of intrinsic stress has been taken at
plating temperature, the helix can be removed and cooled. A second reading can
be taken, providing the total stress, and then the thermal component can be
calculated:
Spiral total stress
= intrinsic stress + thermal stress
The results can be used
to predict when adhesion may be compromised on
aluminum and other alloys, as well as the preference for certain types of
corrosion.
Other methods of analysis for total stress have been developed using a rigid
strip. Strips are first plated on both sides, then one side is removed with a
stripper. The thickness of the strip and the coating are measured, and then the
amount of bow in the strip is measured. Calculations can be made from this
information, and the total stress in the part can be selected.
POST-TREATMENTS
The quality control of an electroless nickel deposit implies that certain post-
treatments have been completed. These are generally used to improve the
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185. 190 ELECTROLESS PLATING
deposit adhesion or increase the hardness of the deposit by precipitation of
phosphorus
to nickel phosphide.
In most specifications, the requirements for heat treatment are described.
Generally there are no tests for these processes, and self-certification must be
used. In some cases, a test may exist that is destructive, and therefore not usable.
MIL-C-26074C and ASTM 8733 both use the same classification system
to
describe the post-heat treatment of the deposit. These documents use the term
class and establish the steps
to reduce the potential for hydrogen embrittlement,
increase the adhesion of the coating, improve the fatigue properties of the
part@), and increase the wear resistance and hardness of the coating.
Classes of Electroless Nickel Coatings
Class 1
As plated, no treatment other than hydrogen embrittlement relief at 190 *lo" C
for steels with a tensile strength of
1051 MPa or greater.
Class 2
Heat treated for .hardness, the coating shall be heat treated to a minimum
hardness of
850 Knoop (100 g load). This hardness can be produced by heat
treating the coating at 400" C for 60 minutes. Higher temperatures for shorter
times may be used.
Class 3
Heat treatment at 180 to 200" C for 2 to 4 hours to improve coating adhesion on
aluminum.
Class 4
Heat treatment at 120 to 130" C for a minimum of 1 hour to improve adhesion on
heat-treatable (age-hardened) aluminum alloys and carburized steels.
Class 5
Heat treatment at 140 to 150" C fora minimum of 1 hour to improve adhesion on
non-age-hardened aluminum and beryllium alloys.
The use of the shop traveler
to record the completion of the heat treatment, as
well as a notation on the chart record of the oven, is a standard practice.
Additional post-treatments such as silicates, water glasses, waxes, and
chromates, are sometimes applied
to prevent staining and oxidation of the
nickel. The analysis of these treatments are complicated and seldom tested.
Contact resistance tests are generally used
if the treatment requires testing.
QUALITY CONTROL
To conclude, the electroless nickel facility must have a quality control system.
The platers need to identify how the parts are to be processed. In addition, they
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186. Quahty Control of Electroless Nlckel Deposits 191
must maintain the chemistry, qualify the processes, and complete the
acceptance tests as required by the specifications.
The system can be operated from a single notebook or from a multiuser
computer system. The quality control program can be highly complex, or just
cover the essentials.
In the end, it is dedicated people who must build and follow the quality control
system.
It is people who will produce higher levelsof qualityand make it possible
to extend the performance of electroless nickel deposits into new markets and
applications.
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187. Chapter 1
Fundamental Properties of X-rays
1.1 Nature of X-rays
X-rays with energies ranging from about 100 eV to 10 MeV are classified as electro-
magnetic waves, which are only different from the radio waves, light, and gamma
rays in wavelength and energy. X-rays show wave nature with wavelength ranging
from about 10 to10
3
nm. According to the quantum theory, the electromag-
netic wave can be treated as particles called photons or light quanta. The essential
characteristics of photons such as energy, momentum, etc., are summarized as
follows.
The propagation velocitycof electromagnetic wave (velocity of photon) with
frequencyand wavelengthis given by the relation.
cD .ms
1
/ (1.1)
The velocity of light in the vacuum is a universal constant given ascD
299792458m=s(2:99810
8
m=s). Each photon has an energyE,whichis
proportional to its frequency,
EDhD
hc

.J/ (1.2)
wherehis the Planck constant (6:626010
34
Js). WithEexpressed in keV, and
in nm, the following relation is obtained:
E.keV/D
1:240
.nm/
(1.3)
The momentumpis given bymv, the product of the massm, and its velocityv.
The de Broglie relation for material wave relates wavelength to momentum.
D
h
p
D
h
mv
(1.4)
1
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188. 2 1 Fundamental Properties of X-rays
The velocity of light can be reduced when traveling through a material medium,
but it does not become zero. Therefore, a photon is never at rest and so has no rest
massm
e. However, it can be calculated using Einstein’s mass-energy equivalence
relationEDmc
2
.
ED
m
e
r
1

v
c

2
c
2
(1.5)
It is worth noting that (1.5) is a relation derived from Lorentz transformation in the
case where the photon velocity can be equally set either from a stationary coordi-
nate or from a coordinate moving at velocity ofv(Lorentz transformation is given
in detail in other books on electromagnetism: for example, P. Cornille, Advanced
Electromagnetism and Vacuum Physics, World Scientific Publishing, Singapore,
(2003)). The increase in mass of a photon with velocity may be estimated in the
following equation using the rest massm
e:
mD
m
e
r
1

v
c

2
(1.6)
For example, an electron increases its mass when the accelerating voltage exceeds
100 kV, so that the common formula of
1
2
mv
2
for kinetic energy cannot be used. In
such case, the velocity of electron should be treated relativistically as follows:
EDmc
2
mec
2
D
m
e
r
1

v
c

2
c
2
mec
2
(1.7)
vDc
s
1

m
ec
2
ECm ec
2

2
(1.8)
The value ofm
eis obtained, in the past, by using the relationship ofmDh=.c/
from precision scattering experiments, such as Compton scattering andm
eD
9:10910
31
kg is usually employed as electron rest mass. This also means that an
electron behaves as a particle with the mass of9:10910
31
kg, and it corresponds
to the energy ofEDmc
2
D8:18710
14
Jor0:510910
6
eV in eV.
There is also a relationship between mass, energy, and momentum.

E
c

2
p
2
D.mec/
2
(1.9)
It is useful to compare the properties of electrons and photons. On the one hand,
the photon is an electromagnetic wave, which moves at the velocity of light some-
times called light quantum with momentum and energy and its energy depends upon
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189. 1.2 Production of X-rays 3
the frequency. The photon can also be treated as particle. On the other hand, the
electron has “mass” and “charge.” It is one of the elementary particles that is a
constituent of all substances. The electron has both particle and wave nature such
as photon. For example, when a metallic filament is heated, the electron inside it
is supplied with energy to jump out of the filament atom. Because of the negative
charge of the electron, (eD1:60210
19
C), it moves toward the anode in an
electric field and its direction of propagation can be changed by a magnetic field.
1.2 Production of X-rays
When a high voltage with several tens of kV is applied between two electrodes,
the high-speed electrons with sufficient kinetic energy, drawn out from the cath-
ode, collide with the anode (metallic target). The electrons rapidly slow down and
lose kinetic energy. Since the slowing down patterns (method of loosing kinetic
energy) vary with electrons, continuous X-rays with various wavelengths are gener-
ated. When an electron loses all its energy in a single collision, the generated X-ray
has the maximum energy (or the shortest wavelengthD
SWL). The value of the
shortest wavelength limit can be estimated from the accelerating voltageVbetween
electrodes.
eVh
max (1.10)

SWLD
c
max
D
hc
eV
(1.11)
The total X-ray intensity released in a fixed time interval is equivalent to the area
under the curve in Fig.1.1. It is related to the atomic number of the anode targetZ
and the tube currenti:
I
contDAiZV
2
(1.12)
whereAis a constant. For obtaining high intensity of white X-rays, (1.12) suggests
that it is better to use tungsten or gold with atomic numberZat the target, increase
accelerating voltageV, and draw larger currentias it corresponds to the number
of electrons that collide with the target in unit time. It may be noted that most of
the kinetic energy of the electrons striking the anode (target metal) is converted into
heat and less than 1% is transformed into X-rays. If the electron has sufficient kinetic
energy to eject an inner-shell electron, for example, a K shell electron, the atom will
become excited with a hole in the electron shell. When such hole is filled by an outer
shell electron, the atom regains its stable state. This process also includes production
of an X-ray photon with energy equal to the difference in the electron energy levels.
As the energy released in this process is avalue specific to the target metal and
related electron shell, it is called characteristic X-ray. A linear relation between the
square root of frequencyof the characteristic X-ray and the atomic numberZof
the target material is given by Moseley’s law.
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190. 4 1 Fundamental Properties of X-rays
Fig. 1.1Schematic
representation of the X-ray
spectrum
p
DB M.Z M/ (1.13)
Here,B
Mand Mare constants. This Moseley’s law can also be given in terms of
wavelengthof emitted characteristic X-ray:
1

DR.ZS
M/
2

1
n
2
1

1
n
2
2

(1.14)
Here,Ris the Rydberg constant (1:097310
7
m
1
),SMis a screening constant, and
usually zero for K˛line and one for Kˇline. Furthermore,n
1andn 2represent the
principal quantum number of the inner shell and outer shell, respectively, involved
in the generation of characteristic X-rays. For example,n
1D1for K shell,n 2D2
for L shell, andn
3D3for M shell. As characteristicX-rays are generated when
the applied voltage exceeds the so-called excitation voltage, corresponding to the
potential required to eject an electronfrom the K shell (e.g., Cu: 8.86 keV, Mo:
20.0 keV), the following approximate relation is available between the intensity of
K˛radiation,I
K, and the tube current,i, the applied voltageV, and the excitation
voltageV
K:
I
KDBSi.VV K/
1:67
(1.15)
Here,B
Sis a constant and the value ofB SD4:2510
8
is usually employed. As it is
clear from (1.15), larger the intensity of characteristic X-rays, the larger the applied
voltage and current.
It can be seen from (1.14), characteristic radiation is emitted as a photoelec-
tron when the electron of a specific shell (the innermost shell of electrons, the
K shell) is released from the atom, when the electrons are pictured as orbiting
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191. 1.3 Absorption of X-rays 5
the nucleus in specific shells. Therefore, this phenomenon occurs with a specific
energy (wavelength) and is called “photoelectric absorption.” The energy,E
ej,of
the photoelectron emitted may be described in the following form as a difference
of the binding energy (E
B) for electrons of the corresponding shell with which the
photoelectron belongs and the energy of incidence X-rays (h):
E
ejDhE B (1.16)
The recoil of atom is necessarily produced in the photoelectric absorption pro-
cess, but its energy variation is known to be negligibly small (see Question 1.6).
Equation (1.16) is based on such condition. Moreover, the value of binding energy
(E
B) is also called absorption edge of the related shell.
1.3 Absorption of X-rays
X-rays which enter a sample are scattered by electrons around the nucleus of atoms
in the sample. The scattering usually occurs in various different directions other than
the direction of the incident X-rays, even if photoelectric absorption does not occur.
As a result, the reduction in intensity of X-rays which penetrate the substance is
necessarily detected. When X-rays with intensityI
0penetrate a uniform substance,
the intensityIafter transmission through distancexis given by.
IDI
0e
x
(1.17)
Here, the proportional factoris called linear absorption coefficient, which is
dependent on the wavelength of X-rays, the physical state (gas, liquid, and solid)
or density of the substance, and its unit is usually inverse of distance. However,
since the linear absorption coefficientis proportional to density,.=/becomes
unique value of the substance, independent upon the state of the substance. The
quantity of.=/is called the mass absorption coefficient and the specific values
for characteristic X-rays frequently-used are compiled (see Appendix A.2). Equa-
tion (1.17) can be re-written as (1.18) in terms of the mass absorption coefficient.
IDI
0e





x
(1.18)
Mass absorption coefficient of the sample of interest containing two or more ele-
ments can be estimated from (1.19) using the bulk density,, and weight ratio ofw
j
for each element j.




Dw
1




1
Cw2




2
CD
X
jD1
wj




j
(1.19)
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192. 6 1 Fundamental Properties of X-rays
Fig. 1.2Wavelength dependences of mass absorption coefficient of X-ray using the La as an
example
Absorption of X-rays becomes small as transmittivity increases with increasing
energy (wavelength becomes shorter). However, if the incident X-ray energy comes
close to a specific value (or wavelength) as shown in Fig.1.2, the photoelectric
absorption takes place by ejecting an electron in K-shell and then discontinu-
ous variation in absorption is found. Such specific energy (wavelength) is called
absorption edge. It may be added that monotonic variation in energy (wavelength)
dependence is again detected when the incident X-ray energy is away from the
absorption edge.
1.4 Solved Problems (12 Examples)
Question 1.1Calculate the energy released per carbon atom when 1 g of
carbon is totally converted to energy.
Answer 1.1EnergyEis expressed by Einstein’s relation ofEDmc
2
wheremis
mass andcis the speed of light. If this relationship is utilized, considering SI unit
that expresses mass in kg,
ED110
3
.2:99810
10
/
2
D8:9910
13
J
The atomic weight per mole (molar mass) for carbon is 12.011 g from reference
table (for example, Appendix A.2). Thus, the number of atoms included in 1 g
carbon is calculated as.1=12:011/0:602210
24
D5:0110
22
because the
numbers of atoms are included in one mole of carbon is the Avogadro’s number
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193. 1.4 Solved Problems 7
.0:602210
24
/. Therefore, the energy release per carbon atom can be estimated as:
.8:9910
13
/
.5:0110
22
/
D1:7910
9
J
Question 1.2Calculate (1) strength of the electric fieldE, (2), force on the
electronF, (3) acceleration of electron˛, when a voltage of 10 kV is applied
between two electrodes separated by an interval of 10 mm.
Answer 1.2The work,W, if electric chargeQ(coulomb, C) moves under voltageV
is expressed byWDVQ. When an electron is accelerated under 1 V of difference
in potential, the energy obtained by the electron is called 1 eV. Since the elementary
chargeeis1:60210
19
(C),
1eVD1:60210
19
1 (C)(V)
D1:60210
19
(J)
Electric fieldEcan be expressed withEDV=d, where the distance,d, between
electrodes and the applied voltage beingV. The forceFon the electron with
elementary chargeeis given by;
FDeE (N)
Here, the unit ofFis Newton. Acceleration˛of electrons is given by the following
equation in whichmis the mass of the electron:
˛D
eE
m
.m=s
2
/
.1/ ED
10 .kV/
10 .mm/
D
10
4
.V/
10
2
.m/
D10
6
.V=m/
.2/ FD1:60210
19
10
6
D1:60210
13
.N/
.3/ ˛D
1:60210
13
9:10910
31
D1:7610
17
.m=s
2
/
Question 1.3X-rays are generated by makingthe electrically charged parti-
cles (electrons) with sufficient kinetic energy in vacuum collide with cathode,
as widely used in the experiment of an X-ray tube. The resultant X-rays can
be divided into two parts: continuous X-rays (also called white X-rays) and
characteristic X-rays. The wavelength distribution and intensity of continu-
ous X-rays are usually depending upon the applied voltage. A clear limit is
recognized on the short wavelength side.
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194. 8 1 Fundamental Properties of X-rays
(1)Estimate the speed of electron before collision when applied voltage is
30,000 V and compare it with the speed of light in vacuum.
(2)In addition, obtain the relation of the shortest wavelength limit
SWLof
X-rays generated with the applied voltageV, when an electron loses all
energy in a single collision.
Answer 1.3Electrons are drawn out from cathode by applying the high voltage of
tens of thousands ofVbetween two metallic electrodes installed in the X-ray tube
in vacuum. The electrons collide with anodeat high speed. The velocity of electrons
is given by,
eVD
mv
2
2
!v
2
D
2eV
m
whereeis the electric charge of the electron,Vthe applied voltage across the
electrodes,mthe mass of the electron, andvthe speed of the electron before the
collision. When values of rest massm
eD9:11010
31
.kg/as mass of electron,
elementary electron chargeeD1:60210
19
.C/andVD310
4
.V/are used for
calculating the speed of electronv.
v
2
D
21:60210
19
310
4
9:11010
31
D1:05510
16
;vD1:00210
8
m=s
Therefore, the speed of electron just before impact is about one-third of the speed
of light in vacuum.2:99810
8
m=s/.
Some electrons release all their energy in a single collision. However, some other
electrons behave differently. The electrons slow down gradually due to successive
collisions. In this case, the energy of electron (eV) which is released partially and
the corresponding X-rays (photon) generated have less energy compared with the
energy (h
max) of the X-rays generated when electrons are stopped with one colli-
sion. This is a factor which shows the maximum strength moves toward the shorter
wavelength sides, as X-rays of various wavelengths generate, and higher the inten-
sity of the applied voltage, higher the strength of the wavelength of X-rays (see
Fig.1). Every photon has the energyh,wherehis the Planck constant andthe
frequency.
The relationship ofeVDh
maxcan be used, when electrons are stopped in one
impact and all energy is released at once. Moreover, frequency () and wavelength
() are described by a relation ofDc=,wherecis the speed of light. Therefore,
the relation between the wavelength
SWLin m and the applied voltageVmay be
given as follows:

SWLDc=maxDhc=eVD
.6:62610
34
/.2:99810
8
/
.1:60210
19
/V
D
.12:4010
7
/
V
This relation can be applied to more general cases, such as the production of electro-
magnetic waves by rapidly decelerating anyelectrically charged particle including
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195. 1.4 Solved Problems 9
electron of sufficient kinetic energy, and it is independent of the anode material.
When wavelength is expressed in nm, voltage in kV, and the relationship becomes
VD1:240.
10
8
6
4
2
0
0.01 0.02 0.05 0.1 0.2 0.4
Wavelength [nm]
Intensity [a.u.]
100kV
80kV
60kV
20kV
40kV
Fig. 1Schematic diagram for X-ray spectrum as a function of applied voltage
Question 1.4K˛ 1radiation of Fe is the characteristic X-rays emitted when
one of the electrons in L shell falls into the vacancy produced by knocking
an electron out of the K-shell, and its wavelength is 0.1936 nm. Obtain the
energy difference related to this process for X-ray emission.
Answer 1.4Consider the process in which an L shell electron moves to the vacancy
created in the K shell of the target (Fe) by collision with highly accelerated electrons
from a filament. The wavelength of the photon released in this process is given by
, (with frequency). We also use Planck’s constanthof.6:62610
34
Js/and
the velocity of lightcof.2:99810
8
ms
1
/. Energy per photon is given by,
EDhD
hc

Using Avogadro’s numberN
A, one can obtain the energy differenceErelated to
the X-ray release process per mole of Fe.
ED
N
Ahc

D
0:602210
24
6:62610
34
2:99810
8
0:193610
9
D
11:9626
0:1936
10
7
D6:197910
8
J=mole
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196. 10 1 Fundamental Properties of X-rays
Reference:The electrons released from a filament have sufficient kinetic energy and
collide with the Fe target. Therefore, an electron of K-shell is readily ejected. This
gives the state of Fe
C
ion left in an excited state with a hole in the K-shell. When
this hole is filled by an electron from an outer shell (L-shell), an X-ray photon is
emitted and its energy is equal to the difference in the two electron energy levels.
This variation responds to the following electron arrangement of Fe
C
.
Before release K1 L8 M14 N2
After release K2 L7 M14 N2
Question 1.5Explain atomic densityand electron density.
Answer 1.5The atomic densityN
aof a substance for one-component system is
given by the following equation, involving atomic weightM, Avogadro’s number
N
A, and the density.
N
aD
N
A
M
: (1)
In the SI system,N
a.m
3
/,N AD0:602210
24
.mol
1
/,.kg=m
3
/,and
M.kg=mol/, respectively.
The electron densityN
eof a substance consisting of single element is given by,
N
eD
N
A
M
Z (2)
Each atom involvesZelectrons (usuallyZis equal to the atomic number) and the
unit ofN
eis also.m
3
/.
The quantityN
aDN A=Min (1) orN eD.NAZ/=Min (2), respectively, gives
the number of atoms or that of electrons per unit mass (kg), when excluding den-
sity,. They are frequently called “atomic density” or “electron density.” However,
it should be kept in mind that the number per m
3
(per unit volume) is completely
different from the number per 1 kg (per unit mass). For example, the following val-
ues of atomic number and electron number per unit mass (D1kg) are obtained for
aluminum with the molar mass of 26.98 g and the atomic number of 13:
N
aD
0:602210
24
26:9810
3
D2:23210
25
.kg
1
/
N
eD
0:602210
2426:9810
3
13D2:910
26
.kg
1
/
Since the density of aluminum is2:70Mg=m
3
D2:7010
3
kg=m
3
from reference
table (Appendix A.2), we can estimate the corresponding values per unit volume as
N
aD6:02610
28
.m
3
/andN eD7:8310
29
.m
3
/, respectively.
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197. 1.4 Solved Problems 11
Reference:Avogadro’s number provides the number of atom (or molecule) included
in one mole of substance. Since the atomic weight is usually expressed by the num-
ber of grams per mole, the factor of10
3
is required for using Avogadro’s number
in the SI unit system.
Question 1.6The energy of a photoelectron,E ej, emitted as the result of pho-
toelectron absorption process may be given in the following with the binding
energyE
Bof the electron in the corresponding shell:
E
ejDhE B
Here,his the energy of incident X-rays, and this relationship has been
obtained with an assumption that the energy accompanying the recoil of atom,
which necessarily occurs in photoelectron absorption, is negligible.
Calculate the energy accompanyingthe recoil of atom in the following
condition for Pb. The photoelectron absorption process of K shell for Pb was
made by irradiating X-rays with the energy of 100 keV against a Pb plate and
assuming that the momentum of the incident X-rays was shared equally by
Pb atom and photoelectron. In addition, the molar mass (atomic weight) of
Pb is 207.2 g and the atomic mass unit is1amuD1:6605410
27
kgD
931:510
3
keV.
Answer 1.6The energy of the incident X-rays is given as 100 keV, so that its
momentum can be described as being100keV=c, using the speed of lightc.Since
the atom and photoelectron shared the momentum equally, the recoil energy of atom
will be50keV=c. Schematic diagram of this process is illustrated in Fig.1.
Fig. 1Schematic diagram for the photo electron absorption process assuming that the momentum
of the incident X-rays was shared equally by atom and photoelectron. Energy of X-ray radiation is
100 keV
On the other hand, one should consider for the atom that1amuD931:510
3
keV
is used in the same way as the energy which is the equivalent energy amount of
the rest mass for electron,m
e. The molar mass of 207.2 g for Pb is equivalent to
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198. 12 1 Fundamental Properties of X-rays
207.2 amu, so that the mass of 1 mole of Pb is equivalent to the energy of207:2
931:510
3
D193006:810
3
keV=c.
When the speed of recoil atom isvand the molar mass of Pb isM
A, its energy
can be expressed by
1
2
MAv
2
. According to the given assumption and the momen-
tum described aspDM
Av, the energy of the recoil atom,E
A
r
, may be obtained
as follows:
E
A
r
D
1
2
M
Av
2
D
p
2
2MA
D
.50/
2
2.193006:810
3
/
D0:006510
3
.keV/
The recoil energy of atom in the photoelectron absorption process shows just a
very small value as mentioned here using the result of Pb as an example, although
the recoil of the atom never fails to take place.
Reference:
Energy of1amuD
1:6605410
27
.2:9979210
8
/
2
1:6021810
19
D9:31510
8
.eV/
On the other hand, the energy of an electron with rest massm
eD9:10910
31
.kg/
can be obtained in the following with the relationship of1.eV/D1:60210
19
.J/:
EDm
ec
2
D
9:10910
31
.2:99810
8
/
2
1:60210
19
D0:510910
6
.eV/
Question 1.7Explain the Rydberg constant in Moseley’s law with respect to
the wavelength of characteristic X-rays, and obtain its value.
Answer 1.7Moseley’s law can be written as,
1

DR.ZS
M/
2

1
n
2
1

1
n
2
2

(1)
The wavelength of the X-ray photon./corresponds to the shifting of an electron
from the shell of the quantum numbern
2to the shell of the quantum number ofn 1.
Here,Zis the atomic number andS
Mis a screening constant.
Using the elementary electron charge ofe, the energy of electron characterized
by the circular movement around the nucleus chargeZein each shell (orbital) may
be given, for example, with respect to an electron of quantum numbern
1shell in
the following form:
E
nD
2
2
me
4
h
2
Z
2
n
2
1
(2)
Here,his a Planck constant andmrepresents the mass of electron. The energy of
this photon is given by,
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199. 1.4 Solved Problems 13
hDE n2
En1
DED
2
2
me
4
h
2
Z
2

1
n
2
1

1
n
2
2

(3)
The following equation will also be obtained, if the relationship ofEDhD
hc

is employed while using the velocity of photon,c:
1

D
2
2
me
4
ch
3
Z
2

1
n
2
1

1
n
2
2

(4)
If the value of electron mass is assumed to be rest mass of electron and a compar-
ison of (1) with (4) is made, the Rydberg constantRcan be estimated. It may be
noted that the term of.ZS
M/
2
in (1) could be empirically obtained from the
measurements on various characteristic X-rays as reported by H.G.J. Moseley in
1913.
RD
2
2
me
4
ch
3
D
2.3:142/
2
.9:10910
28
/.4:80310
10
/
4
.2:99810
10
/.6:62610
27
/
3
D109:74310
3
.cm
1
/D1:09710
7
.m
1
/ (5)
The experimental value ofRcan be obtained from the ionization energy (13.6 eV)
of hydrogen (H). The corresponding wave number (frequency) is109737:31cm
1
,
in good agreement with the value obtainedfrom (5). In addition, since Moseley’s
law and the experimental results are all described by using the cgs unit system (gauss
system),4:80310
10
esu has been used for the elemental electron chargee. Con-
version into the SI unit system is given by (SI unitvelocity of light10
1
)(e.g.,
5th edition of the Iwanami Physics-and-Chemistry Dictionary p. 1526 (1985)). That
is to say, the amount of elementary electron chargeecan be expressed as:
1:60210
19
Coulomb2:99810
10
cm=s10
1
D4:80310
10
esu
The Rydberg constant is more strictly defined by the following equation:
RD
2
2
e
4
ch
3
(6)
1

D
1
m
C
1
mp
(7)
Here,mis electron mass andm
Pis nucleus (proton) mass.The detected difference
is quite small, but the value ofm
Pdepends on the element. Then, it can be seen
from the relation of (6) and (7) that a slightly different value ofRis obtained for
each element. However, if a comparison is made with a hydrogen atom, there is a
difference of about 1,800 times between the electron massm
eD9:10910
31
kg
and the proton mass which ism
PD1:6710
27
kg. Therefore, the relationship of
(6) is usually treated asDm, becausem
Pis very large in comparison withm e.
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200. 14 1 Fundamental Properties of X-rays
Reference:The definition of the Rydberg constant in the SI unit is given in the form
where the factor of.1=4
0/is included by using the dielectric constant 0.8:854
10
12
F=m/in vacuum for correlating with nucleus chargeZ e.
RD
2
2
e
4
ch
3


1
4 0

2
D
me
4
8
2
0
ch
3
D
9:10910
31
.1:60210
19
/
4
8.8:85410
12
/
2
.2:99810
8
/.6:62610
34
/
3
D
9:109.1:602/
4
10
107
8.8:854/
2
.2:998/.6:626/
3
10
118
D1:09710
7
.m
1
/
Question 1.8When the X-ray diffraction experiment is made for a plate
sample in the transmission mode, it is readily expected that absorption
becomes large and diffraction intensity becomes weak as the sample thickness
increases. Obtain the thickness of a plate sample which makes the diffrac-
tion intensity maximum and calculate the value of aluminum for the Cu-K˛
radiation.
Back
side
Surface
side
x
dx
t
t-x
I
0 I
Fig. AGeometry for a case where X-ray penetrates a plate sample
Answer 1.8X-ray diffraction experiment in the transmission mode includes both
absorption and scattering of X-rays. Let us consider the case where the sample
thickness ist, the linear absorption coefficient, the scattering coefficientS,and
the intensity of incident X-raysI
0as referred to Fig. A.
Since the intensity of the incident X-rays reaching the thin layer dxwhich is at
distance ofxfrom the sample surface is given byI
0e
x
, the scattering intensity
dI
0
x
from the thin layer dx(with scattering coefficientS) is given by the following
equation:
dI
0
x
DSI0e
x
dx (1)
The scattering intensity dI
xpasses through the distance of.tx/in the sample
and the absorption during this passage is expressed by the form of e
.tx/
.There-
fore, the scattering intensity of the thin layer dxafter passing through the sample
may be given in the following form:
dI
xD.SI0e
x
dx/e
.tx/
DSI0e
t
dx (2)
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201. 1.4 Solved Problems 15
The scattering intensity of the overall sample will be equal to the result obtained
by integrating the intensity of the thin layer dxwith respect to the sample thickness
from zero tot.
ID
Z
t
0
SI0e
t
dxDSI 0te
t
(3)
The maximum value ofIis given under the condition of dI=dtD0.
dI
dt
DSI
0.e
t
te
t
/D0; tD1!tD
1

(4)
We can find the values of mass absorption coefficient.=/and density./
of aluminum for Cu-K˛radiation in the reference table (e.g., Appendix A.2). The
results are.=/D49:6cm
2
=gandD2:70g=cm
3
, respectively. The linear
absorption coefficient of aluminum is calculated in the following:
D




D49:62:70D133:92 .cm
1
/
Therefore, the desired sample thicknesstcan be estimated as follows:
tD
1

D

1
133:92

D7:4710
3
.cm/D74:7 .m/
Question 1.9There is a substance of linear absorption coefficient.
(1)Obtain a simple relation to give the sample thicknessxrequired to reduce
the amount of transmitted X-ray intensity by half.
(2)Calculate also the corresponding thickness of Fe-17 mass % Cr alloy
.densityD7:7610
6
g=m
3
/for Mo-K˛radiation, using the relation
obtained in (1).
Answer 1.9Let us consider the intensity of the incident X-rays asI
0and that of the
transmitted X-rays asI. Then,
IDI
0e
x
(1)
If the condition ofID
I0
2
is imposed, taken into account, one obtains,
I
0
2
DI
x
e
(2)
1
2
De
x
(3)
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202. 16 1 Fundamental Properties of X-rays
When the logarithm of both sides is taken, we obtain log1log2Dxloge.
The result islog2Dx, as they are log1D0,andlogeD1. Here, natural
logarithm is used and the required relation is given as follows:
xD
log2

'
0:693

(4)
The values of mass absorption coefficients of Fe and Cr for the Mo-K˛radiation
are37:6and29:9cm
2
=g obtained from Appendix A.2, respectively. The concentra-
tion of Cr is given by 17 mass %, so that the weight ratio of two alloy components
can be set asw
FeD0:83andw CrD0:17. Then, the mass absorption coefficient of
the alloy is expressed in the following:




Alloy
DwFe




Fe
CwCr




Cr
D0:83.37:6/C0:17.29:9/D36:3 .cm
2
=g/
Next, note that the unit of the density of the Fe–Cr alloy is expressed in cgs,
7:7610
6
g=m
3
D7:76g=cm
3
. We obtain,

AlloyD36:37:76 .cm
1
/D281:7 .cm
1
/
xD
0:693
281:7
D0:0025cmD0:025mmD25 m
Question 1.10Calculate the mass absorption coefficient of lithium niobate
.LiNbO
3/for Cu-K˛radiation.
Answer 1.10The atomic weight of Li, Nb, and oxygen (O) and their mass absorp-
tion coefficients for Cu-K˛radiation are obtained from Appendix A.2, as follows:
Atomic weightMass-absorption coefficient
(g) = .cm
2
=g/
Li 6.941 0.5
Nb 92.906 145
O 15.999 11.5
The molar weight(molar mass)Mper 1 mole of LiNbO 3is given in the following:
MD6:941C92:906C.15:9993/D147:844 .g/
The weight ratiow
jof three components of Li, Nb, and O is to be obtained.
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203. 1.4 Solved Problems 17
wLiD
6:941
147:844
D0:047;w
NbD
92:906
147:844
D0:628;w
OD
47:997
147:844
D0:325
Then, the mass absorption coefficient of lithium niobate can be obtained as follows:




LiNbO3
D
X
w j




j
D0:0470:5C0:628145C0:32511:5
D94:8 .cm
2
=g/
Question 1.11A thin plate of pure iron is suitable for a filter for Co-K˛
radiation, but it is also known to easily oxidize in air. For excluding such
difficulty,we frequently utilize crystalline hematite powder (Fe
2O3:density
5:2410
6
g=m
3
). Obtain the thickness of a filter consisting of hematite powder
which reduces the intensity of Co-Kˇradiation to 1/500 of the K˛radiation
case. Given condition is as follows. The intensity ratio between Co-K˛and
Co-Kˇis found to be given by 5:1 without a filter. The packing density of
powder sample is known usually about 70% of the bulk crystal.
Answer 1.11The atomic weight of Fe and oxygen (O) and their mass absorption
coefficients for Co-K˛and Co-Kˇradiations are obtained from Appendix A.2, as
follows:
Atomic/for Co-K˛/for Co-Kˇ
weight (g)(cm
2
=g) (cm
2
=g)
Fe55.845 57.2 342
O15.999 18.0 13.3
The weight ratio of Fe and O in hematite crystal is estimated in the following:
M
Fe2O3
D55:8452C15:9993D159:687
w
FeD
55:8452
159:687
D0:699;w
OD0:301
The mass absorption coefficients of hematite crystals for Co-K˛and Co-Kˇradia-
tions are, respectively, to be calculated.




˛
Fe
2O3
D0:69957:2C0:30118:0D45:4 .cm
2
=g/




ˇ
Fe
2O3
D0:699342C0:30113:3D243:1 .cm
2
=g/
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204. 18 1 Fundamental Properties of X-rays
It is noteworthy that the density of hematite in the filter presently prepared is equiv-
alent to 70% of the value of bulk crystal by considering the packing density, so that
we have to use the density value of
fD5:240:70D3:67g=cm
3
Therefore, the
value of the linear absorption coefficient of hematite powder packed into the filter
for Co-K˛and Co-Kˇradiations will be, respectively, as follows:

˛D




˛
Fe
2O3
fD45:43:67D166:6 .cm
1
/

ˇD




ˇ
Fe
2O3
fD24:13:67D892:2 .cm
1
/
The intensity ratio of Co-K˛and Co-Kˇradiations before and after passing through
the filter consisting of hematite powder may be described in the following equation:
I
CoKˇ
ICoK˛
D
I
ˇ
0
e

ˇt
I
˛
0
e
˛t
From the given condition, the ratio betweenI
˛
0
andI
ˇ
0
is 5:1 without filter, and it
should be 500:1 after passing through the filter. They are expressed as follows:
1
500
D
1
5
e

ˇt
e
˛t
!
1
100
De
.˛
ˇ/t
Take the logarithm of both sides and obtain the thickness by using the values of ˛
and ˇ.
.
˛ˇ/tDlog100 . *logeD1;log1D0/
.166:6892:2/tD4:605
tD0:0063 .cm
1
/D63 .m/
Question 1.12For discussing the influence of X-rays on the human body
etc., it would be convenient if the effect of a substance consisting of multi-
elements, such as water (H
2O) and air (N2,O2, others), can be described by
information of each constituent element (H, O, N, and others) with an appro-
priate factor. For this purpose, the value of effective element numberNZis
often used and it is given by the following equation:
NZD
2:94
q
a1Z
2:94
1
Ca2Z
2:94
2
C
wherea
1;a2:::is the electron component ratio which corresponds to the rate
of the number of electrons belonging to each element with the atomic number
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205. 1.4 Solved Problems 19
Z1;Z2;:::to the total number of electrons of a substance. Find the effective
atomic number of water and air. Here, the air composition is given by 75.5%
of nitrogen, 23.2% of oxygen, and 1.3% of argon in weight ratio.
Answer 1.12Water (H
2O) consists of two hydrogen atoms and one oxygen atom,
whereas the number of electrons are one for hydrogen and eight for oxygen. The
values of atomic weight per mole (molar mass) of hydrogen and oxygen (molar
mass) are 1.008 and 15.999 g, respectively.Each electron density per unit mass is
given as follows:
For hydrogenN
H
e
D
0:602210
24
1:008
1D0:59710
24
.g
1
/
For oxygenN
O
e
D
0:602210
2415:999
8D0:30110
24
.g
1
/
In water (H
2O), the weight ratio can be approximated by2=18for hydrogen and
16=18for oxygen, respectively. Then, the number of electrons in hydrogen
and oxygen contained in 1 g water are0:59710
24
.2=18/D0:066310
24
and0:30110
24
.16=18/D0:267610
24
,respectively, so that the number
of electrons contained in 1 g water is estimated to be.0:0663C0:2676/10
24
D
0:333910
24
. Therefore, the electron component ratio of water is found as follows:
a
H
D
0:0663
0:3339
D0:199
a
O
D
0:2262
0:3339
D0:801
NZD
2:94
p
0:1991
2:94
C0:8018
2:94
D
2:94
p
0:199C362:007D
2:94
p
362:206D7:42
Here, we use the relationship ofNZDX
1
y!lnNZD
1
y
lnX!NZDe
1
y
lnX
On the other hand, the molar masses of nitrogen, oxygen, and argon are 14.01,
15.999, and 39.948 g, respectively. Since 75.5% of nitrogen (7 electrons), 23.2% of
oxygen (8 electrons), and 1.3% of argon (18 electrons) in weight ratio are contained
in 1 g of air, each electron numbers is estimated in the following:
For nitrogenN
N
e
D
0:602210
24
14:01
0:7557D0:227210
24
For oxygen N
O
e
D
0:602210
24
15:999
0:2328D0:069910
24
For argon N
Ar
e
D
0:602210
24
39:948
0:01318D0:003510
24
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206. 20 1 Fundamental Properties of X-rays
Therefore, the value of.0:2272C0:0699C0:0035/10
24
D0:300610
24
is
corresponding to the number of electrons in 1 g of air. The rate to the total number
of electrons of each element is as follows:
a
N
D
0:2272
0:3006
D0:756
a
O
D
0:0699
0:3006
D0:232
a
Ar
D
0:0035
0:3006
D0:012
Accordingly, the effective atomic number of air is estimated in the following:
NZD
2:94
p
0:7567
2:94
C0:2328
2:94
C0:01218
2:94
D
2:94
p
230:73C104:85C58:84D
2:94
p
394:42D7:64
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207. http://www.springer.com/978-3-642-16634-1
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208. Introduction to Non-Destructive Testing Techniques
Radiographic Testing Page 1 of 47
Radiographic Testing
Radiography is used in a very wide range of aplications including medicine,
engineering, forensics, security, etc. In NDT, radiography is one of the most
important and widely used methods. Radiographic testing (RT) offers a
number of advantages over other NDT methods, however, one of its major
disadvantages is the health risk associated with the radiation.
In general, RT is method of inspecting materials for hidden flaws by
using the ability of short wavelength electromagnetic radiation
(high energy photons) to penetrate various materials. The intensity
of the radiation that penetrates and passes through the material is
either captured by a radiation sensitive film (Film Radiography) or
by a planer array of radiation sensitive sensors (Real-time
Radiography). Film radiography is the oldest approach, yet it is still
the most widely used in NDT.

Basic Principles
In radiographic testing, the part to be inspected is placed
between the radiation source and a piece of radiation
sensitive film. The radiation source can either be an X-
ray machine or a radioactive source (Ir-192, Co-60, or in
rare cases Cs-137). The part will stop some of the
radiation where thicker and more dense areas will stop
more of the radiation. The radiation that passes through
the part will expose the film and forms a shadowgraph
of the part. The film darkness (density) will vary with the
amount of radiation reaching the film through the test
object where darker areas indicate more exposure
(higher radiation intensity) and liter areas indicate less
exposure (higher radiation intensity).
This variation in the image darkness can be used to
determine thickness or composition of material and
would also reveal the presence of any flaws or
discontinuities inside the material.

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Advantages and Disadvantages
The primary advantages and disadvantages in comparison to other NDT methods are:
Advantages
 Both surface and internal discontinuities can be detected.
 Significant variations in composition can be detected.
 It has a very few material limitations.
 Can be used for inspecting hidden areas (direct access to surface is not required)
 Very minimal or no part preparation is required.
 Permanent test record is obtained.
 Good portability especially for gamma-ray sources.
Disadvantages
 Hazardous to operators and other nearby personnel.
 High degree of skill and experience is required for exposure and interpretation.
 The equipment is relatively expensive (especially for x-ray sources).
 The process is generally slow.
 Highly directional (sensitive to flaw orientation).
 Depth of discontinuity is not indicated.
 It requires a two-sided access to the component.


PHYSICS OF RADIATION

Nature of Penetrating Radiation
Both X-rays and gamma rays are electromagnetic waves and on the electromagnetic
spectrum they ocupy frequency ranges that are higher than ultraviolate radiation. In
terms of frequency, gamma rays generaly have higher frequencies than X-rays as seen
in the figure . The major distenction between X-rays and gamma rays is the origion
where X-rays are usually artificially produced using an X-ray generator and gamma
radiation is the product of radioactive materials. Both X-rays and gamma rays are
waveforms, as are light rays, microwaves, and radio waves. X-rays and gamma rays
cannot been seen, felt, or heard. They possess no charge and no mass and, therefore,
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are not influenced by electrical and magnetic fields and will generally travel in straight
lines. However, they can be diffracted (bent) in a manner similar to light.







Electromagentic radiation act somewhat like a particle at times in that they occur as
small “packets” of energy and are referred to as “photons”. Each photon contains a
certain amount (or bundle) of energy, and all electromagnetic radiation consists of
these photons. The only difference between the various types of electromagnetic
radiation is the amount of energy found in the photons. Due to the short wavelength
of X-rays and gamma rays, they have more energy to pass through matter than do the
other forms of energy in the electromagnetic spectrum. As they pass through matter,
they are scattered and absorbed and the degree of penetration depends on the kind of
matter and the energy of the rays.
Properties of X-Rays and Gamma Rays
 They are not detected by human senses (cannot be seen, heard, felt, etc.).
 They travel in straight lines at the speed of light.
 Their paths cannot be changed by electrical or magnetic fields.
 They can be diffracted, refracted to a small degree at interfaces between two
different materials, and in some cases be reflected.
 They pass through matter until they have a chance to encounter with an atomic
particle.
 Their degree of penetration depends on their energy and the matter they are
traveling through.
 They have enough energy to ionize matter and can damage or destroy living cells.
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X-Radiation
X-rays are just like any other kind of electromagnetic radiation. They can be produced
in packets of energy called photons, just like light. There are two different atomic
processes that can produce X-ray photons. One is called Bremsstrahlung (a German
term meaning “braking radiation”) and the other is called K-shell emission. They can
both occur in the heavy atoms of tungsten which is often the material chosen for the
target or anode of the X-ray tube.
Both ways of making X-rays involve a change in the state of electrons. However,
Bremsstrahlung is easier to understand using the classical idea that radiation is emitted
when the velocity of the electron shot at the tungsten target changes. The negatively
charged electron slows down after swinging around the nucleus of a positively charged
tungsten atom and this energy loss produces X-radiation. Electrons are scattered
elastically or inelastically by the positively charged nucleus. The inelastically scattered
electron loses energy, and thus produces X-ray photon, while the elastically scattered
electrons generally change their direction significantly but without loosing much of
their energy.

Bremsstrahlung Radiation
X-ray tubes produce X-ray photons by accelerating a
stream of electrons to energies of several hundred
kiloelectronvolts with velocities of several hundred
kilometers per hour and colliding them into a heavy
target material. The abrupt acceleration of the charged
particles (electrons) produces Bremsstrahlung photons.
X-ray radiation with a continuous spectrum of energies is
produced with a range from a few keV to a maximum of
the energy of the electron beam.
The Bremsstrahlung photons generated within the target material are attenuated as
they pass through, typically, 50 microns of target material. The beam is further
attenuated by the aluminum or beryllium vacuum window. The results are the
elimination of the low energy photons, 1 keV through 15 keV, and a significant
reduction in the portion of the spectrum from 15 keV through 50 keV. The spectrum
from an X-ray tube is further modified by the filtration caused by the selection of filters
used in the setup.

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K-shell Emission Radiation
Remember that atoms have their electrons arranged in
closed “shells” of different energies. The K-shell is the
lowest energy state of an atom. An incoming electron
can give a K-shell electron enough energy to knock it out
of its energy state. About 0.1% of the electrons produce
K-shell vacancies; most produce heat. Then, a tungsten
electron of higher energy (from an outer shell) can fall
into the K-shell. The energy lost by the falling electron
shows up as an emitted X-ray photon. Meanwhile,
higher energy electrons fall into the vacated energy state in the outer shell, and so on.
After losing an electron, an atom remains ionized for a very short time (about 10
-14

second) and thus an atom can be repeatedly ionized by the incident electrons which
arrive about every 10
-12
second. Generally, K-shell emission produces higher-intensity
X-rays than Bremsstrahlung, and the X-ray photon comes out at a single wavelength.

Gamma Radiation
Gamma radiation is one of the three types of natural radioactivity. Gamma rays are
electromagnetic radiation just like X-rays. The other two types of natural radioactivity
are alpha and beta radiation, which are in the form of particles. Gamma rays are the
most energetic form of electromagnetic radiation.
Gamma radiation is the product of radioactive atoms. Depending upon the ratio of
neutrons to protons within its nucleus, an isotope of a particular element may be
stable or unstable. When the binding energy is not strong enough to hold the nucleus
of an atom together, the atom is said to be unstable. Atoms with unstable nuclei are
constantly changing as a result of the imbalance of energy within the nucleus. Over
time, the nuclei of unstable isotopes spontaneously disintegrate, or transform, in a
process known as “radioactive decay” and such material is called “radioactive
material”.

Types of Radiation Produced by Radioactive Decay
When an atom undergoes radioactive decay, it emits one or more forms of high speed
subatomic particles ejected from the nucleus or electromagnetic radiation (gamma-
rays) emitted by either the nucleus or orbital electrons.
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Alpha Particles
Certain radioactive materials of high atomic mass
(such as Ra-226, U-238, Pu-239), decay by the
emission of alpha particles. These alpha particles are
tightly bound units of two neutrons and two protons
each (He-4 nucleus) and have a positive charge.
Emission of an alpha particle from the nucleus results
in a decrease of two units of atomic number (Z) and
four units of mass number (A). Alpha particles are
emitted with discrete energies characteristic of the
particular transformation from which they originate.
All alpha particles from a particular radionuclide
transformation will have identical energies.

Beta Particles
A nucleus with an unstable ratio of neutrons to
protons may decay through the emission of a high
speed electron called a beta particle. In beta
decay a neutron will split into a positively charged
proton and a negatively charged electron. This
results in a net change of one unit of atomic
number (Z) and no change in the mass number
(A). Beta particles have a negative charge and the
beta particles emitted by a specific radioactive
material will range in energy from near zero up to
a maximum value, which is characteristic of the
particular transformation.

Gamma-rays
A nucleus which is in an excited state (unstable nucleus) may emit one or more
photons of discrete energies. The emission of gamma rays does not alter the number
of protons or neutrons in the nucleus but instead has the effect of moving the nucleus
from a higher to a lower energy state (unstable to stable). Gamma ray emission
frequently follows beta decay, alpha decay, and other nuclear decay processes.

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Activity (of Radioactive Materials)
The quantity which expresses the radiation producing potential of a given amount of
radioactive material is called “Activity”. The Curie (Ci) was originally defined as that
amount of any radioactive material that disintegrates at the same rate as one gram of
pure radium. The International System (SI)
unit for activity is the Becquerel (Bq), which is
that quantity of radioactive material in which
one atom is transformed per second. The
radioactivity of a given amount of radioactive
material does not depend upon the mass of
material present. For example, two one-curie
sources of the same radioactive material
might have very different masses depending
upon the relative proportion of non-
radioactive atoms present in each source.
The concentration of radioactivity, or the relationship between the mass of radioactive
material and the activity, is called “specific activity”. Specific activity is expressed as the
number of Curies or Becquerels per unit mass or volume. Each gram of Cobalt-60 will
contain approximately 50 Ci. Iridium-192 will contain 350 Ci for every gram of material.
The higher specific activity of iridium results in physically smaller sources. This allows
technicians to place the source in closer proximity to the film while maintaining the
sharpness of the radiograph.

Isotope Decay Rate (Half-Life)
Each radioactive material decays at its own unique rate which cannot be altered by any
chemical or physical process. A useful measure of this rate is the “half-life” of the
radioactivity. Half-life is defined as the time required
for the activity of any particular radionuclide to
decrease to one-half of its initial value. In other words
one-half of the atoms have reverted to a more stable
state material. Half-lives of radioactive materials
range from microseconds to billions of years. Half-life
of two widely used industrial isotopes are; 74 days for
Iridium-192, and 5.3 years for Cobalt-60.
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In order to find the remaining activity of a certain material with a known half-life value
after a certain period of time, the following formula may be used. The formula
calculates the decay fraction (or the remaining fraction of the initial activity) as:









Where;

: decay fraction (i.e., remaining fraction of the initial activity)

: Half-Life value (hours, days, years, etc.)
: Elapsed time (hours, days, years, etc.)

Or alternatively, the equitation can be solved to find the time required for activity to
decay to a certain level as:







Radiation Energy, Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different
energy levels and at different rates. It is important to understand the terms used to
describe the energy and intensity of the radiation.

Radiation Energy
The energy of the radiation is responsible for its ability to penetrate matter. Higher
energy radiation can penetrate more and higher density matter than low energy
radiation. The energy of ionizing radiation is measured in electronvolts (eV). One
electronvolt is an extremely small amount of energy so it is common to use
kiloelectronvolts (keV) and megaelectronvolt (MeV). An electronvolt is a measure of
energy, which is different from a volt which is a measure of the electrical potential
between two positions. Specifically, an electronvolt is the kinetic energy gained by an
electron passing through a potential difference of one volt. X-ray generators have a
control to adjust the radiation energy, keV (or kV).
The energy of a radioisotope is a characteristic of the atomic structure of the material.
Consider, for example, Iridium-192 and Cobalt-60, which are two of the more common
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industrial Gamma ray sources. These isotopes emit radiation in two or three discreet
wavelengths. Cobalt-60 will emit 1.17 and 1.33 MeV gamma rays, and Iridium-192 will
emit 0.31, 0.47, and 0.60 MeV gamma rays. It can be seen from these values that the
energy of radiation coming from Co-60 is more than twice the energy of the radiation
coming from the Ir-192. From a radiation safety point of view, this difference in energy
is important because the Co-60 has more material penetrating power and, therefore, is
more dangerous and requires more shielding.

Intensity and Exposure
Radiation intensity is the amount of energy passing through a given area that is
perpendicular to the direction of radiation travel in a given unit of time. One way to
measure the intensity of X-rays or gamma rays is to measure the amount of ionization
they cause in air. The amount of ionization in air produced by the radiation is called the
exposure. Exposure is expressed in terms of a scientific unit called a Roentgen (R). The
unit roentgen is equal to the amount of radiation that ionizes 1 cm
3
of dry air (at 0°C
and standard atmospheric pressure) to one electrostatic unit of charge, of either sign.
Most portable radiation detection safety devices used by radiographers measure
exposure and present the reading in terms of Roentgens or Roentgens/hour, which is
known as the “dose rate”.

Ionization
As penetrating radiation moves from point to point in matter, it loses its energy
through various interactions with the atoms it encounters. The rate at which this
energy loss occurs depends upon the type and energy of the radiation and the density
and atomic composition of the matter through which it is passing.
The various types of penetrating radiation impart their energy to matter primarily
through excitation and ionization of orbital electrons. The term “excitation” is used to
describe an interaction where electrons acquire energy from a passing charged particle
but are not removed completely from their atom. Excited electrons may subsequently
emit energy in the form of X-rays during the process of returning to a lower energy
state. The term “ionization” refers to the complete removal of an electron from an
atom following the transfer of energy from a passing charged particle.
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Because of their double charge and relatively slow velocity, alpha particles have a
relatively short range in matter (a few centimeters in air and only fractions of a
millimeter in tissue). Beta particles have, generally, a greater range.
Since they have no charge, gamma-rays and X-rays
proceeds through matter until there is a chance of
interaction with a particle. If the particle is an
electron, it may receive enough energy to be
ionized, whereupon it causes further ionization by
direct interactions with other electrons. As a result,
gamma-rays and X-rays can cause the liberation of
electrons deep inside a medium. As a result, a given
gamma or X-ray has a definite probability of
passing through any medium of any depth.

Newton's Inverse Square Law
Any point source which spreads its influence equally in
all directions without a limit to its range will obey the
inverse square law. This comes from strictly geometrical
considerations. The intensity of the influence at any
given distance (d) is the source strength divided by the
area of a sphere having a radius equal to the distance
(d). Being strictly geometric in its origin, the inverse
square law applies to diverse phenomena. Point sources
of gravitational force, electric field, light, sound, and
radiation obey the inverse square law.
As one of the fields which obey the general inverse square law, the intensity of the
radiation received from a point radiation source can be characterized by the diagram
above. The relation between intensity and distance according to the inverse square law
can be expresses as:









Where

are the intensities at distances

form the source, respectively.
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All measures of exposure or dose rate will drop off by the inverse square law. For
example, if the received dose of radiation is 100 mR/hr at 2 cm from a source, it will be
0.01 mR/hr at 2 m.

Interaction between Penetrating Radiation and Matter (Attenuation)
When X-rays or gamma rays are directed into an
object, some of the photons interact with the
particles of the matter and their energy can be
absorbed or scattered. This absorption and
scattering is called “Attenuation”. Other photons
travel completely through the object without
interacting with any of the material's particles. The
number of photons transmitted through a material
depends on the thickness, density and atomic
number of the material, and the energy of the
individual photons.
Even when they have the same energy, photons travel different distances within a
material simply based on the probability of their encounter with one or more of the
particles of the matter and the type of
encounter that occurs. Since the probability of
an encounter increases with the distance
traveled, the number of photons reaching a
specific point within the matter decreases
exponentially with distance traveled. As shown
in the graphic to the right, if 1000 photons are
aimed at ten 1 cm layers of a material and
there is a 10% chance of a photon being
attenuated in this layer, then there will be 100
photons attenuated. This leaves 900 photos to
travel into the next layer where 10% of these
photos will be attenuated. By continuing this
progression, the exponential shape of the curve
becomes apparent.
The formula that describes this curve is:
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Where;

: initial (unattenuated) intensity
: linear attenuation coefficient per unit distance
: distance traveled through the matter

Linear and Mass Attenuation Coefficients
The “linear attenuation coefficient” ( ) describes the fraction of a beam of X-rays or
gamma rays that is absorbed or scattered per unit thickness of the absorber (10% per
cm thickness for the previous example).
Using the transmitted intensity equation above, linear attenuation coefficients can be
used to make a number of calculations. These include:
 The intensity of the energy transmitted through a material when the incident X-
ray intensity, the material and the material thickness are known.
 The intensity of the incident X-ray energy when the transmitted X-ray intensity,
material, and material thickness are known.
 The thickness of the material when the incident and transmitted intensity, and the
material are known.
 The material can be determined from the value of when the incident and
transmitted intensity, and the material thickness are known.
Linear attenuation coefficients can sometimes be found in the literature. However, it is
often easier to locate attenuation data in terms of the “mass attenuation coefficient”.
Tables and graphs of the mass attenuation coefficients for chemical elements and for
several compounds and mixtures as a function of radiation energy (in keV) are
available in literature (such information can be found at the National Institute for
Standards and Technology website).
Since a linear attenuation coefficient is dependent on the density of a material, the
mass attenuation coefficient is often reported for convenience. Consider water for
example. The linear attenuation for water vapor is much lower than it is for ice
because the molecules are more spread out in vapor so the chance of a photon
encounter with a water particle is less. Normalizing by dividing it by the density of
the element or compound will produce a value that is constant for a particular element
or compound. This constant ( ) is known as the mass attenuation coefficient and
has units of cm
2
/gm. The mass attenuation coefficient can simply be converted to a
linear attenuation coefficient by multiplying it by the density ( ) of the material.
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Sometimes instead of specifying the
HVL, the Tenth Value Layer (TVL) is
specified. The TVL is the thickness
that attenuates 90% of the intensity
(only 10% passes through).
In that case, the equation becomes:








Half-Value Layer
The thickness of any given material where 50% of the incident energy has been
attenuated is known as the half-value layer (HVL). The HVL is expressed in units of
distance (mm or cm). Like the attenuation coefficient, it is photon energy dependant.
Increasing the penetrating energy of a stream of photons will result in an increase in a
material's HVL.
The HVL is inversely proportional to the attenuation coefficient. If an incident energy of
1 and a transmitted energy of 0.5 are plugged into the intensity attenuation equation
introduced earlier, solving for which will correspond to the HVL gives:







The HVL is often used in radiography simply because
it is easier to remember values and perform simple
calculations. In a shielding calculation, such as
illustrated to the right, it can be seen that if the
thickness of one HVL is known, it is possible to quickly
determine how much material is needed to reduce
the intensity to less than 1%.
In order to calculate the ratio of intensity attenuation
(or reduction) resulting from passing through a certain
thickness of a material for which the HVL is known,
the following equation may be used:









Where
is intensity reduction ratio.
Or alternatively, the equitation can be solved to find the
material thickness required for reducing the intensity to
a certain level as:





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Approximate HVL for various materials when radiation is from a gamma-ray source:
Half-Value Layer (mm)
Source

Concrete Steel Lead Tungsten Uranium
Iridium-192 44.5 12.7 4.8 3.3 2.8
Cobalt-60 60.5 21.6 12.5 7.9 6.9

Approximate HVL for some materials when radiation is from an X-ray source:


Half-Value Layer (mm)

X-ray Tube Voltage
(kV)

Lead Concrete
50 0.06 4.32
100 0.27 15.10
150 0.30 22.32
200 0.52 25.0
250 0.88 28.0
300 1.47 31.21
400 2.5 33.0
1000 7.9 44.45

Sources of Attenuation
The attenuation that results due to the interaction between penetrating radiation and
matter is not a simple process. A single interaction event between a primary X-ray
photon and a particle of matter does not usually result in the photon changing to some
other form of energy and effectively disappearing. Several interaction events are
usually involved and the total attenuation is the sum of the attenuation due to
different types of interactions. These interactions include the photoelectric effect,
scattering, and pair production.
 Photoelectric (PE) absorption of X-rays occurs when the X-ray photon is absorbed,
resulting in the ejection of electrons from the outer shell of the atom, and hence
the ionization of the atom. Subsequently, the ionized atom returns to the neutral
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state with the emission of an X-ray characteristic of the atom. This subsequent
emission of lower energy photons is generally
absorbed and does not contribute to (or hinder) the
image making process. Photoelectron absorption is
the dominant process for X-ray absorption up to
energies of about 500 keV. Photoelectric absorption
is also dominant for atoms of high atomic numbers.

 Compton scattering (C) occurs when the incident X-ray photon is deflected from its
original path by an interaction with an electron. The electron gains energy and is
ejected from its orbital position. The X-ray photon
loses energy due to the interaction but continues
to travel through the material along an altered
path. Since the scattered X-ray photon has less
energy, it, therefore, has a longer wavelength
than the incident photon.

 Pair production (PP) can occur when the X-ray photon energy is greater than 1.02
MeV, but really only becomes significant at energies around 10 MeV. Pair
production occurs when an electron and positron are created with the annihilation
of the X-ray photon. Positrons are very short lived and disappear (positron
annihilation) with the formation of two
photons of 0.51 MeV energy. Pair production
is of particular importance when high-energy
photons pass through materials of a high
atomic number.

EQUIPMENT & MATERIALS

X-ray Generators
The major components of an X-ray generator are the tube, the high voltage generator,
the control console, and the cooling system. As discussed earlier in this material, X-rays
are generated by directing a stream of high speed electrons at a target material such as
tungsten, which has a high atomic number. When the electrons are slowed or stopped
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by the interaction with the atomic particles of the target, X-radiation is produced. This
is accomplished in an X-ray tube such as the one shown in the figure.
The tube cathode (filament) is heated with a low-voltage current of a few amps. The
filament heats up and the electrons in the wire become loosely held. A large electrical
potential is created between the cathode and the anode by the high-voltage
generator. Electrons that break free of the cathode are strongly attracted to the anode
target. The stream of electrons between the cathode and the anode is the tube
current. The tube current is measured in
milliamps and is controlled by regulating
the low-voltage heating current applied to
the cathode. The higher the temperature
of the filament, the larger the number of
electrons that leave the cathode and travel
to the anode. The milliamp or current
setting on the control console regulates
the filament temperature, which relates to
the intensity of the X-ray output.
The high-voltage between the cathode and the anode affects
the speed at which the electrons travel and strike the anode.
The higher the kilovoltage, the more speed and, therefore,
energy the electrons have when they strike the anode.
Electrons striking with more energy results in X-rays with
more penetrating power. The high-voltage potential is
measured in kilovolts, and this is controlled with the voltage
or kilovoltage control on the control console. An increase in
the kilovoltage will also result in an increase in the intensity of
the radiation. The figure shows the spectrum of the radiated
X-rays associated with the voltage and current settings. The
top figure shows that increasing the kV increases both the
energy of X-rays and also increases the intensity of radiation
(number of photons). Increasing the current, on the other
hand, only increases the intensity without shifting the spectrum.
A focusing cup is used to concentrate the stream of electrons to a small area of the
target called the “focal spot”. The focal spot size is an important factor in the system's
ability to produce a sharp image. Much of the energy applied to the tube is
transformed into heat at the focal spot of the anode. As mentioned above, the anode
target is commonly made from tungsten, which has a high melting point in addition to
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a high atomic number. However, cooling of the anode by active or passive means is
necessary. Water or oil re-circulating systems are often used to cool tubes. Some low
power tubes are cooled simply with the use of thermally conductive materials and heat
radiating fins.
In order to prevent the cathode from burning up and to prevent arcing between the
anode and the cathode, all of the oxygen is removed from the tube by pulling a
vacuum. Some systems have external vacuum pumps to remove any oxygen that may
have leaked into the tube. However, most industrial X-ray tubes simply require a
warm-up procedure to be followed. This warm-up procedure carefully raises the tube
current and voltage to slowly burn any of the available oxygen before the tube is
operated at high power.
In addition, X-ray generators usually have a filter along the beam path (placed at or
near the x-ray port). Filters consist of a thin sheet of material (often high atomic
number materials such as lead, copper, or brass) placed in the useful beam to modify
the spatial distribution of the beam. Filtration is required to absorb the lower-energy
X-ray photons emitted by the tube before they reach the target in order to produce a
cleaner image (since lower energy X-ray photons tend to scatter more).
The other important component of an X-ray generating
system is the control console. Consoles typically have a
keyed lock to prevent unauthorized use of the system.
They will have a button to start the generation of X-rays
and a button to manually stop the generation of X-rays.
The three main adjustable controls regulate the tube
voltage in kilovolts, the tube amperage in milliamps, and
the exposure time in minutes and seconds. Some systems
also have a switch to change the focal spot size of the tube.

Radio Isotope (Gamma-ray) Sources
Manmade radioactive sources are produced by introducing an extra neutron to atoms
of the source material. As the material gets rid of the neutron, energy is released in the
form of gamma rays. Two of the most common industrial gamma-ray sources for
industrial radiography are Iridium-192 and Cobalt-60. In comparison to an X-ray
generator, Cobalt-60 produces energies comparable to a 1.25 MV X-ray system and
Iridium-192 to a 460 kV X-ray system. These high energies make it possible to
penetrate thick materials with a relatively short exposure time. This and the fact that
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sources are very portable are the main reasons that gamma sources are widely used
for field radiography. Of course, the disadvantage of a radioactive source is that it can
never be turned off and safely managing the source is a constant responsibility.
Physical size of isotope materials varies between manufacturers, but generally an
isotope material is a pellet that measures 1.5 mm x 1.5 mm. Depending on the level of
activity desired, a pellet or pellets are
loaded into a stainless steel capsule
and sealed by welding. The capsule is
attached to short flexible cable called
a pigtail.
The source capsule and the pigtail are housed in a
shielding device referred to as a exposure device or
camera. Depleted uranium is often used as a
shielding material for sources. The exposure device
for Iridium-192 and Cobalt-60 sources will contain
22 kg and 225 kg of shielding materials,
respectively. Cobalt cameras are often fixed to a
trailer and transported to and from inspection sites.
When the source is not being used to make an exposure, it is locked inside the
exposure device.
To make a radiographic exposure, a crank-out mechanism and a guide tube are
attached to opposite ends of the exposure device. The guide tube often has a
collimator (usually made of tungsten) at the
end to shield the radiation except in the
direction necessary to make the exposure.
The end of the guide tube is secured in the
location where the radiation source needs to
be to produce the radiograph. The crank-out
cable is stretched as far as possible to put as
much distance as possible between the exposure device and the radiographer. To
make the exposure, the radiographer quickly cranks the source out of the exposure
device and into position in the collimator at the end of the guide tube. At the end of
the exposure time, the source is cranked back into the exposure device. There is a
series of safety procedures, which include several radiation surveys, that must be
accomplished when making an exposure with a gamma source.
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Radiographic Film
X-ray films for general radiography basically consist of an emulsion-gelatin containing
radiation-sensitive silver halide crystals (such as silver bromide or silver chloride). The
emulsion is usually coated on both sides of a flexible, transparent, blue-tinted base in
layers about 0.012 mm thick. An adhesive undercoat fastens the emulsion to the film
base and a very thin but tough coating covers the emulsion to protect it against minor
abrasion. The typical total thickness of the X-ray film is approximately 0.23 mm.
Though films are made to be sensitive for X-ray or gamma-ray, yet they are also
sensitive to visible light. When X-rays,
gamma-rays, or light strike the film, some
of the halogen atoms are liberated from
the silver halide crystal and thus leaving
the silver atoms alone. This change is of
such a small nature that it cannot be
detected by ordinary physical methods
and is called a “latent (hidden) image”.
When the film is exposed to a chemical
solution (developer) the reaction results
in the formation of black, metallic silver.

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Film Selection
Selecting the proper film and developing the optimal radiographic technique for a
particular component depends on a number of different factors;
 Composition, shape, and size of the part being examined and, in some cases, its
weight and location.
 Type of radiation used, whether X-rays from an X-ray generator or gamma rays
from a radioactive source.
 Kilovoltage available with the X-ray equipment or the intensity of the gamma
radiation.
 Relative importance of high radiographic detail or quick and economical results.

Film Packaging
Radiographic film can be purchased in a number of different packaging options and
they are available in a variety of sizes. The most basic form is as individual sheets in a
box. In preparation for use, each sheet must be loaded into a cassette or film holder in
a darkroom to protect it from exposure to light.
Industrial X-ray films are also available in a form in which each sheet is enclosed in a
light-tight envelope. The film can be exposed from either side without removing it
from the protective packaging. A rip strip makes it easy to remove the film in the
darkroom for processing.
Packaged film is also available in the form of rolls where that allows the radiographer
to cut the film to any length. The ends of the packaging are sealed with electrical tape
in the darkroom. In applications such as the radiography of circumferential welds and
the examination of long joints on an aircraft fuselage, long lengths of film offer great
economic advantage.

Film Handling
X-ray film should always be handled carefully to avoid physical strains, such as
pressure, creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible
holders and external clamping devices are used, care should be taken to be sure
pressure is uniform. Marks resulting from contact with fingers that are moist or
contaminated with processing chemicals, as well as crimp marks, are avoided if large
films are always grasped by the edges and allowed to hang free. Use of envelope-
packed films avoids many of these problems until the envelope is opened for
processing.
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RADIOGRAPHY CONSIDERATIONS & TECHNIQUES

Radiographic Sensitivity
The usual objective in radiography is to produce an image showing the highest amount
of detail possible. This requires careful control of a number of different variables that
can affect image quality. Radiographic sensitivity is a measure of the quality of an
image in terms of the smallest detail or discontinuity that may be detected.
Radiographic sensitivity is dependant on the contrast and the definition of the image.
Radiographic contrast is the degree of density (darkness)
difference between two areas on a radiograph. Contrast makes
it easier to distinguish features of interest, such as defects, from
the surrounding area. The image to the right shows two
radiographs of the same stepwedge. The upper radiograph has a
high level of contrast and the lower radiograph has a lower level
of contrast. While they are both imaging the same change in
thickness, the high contrast image uses a larger change in
radiographic density to show this change. In each of the two
radiographs, there is a small dot, which is of equal density in
both radiographs. It is much easier to see in the high contrast
radiograph.
Radiographic definition is the abruptness of change in going from one area of a given
radiographic density to another. Like contrast, definition also makes it easier to see
features of interest, such as defects, but in a totally different
way. In the image to the right, the upper radiograph has a high
level of definition and the lower radiograph has a lower level of
definition. In the high definition radiograph it can be seen that
a change in the thickness of the stepwedge translates to an
abrupt change in radiographic density. It can be seen that the
details, particularly the small dot, are much easier to see in the
high definition radiograph. It can be said that a faithful visual
reproduction of the stepwedge was produced. In the lower
image, the radiographic setup did not produce a faithful visual
reproduction. The edge line between the steps is blurred. This
is evidenced by the gradual transition between the high and
low density areas on the radiograph.
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Radiographic “Image” Density
After taking a radiographic image of a part and processing the film, the resulting
darkness of the film will vary according to the amount of radiation that has reached
the film through the test object. As mentioned earlier, the darker areas indicate more
exposure and liter areas indicate less exposure. The processed film (or image) is usually
viewed by placing it in front of a screen providing white light illumination of uniform
intensity such that the light is transmitted through the film such that the image can be
clearly seen. The term “radiographic density” is a measure of the degree of film
darkening (darkness of the image). Technically it should be called “transmitted density”
when associated with transparent-base film since it is a measure of the light
transmitted through the film. Radiographic density is the logarithm of two
measurements: the intensity of light incident on the film (
) and the intensity of light
transmitted through the film (
). This ratio is the inverse of transmittance.






Similar to the decibel, using the log of the ratio allows ratios of significantly different
sizes to be described using easy to work with numbers. The following table shows
numeric examples of the relationship between the amount of transmitted light and the
calculated film density.
Transmittance
(It/I0)
Transmittance (%) Inverse of Transmittance
(I0/It)
Density
(Log(I0/It))
1.0 100% 1 0
0.1 10% 10 1
0.01 1% 100 2
0.001 0.1% 1000 3
0.0001 0.01% 10000 4

From the table, it can be seen that a density reading of 2.0 is the result of only one
percent of the incident light making it through the film. At a density of 4.0 only 0.01%
of transmitted light reaches the far side of the film. Industrial codes and standards
typically require a radiograph to have a density between 2.0 and 4.0 for acceptable
viewing with common film viewers. Above 4.0, extremely bright viewing lights is
necessary for evaluation.
Film density is measured with a densitometer which simply measures the amount of
light transmitted through a piece of film using a photovoltic sensor.
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Secondary (Scatter) Radiation Control
Secondary or scatter radiation must often be taken into
consideration when producing a radiograph. The scattered
photons create a loss of contrast and definition. Often, secondary
radiation is thought of as radiation striking the film reflected from
an object in the immediate area, such as a wall, or from the table
or floor where the part is resting.
Control of side scatter can be achieved by moving objects in the
room away from the film, moving the X-ray tube to the center of
the vault, or placing a collimator at the exit port, thus reducing
the diverging radiation surrounding the central beam.
When scarered radiation comes from objects behind the film, it is often called
“backscatter”. Industry codes and standards often require
that a lead letter “B” be placed on the back of the cassette
to verify the control of backscatter. If the letter “B” shows
as a “ghost” image on the film, a significant amount of
backscatter radiation is reaching the film. The image of the
“B” is often very nondistinct as shown in the image to the
right. The arrow points to the area of backscatter radiation
from the lead “B” located on the back side of the film.
The control of backscatter radiation is achieved by backing the film in the cassette with
a sheet of lead that is at least 0.25 mm thick such that the sheet will be behind the film
when it is exposed. It is a common practice in industry to place thin sheets of lead
(called “lead screens”) in front and behind the film (0.125 mm thick in front and 0.25
mm thick behind).

Radiographic Contrast
As mentioned previously, radiographic contrast describes the differences in
photographic density in a radiograph. The contrast between different parts of the
image is what forms the image and the greater the contrast, the more visible features
become. Radiographic contrast has two main contributors; subject contrast and film
(or detector) contrast.

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Subject Contrast
Subject contrast is the ratio of radiation intensities transmitted through different areas
of the component being evaluated. It is dependant on the absorption differences in the
component, the wavelength of the primary radiation, and intensity and distribution of
secondary radiation due to scattering.
It should be no surprise that absorption differences within the subject will affect the
level of contrast in a radiograph. The larger the difference in thickness or density
between two areas of the subject, the larger the difference in radiographic density or
contrast. However, it is also possible to radiograph a
particular subject and produce two radiographs
having entirely different contrast levels. Generating
X-rays using a low kilovoltage will generally result in
a radiograph with high contrast. This occurs because
low energy radiation is more easily attenuated.
Therefore, the ratio of photons that are transmitted
through a thick and thin area will be greater with
low energy radiation.
There is a tradeoff, however. Generally, as contrast sensitivity increases, the latitude of
the radiograph decreases. Radiographic latitude refers to the range of material
thickness that can be imaged. This means that more areas of different thicknesses will
be visible in the image. Therefore, the goal is to balance radiographic contrast and
latitude so that there is enough contrast to identify the features of interest but also to
make sure the latitude is great enough so that all areas of interest can be inspected
with one radiograph. In thick parts with a large range of thicknesses, multiple
radiographs will likely be necessary to get the necessary density levels in all areas.

Film Contrast
Film contrast refers to density differences that result due to the type of film being
used, how it was exposed, and how it was processed. Since there are other detectors
besides film, this could be called detector contrast, but the focus here will be on film.
Exposing a film to produce higher film densities will generally increase the contrast in
the radiograph.
A typical film characteristic curve, which shows how a film responds to different
amounts of radiation exposure, is shown in the figue. From the shape of the curves, it
can be seen that when the film has not seen many photon interactions (which will
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result in a low film density) the slope of the curve is low. In this region of the curve, it
takes a large change in exposure to produce a small change in film density. Therefore,
the sensitivity of the film is relatively low. It can be
seen that changing the log of the relative exposure
from 0.75 to 1.4 only changes the film density from
0.20 to about 0.30. However, at film densities above
2.0, the slope of the characteristic curve for most
films is at its maximum. In this region of the curve, a
relatively small change in exposure will result in a
relatively large change in film density. For example,
changing the log of relative exposure from 2.4 to 2.6
would change the film density from 1.75 to 2.75.
Therefore, the sensitivity of the film is high in this
region of the curve. In general, the highest overall
film density that can be conveniently viewed or
digitized will have the highest level of contrast and
contain the most useful information.
As mentioned previously, thin lead sheets (called “lead screens”) are typically placed
on both sides of the radiographic film during the exposure (the film is placed between
the lead screens and inserted inside the cassette). Lead screens in the thickness range
of 0.1 to 0.4 mm typically reduce scatter radiation at energy levels below 150 kV.
Above this energy level, they will emit electrons to provide more exposure of the film,
thus increasing the density and contrast of the radiograph.
Other type of screens called “fluorescent screens” can alternatively be used where
they produce visible light when exposed to radiation and this light further exposes the
film and increases density and contrast.

Radiographic Definition
As mentioned previously, radiographic definition is the abruptness of change from one
density to another. Both geometric factors of the equipment and the radiographic
setup, and film and screen factors have an effect on definition.

Geometric Factors
The loss of definition resulting from geometric factors of the radiographic equipment
and setup is refered to as “geometric unsharpness”. It occurs because the radiation
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does not originate from a single point but rather over an area. The three factors
controlling unsharpness are source size, source to object distance, and object to
detector (film) distance. The effects of these three factors on image defenetion is
illustrated by the images below (source size effect; compare A & B, source to object
distance; compare B & D, and object to detector distance; compare B & C).

The source size is obtained by referencing manufacturers specifications for a given X-
ray or gamma ray source. Industrial X-ray tubes often have focal spot sizes of 1.5 mm
squared but microfocus systems have spot sizes in the 30 micron range. As the source
size decreases, the geometric unsharpness also decreases. For a given size source, the
unsharpness can also be decreased by increasing the source to object distance, but this
comes with a reduction in radiation intensity. The object to detector distance is usually
kept as small as possible to help minimize unsharpness. However, there are situations,
such as when using geometric enlargement, when the object is separated from the
detector, which will reduce the definition.
In general, in order to produce the highest level of definition, the focal-spot or source
size should be as close to a point source as possible, the source-to-object distance
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should be as large as practical, and the object-to-detector distance should be a small as
practical.
Codes and standards used in industrial radiography require that geometric
unsharpness be limited. In general, the allowable amount is 1/100 of the material
thickness up to a maximum of 1 mm. These values refer to the width of penumbra
shadow in a radiographic image.
The amount of geometric unsharpness (
) can be
calculated using the following geometric formula:






Where;

: source focal-spot size
: distance from the source to the front surface
of the object
: distance from the front surface of the object
to the detector (or the thickness of the object
if a thick object is placed immediately on top of the detector)

The angle between the radiation and some features will
also have an effect on definition. If the radiation is parallel
to an edge or linear discontinuity, a sharp distinct
boundary will be seen in the image. However, if the
radiation is not parallel with the discontinuity, the feature
will appear distorted, out of position and less defined in
the image.
Abrupt changes in thickness and/or density will appear more defined in a radiograph
than will areas of gradual change. For example, consider a circle. Its largest dimension
will be a cord that passes through its centerline. As the cord is moved away from the
centerline, the thickness gradually decreases. It is sometimes difficult to locate the
edge of a void due to this gradual change in thickness.
Lastly, any movement of the specimen, source or detector during the exposure will
reduce definition. Similar to photography, any movement will result in blurring of the
image. Vibration from nearby equipment may be an issue in some inspection
situations.
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Film and Screen Factors
The last set of factors concern the film and the use of fluorescent screens. A fine grain
film is capable of producing an image with a higher level of definition than is a coarse
grain film. Wavelength of the radiation will influence apparent graininess. As the
wavelength shortens and penetration increases, the apparent graininess of the film will
increase. Also, increased development of the film will increase the apparent graininess
of the radiograph.
The use of fluorescent screens also results in lower definition. This occurs for a couple
of different reasons. The reason that fluorescent screens are sometimes used is
because incident radiation causes them to give off light that helps to expose the film.
However, the light they produce spreads in all directions, exposing the film in adjacent
areas, as well as in the areas which are in direct contact with the incident radiation.
Fluorescent screens also produce screen mottle on radiographs. Screen mottle is
associated with the statistical variation in the numbers of photons that interact with
the screen from one area to the next.

Film Characteristic Curves
In film radiography, the number of photons reaching the film determines how dense
the film will become when other factors such as the developing time are held constant.
The number of photons reaching the film is a function of the intensity of the radiation
and the time that the film is exposed to the radiation. The term used to describe the
control of the number of photons reaching the film is “exposure”.
Different types of radiographic films respond differently to a given amount of
exposure. Film manufacturers commonly characterize their film to determine the
relationship between the applied exposure and
the resulting film density. This relationship
commonly varies over a range of film densities,
so the data is presented in the form of a curve
such as the one for Kodak AA400 shown to the
right. This plot is usually called a film
characteristic curve or density curve. A log scale
is sometimes used for the x-axis or it is more
common that the values are reported in log
units on a linear scale as seen in the figure. Also,
relative exposure values (unitless) are often
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used. Relative exposure is the ratio of two exposures. For example, if one film is
exposed at 100 kV for 6 mA.min and a second film is exposed at the same energy for 3
mA.min, then the relative exposure would be 2.
The location of the characteristic curves of different films along the x-axis relates to the
speed of the film. The farther to the right that a curve is on the chart, the slower the
film speed (Film A has the highest speed while film C has the lowest speed). The shape
of the characteristic curve is largely independent of the wavelength of the X-ray or
gamma ray, but the location of the curve along the x-axis, with respect to the curve of
another film, does depend on radiation quality.
Film characteristic curves can be used to adjust the exposure used to produce a
radiograph with a certain density to an exposure that will produce a second radiograph
of higher or lower film density. The curves can also be used to relate the exposure
produced with one type of film to exposure needed to produce a radiograph of the
same density with a second type of film.
Example 1: Adjusting the Exposure to Produce a Different Film Density
A type B Film was exposed with 140 kV at 1 mA for 10 seconds (i.e., 10 mA.s) and
the resulting radiograph had a density of 1.0. If the desired density is 2.5, what
should be the exposure?
From the graph, the log of the relative exposure of a density of 1.0 is 1.62 and the log
of the relative exposure when the density of the film is 2.5 is 2.12.
The difference between the two values is 0.5.
10
0.5
= 3.16
Therefore, the exposure used to produce the initial
radiograph with a 1.0 density needs to be multiplied
by 3.16 to produce a radiograph with the desired
density of 2.5.
So the new exposure must be:
10 mA.s x 3.16 = 31.6 mA.s (at 140 kV)


Example 2: Adjusting the Exposure to Allow Use of a Different Film Type
Suppose an acceptable radiograph with a density of 2.5 was produced by exposing
Film A for 30 seconds at 1mA and 130 kV. What should be the exposure if we want
to produce the same density using Film B?
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From the graph, the log of the relative exposure that
produced a density of 2.5 on Film A is 1.82 and the
log of the relative exposure that produces the same
density on Film B is 2.12.
The difference between the two values is 0.3.
10
0.3
= 2
So the exposure for Film B must be:
30 mA.s x 2 = 60 mA.s (at 130 kV)

Exposure Calculations
Properly exposing a radiograph is often a trial and error process, as there are many
variables that affect the final radiograph. Some of the variables that affect the density
of the radiograph include:
 The spectrum of radiation produced by the X-ray generator.
 The voltage potential used to generate the X-rays (kV).
 The amperage used to generate the X-rays (mA).
 The exposure time.
 The distance between the radiation source and the film.
 The material of the component being radiographed.
 The thickness of the material that the radiation must travel through.
 The amount of scattered radiation reaching the film.
 The film being used.
 The use of lead screens or fluorescent screens.
 The concentration of the film processing chemicals and the contact time.
The current industrial practice is to develop a procedure that produces an acceptable
density by trail for each specific X-ray generator. This process may begin using
published exposure charts to determine a starting exposure, which usually requires
some refinement.
However, it is possible to calculate the density of a radiograph to an acceptable degree
of accuracy when the spectrum of an X-ray generator has been characterized. The
calculation cannot completely account for scattering but, otherwise, the relationship
between many of the variables and their effect on film density is known. Therefore,
the change in film density can be estimated for any given variable change. For
example, from Newton's Inverse Square Law, it is known that the intensity of the
radiation varies inversely with the square of the distance from the source. It is also
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known that the intensity of the radiation transmitted through a material varies
exponentially with the linear attenuation coefficient and the thickness of the material.
By calculating the intensity from these equations one can directly calculate the
required exposure knowing that the exposure is inversely related to the intensity as:





The figure below shows exemplary exposure charts for two materials for a specific X-
ray generator for the flowing parameters: film density of 2.0 without screens, 910 mm
source-to-film distance, Industrex AA film & 7 minutes development time.

For gamma-ray sources, however, the required exposure can be more easily calculated
since the radiation spectrum is well known for each different radiation source. The
exposure is usually expressed in
Curie-Time units and the data
can be represented in the form
of chars or in tabulated form.
The figure shows a typical
exposure chart for Ir-192 at the
following parameters: film
density of 1.75 without screens,
455 mm source-to-film
distance, II-ford film & 6
minutes development time.
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It should be noted that such charts are valid for the specified parameters, but of course
using the data in the charts one can calculate the exposure for different set of
parameters such as different source-to-film distance, different type of film, or different
density.

To make such calculations more easy, radiographic modeling calculators and programs
can be used. A number of such
programs are available from
different sources and some are
available online. These programs
can provide a fair representation
of the radiograph that will be
produce with a specific setup and
parameters. The figure shows a
screen shot of an online calculator
available at the (www.ndt-ed.org)
website.


Example 1:
A 25 mm thick Aluminum plate is to be radiographed on type C film without screens
using X-ray generator at 80 kV and 500 mm distance. What is the minimum required
exposure time to get 3.0 density (for same development parameters as used for the
chart, and considering the film used for the chart to be type A) knowing that the max
current setting for the X-ray machine is 20 mA?
Answer: 190 s
Example 2:
A 12.5 mm thick Steel plate is to be radiographed without screens using Ir-192 source
at 455 mm distance. Knowing that the source activity was 100 Ci before 30 days, what
is the required exposure time (for same density, film type, and development parameters
as used for the chart) if the plate is to be place behind a 50 mm thick concrete wall
while it is being exposed?
Answer: 104 s

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Controlling Radiographic Quality
One of the methods of controlling the quality of a radiograph is through the use of
image quality indicators (IQIs), which are also referred to as penetrameters. IQIs
provide means of visually informing the film interpreter of the contrast sensitivity and
definition of the radiograph. The IQI indicates that a specified amount of change in
material thickness will be detectable in the radiograph, and that the radiograph has a
certain level of definition so that the density changes are not lost due to unsharpness.
Without such a reference point, consistency and quality could not be maintained and
defects could go undetected.
IQIs should be placed on the source side of the part over a section with a material
thickness equivalent to the region of interest. If this is not possible, the IQI may be
placed on a block of similar material and thickness to the region of interest. When a
block is used, the IQI should be the same distance from the film as it would be if placed
directly on the part in the region of interest. The IQI
should also be placed slightly away from the edge of
the part so that at least three of its edges are visible
in the radiograph.
Image quality indicators take many shapes and forms due to the various codes or
standards that invoke their use. The two most commonly used IQI types are: the hole-
type and the wire IQIs. IQIs come in a variety of material types so that one with
radiation absorption characteristics similar to the material being radiographed can be
used.

Hole-Type IQIs
ASTM Standard E1025 gives detailed requirements for the design and material group
classification of hole-type image quality indicators. Hole-type IQIs are classified in eight
groups based on their radiation absorption characteristics.
A notching system is used to indicate the IQI material. The
numbers on the IQI indicate the sample thickness that the
IQI would typically be placed on. Also, holes of different
sizes are present where these holes should be visible on
the radiograph. It should be noted however that the IQI is
used to indicate the quality of the radiographic technique
and not intended to be used as a measure of the size of a
cavity that can be located on the radiograph.
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Wire IQIs
ASTM Standard E747 covers the radiographic examination of
materials using wire IQIs to control image quality. Wire IQIs consist
of a set of six wires arranged in order of increasing diameter and
encapsulated between two sheets of clear plastic. Wire IQIs are
grouped in four sets each having different range of wire diameters.
The set letter (A, B, C or D) is shown in the lower right corner of
the IQI. The number in the lower left corner indicates the material
group.

Film Processing
As mentioned previously, radiographic film consists of a transparent, blue-tinted base
coated on both sides with an emulsion. The emulsion consists of gelatin containing
microscopic, radiation sensitive silver halide crystals, such as silver bromide and silver
chloride. When X-rays, gamma rays or light rays strike the crystals or grains, some of
the Br- ions are liberated leaving the Ag+ ions. In this condition, the radiograph is said
to contain a latent (hidden) image because the change in the grains is virtually
undetectable, but the exposed grains are now more sensitive to reaction with the
developer.
When the film is processed, it is exposed to several different chemical solutions for
controlled periods of time. Film processing basically involves the following five steps:
Development: The developing agent gives up electrons to convert the silver halide
grains to metallic silver. Grains that have been exposed to the radiation develop
more rapidly, but given enough time the developer will convert all the silver ions
into silver metal. Proper temperature control is needed to convert exposed grains
to pure silver while keeping unexposed grains as silver halide crystals.
Stopping the development: The stop bath simply stops the development process by
diluting and washing the developer away with water.
Fixing: Unexposed silver halide crystals are removed by the fixing bath. The fixer
dissolves only silver halide crystals, leaving the silver metal behind.
Washing: The film is washed with water to remove all the processing chemicals.
Drying: The film is dried for viewing.
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Film processing is a strict science governed by rigid rules of chemical concentration,
temperature, time, and physical movement. Whether processing is done by hand or
automatically by machine, excellent radiographs require a high degree of consistency
and quality control.

Viewing Radiographs
After the film processing, radiographs are viewed
using a light-box (or they can be digitized and
viewed on a high resolution monitor) in order to be
interpreted. In addition to providing diffused,
adjustable white illumination of uniform intensity,
specialized industrial radiography light-boxes
include magnifying and masking aids. When
handing the radiographs, thin cotton gloves should
be worn to prevent fingerprints on the radiographs.













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RADIATION SAFETY

Radiation Health Risks
As mentioned previously, the health risks associated with the radiation is considered to
be one the major disadvantages of radiogaphy. The amount of risk depends on the
amount of radiation dose received, the time over which the dose is received, and the
body parts exposed. The fact that X-ray and gamma-ray radiation are not detectable by
the human senses complicates matters further. However, the risks can be minimized
and controlled when the radiation is handled and managed properly in accordance to
the radiation safety rules. The active laws all over the world require that individuals
working in the field of radiography receive training on the safe handling and use of
radioactive materials and radiation producing devices.
Today, it can be said that radiation ranks among the most thoroughly investigated (and
somehow understood) causes of disease. The primary risk from occupational radiation
exposure is an increased risk of cancer. Although scientists assume low-level radiation
exposure increases one's risk of cancer, medical studies have not demonstrated
adverse health effects in individuals exposed to small chronic radiation doses.
The occurrence of particular health effects from exposure to ionizing radiation is a
complicated function of numerous factors including:
 Type of radiation involved. All kinds of ionizing radiation can produce health effects.
The main difference in the ability of alpha and beta particles and gamma and X-rays
to cause health effects is the amount of energy they have. Their energy determines
how far they can penetrate into tissue and how much energy they are able to
transmit directly or indirectly to tissues.
 Size of dose received. The higher the dose of radiation received, the higher the
likelihood of health effects.
 Rate at which the dose is received. Tissue can receive larger dosages over a period
of time. If the dosage occurs over a number of days or weeks, the results are often
not as serious if a similar dose was received in a matter of minutes.
 Part of the body exposed. Extremities such as the hands or feet are able to receive a
greater amount of radiation with less resulting damage than blood forming organs
housed in the upper body.
 The age of the individual. As a person ages, cell division slows and the body is less
sensitive to the effects of ionizing radiation. Once cell division has slowed, the
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effects of radiation are somewhat less damaging than when cells were rapidly
dividing.
 Biological differences. Some individuals are more sensitive to radiation than others.
Studies have not been able to conclusively determine the cause of such differences.

Sources of High Energy Radiation
There are many sources of harmful, high energy radiation. Industrial radiographers are
mainly concerned with exposure from X-ray generators and radioactive isotopes.
However, it is important to understand
that eighty percent of human exposure
comes from natural sources such as
radon gas, outer space, rocks and soil,
and the human body. The remaining
twenty percent comes from man-made
radiation sources, such as those used
in medical and dental diagnostic
procedures.
One source of natural radiation is cosmic radiation. The earth and all living things on it
are constantly being bombarded by radiation from space. The sun and stars emit
electromagnetic radiation of all wavelengths. The dose from cosmic radiation varies in
different parts of the world due to differences in elevation and the effects of the
earth’s magnetic field. Radioactive materials are also found throughout nature where
they occur naturally in soil, water, plants and animals. The major isotopes of concern
for terrestrial radiation are uranium and the decay products of uranium, such as
thorium, radium, and radon. Low levels of uranium, thorium, and their decay products
are found everywhere. Some of these materials are ingested with food and water,
while others, such as radon, are inhaled. The dose from terrestrial sources varies in
different parts of the world. Locations with higher concentrations of uranium and
thorium in their soil have higher dose levels. All people also have radioactive isotopes,
such as potassium-40 and carbon-14, inside their bodies. The variation in dose from
one person to another is not as great as the variation in dose from cosmic and
terrestrial sources.
There are also a number of manmade radiation sources that present some exposure to
the public. Some of these sources include tobacco, television sets, smoke detectors,
combustible fuels, certain building materials, nuclear fuel for energy production,
nuclear weapons, medical and dental X-rays, nuclear medicine, X-ray security systems
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and industrial radiography. By far, the most significant source of man-made radiation
exposure to the average person is from medical procedures, such as diagnostic X-rays,
nuclear medicine, and radiation therapy.

Measures Relative to the Biological Effects of Radiation Exposure
There are four measures of radiation that radiographers will commonly encounter
when addressing the biological effects of working with X-rays or gamma-rays. These
measures are: Exposure, Dose, Dose Equivalent, and Dose Rate. A short description of
these measures and their units is given below
Exposure: Exposure is a measure of the strength of a radiation field at some point in
air (the amount of charge produced in a unit mass of air when the interacting photons
are completely absorbed in that mass). This is the measure made by radiation survey
meters since it can be easily and directly measured. The most commonly used unit of
exposure is the “roentgen” (R).
Dose or Absorbed Dose: While exposure is defined for air, the absorbed dose is the
amount of energy that ionizing radiation imparts to a given mass of matter. In other
words, the dose is the amount of radiation absorbed by and object. The SI unit for
absorbed dose is the “gray” (Gy), but the “rad” (Radiation Absorbed Dose) is
commonly used (1 Gy = 100 rad). Different materials that receive the same exposure
may not absorb the same amount of radiation. In human tissue, one Roentgen of X-ray
or gamma radiation exposure results in about one rad of absorbed dose. The size of
the absorbed dose is dependent upon the intensity (or activity) of the radiation source,
the distance from the source, and the time of exposure to radiation.
Dose Equivalent: The dose equivalent relates the absorbed dose to the biological
effect of that dose. The absorbed dose of specific types of radiation is multiplied by a
“quality factor” to arrive at the dose equivalent. The SI unit is the “Sievert” (Sv), but the
“rem” (Roentgen Equivalent in Man) is commonly used (1 Sv = 100 rem). The table
below presents the “Q factors” for several types of radiation.
Type of Radiation Rad Q Factor Rem
X-Ray 1 1 1
Gamma Ray 1 1 1
Beta Particles 1 1 1
Thermal Neutrons 1 5 5
Fast Neutrons 1 10 10
Alpha Particles 1 20 20
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Dose Rate: The dose rate is a measure of how fast a radiation dose is being received.
Dose rate is usually presented in terms of mR/hr, mrem/hr, rad/min, mGy/sec, etc.
Knowing the dose rate, allows the dose to be calculated for a period of time.

Controlling Radiation Exposure
When working with radiation, there is a concern for two types of exposure: acute and
chronic. An acute exposure is a single accidental exposure to a high dose of radiation
during a short period of time. An acute exposure has the potential for producing both
non-stochastic and stochastic effects. Chronic exposure, which is also sometimes called
“continuous exposure”, is long-term, low level overexposure. Chronic exposure may
result in stochastic health effects and is likely to be the result of improper or
inadequate protective measures.
The three basic ways of controlling exposure to harmful radiation are: 1) limiting the
time spent near a source of radiation, 2) increasing the distance away from the source,
3) and using shielding to stop or reduce the level of radiation.

Time
The radiation dose is directly proportional to the time spent in the radiation.
Therefore, a person should not stay near a source of radiation any longer than
necessary. If a survey meter reads 4 mR/h at a particular location, a total dose of 4 mR
will be received if a person remains at that location for one hour. The
received dose can be simply calculated as: Dose = Dose Rate x Time
When using a gamma camera, it is important to get the source from the
shielded camera to the collimator (a device that shields radiation in some
directions but allow it pass in one or more other directions) as quickly as
possible to limit the time of exposure to the unshielded source.
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Distance
Increasing distance from the source of radiation will reduce
the amount of radiation received. As radiation travels from
the source, it spreads out becoming less intense. This
phenomenon can be expressed by the Newton inverse
square law, which states that as the radiation travels out
from the source, the dosage decreases inversely with the
square of the distance: I1 / I2 = D2
2
/ D1
2


Shielding
The third way to reduce exposure to radiation is to place something between the
radiographer and the source of radiation. In general, the more
dense the material the more shielding it will provide. Lead and
concrete are the most commonly used radiation shielding materials
primarily because they are easy to work with and are readily
available materials. Concrete is commonly used in the construction
of radiation vaults. Some vaults will also be lined with lead sheeting
to help reduce the radiation to acceptable levels on the outside.

Exposure Limits
Over the years, numerous recommendations regarding occupational exposure limits
have been developed by international radiation safety commissions. In general, the
guidelines established for radiation exposure have had two principal objectives: 1) to
prevent acute exposure; and 2) to limit chronic exposure to “acceptable” levels.
Current guidelines are based on the conservative assumption that there is no safe level
of exposure. This assumption has led to the general philosophy of not only keeping
exposures below recommended levels or regulation limits but also maintaining all
exposure “as low as reasonably achievable” (ALARA). ALARA is a basic requirement of
current radiation safety practices. It means that every reasonable effort must be made
to keep the dose to workers and the public as far below the required limits as possible.
In general, most international radiation safety codes specify that the dose rate must
not exceed 2mR/hour in any unrestricted area. The specifications for the accumulated
dose per year differ between radiation workers and non-workers. The limits are as
follows:
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Regulatory Limits for Occupational Exposure
Most international codes set the annual limit of exposure for industrial radiographers
who generally are not concerned with an intake of
radioactive material as follows:
1) the more limiting of:
 A total effective dose equivalent of 5 rem
(0.05 Sv)
or
 The sum of the deep-dose equivalent to
any individual organ or tissue other than
the lens of the eye being equal to 50 rem
(0.5 Sv).

2) The annual limits to the lens of the eye, to
the skin, and to the extremities, which are:
 A lens dose equivalent of 15 rem (0.15 Sv)
 A shallow-dose equivalent of 50 rem (0.50
Sv) to the skin or to any extremity.

The shallow-dose equivalent is the external dose to the skin of the whole-body or
extremities from an external source of ionizing radiation. This value is the dose
equivalent at a tissue depth of 0.007 cm averaged over an area of 10 cm
2
.
The lens dose equivalent is the dose equivalent to the lens of the eye from an
external source of ionizing radiation. This value is the dose equivalent at a tissue
depth of 0.3 cm.
The deep-dose equivalent is the whole-body dose from an external source of ionizing
radiation. This value is the dose equivalent at a tissue depth of 1 cm.
The total effective dose equivalent is the dose equivalent to the whole-body.

Declared Pregnant Workers and Minors
Because of the increased health risks to the rapidly developing embryo and fetus,
pregnant women can receive no more than 0.5 rem during the entire gestation period
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(this is 10% of the dose limit that normally applies to radiation workers). The same limit
also applies to persons under the age of 18 years.

Non-radiation Workers and the General Public
The dose limit to non-radiation workers and members of the public is only 2% of the
annual occupational dose limit. Therefore, a non-radiation worker can receive a whole
body dose of no more that 0.1 rem/year from industrial ionizing radiation. This
exposure would be in addition to the 0.3 rem/year from natural background radiation
and the 0.05 rem/year from man-made sources such as medical X-rays.

Over-Dose Health Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of
radiation when the doses are received by an individual within a twenty-four hour
period.
 0-25 rem No injury evident. First detectable blood change at 5 rem.
 25-50 rem Definite blood change at 25 rem. No serious injury.
 50-100 rem Some injury possible.
 100-200 rem Injury and possible disability.
 200-400 rem Injury and disability likely, death possible.
 400-500 rem Median Lethal Dose (MLD) 50% of exposures are fatal.
 500-1,000 rem Up to 100% of exposures are fatal.
 Over 1,000 rem 100% likely fatal.
The delayed effects of radiation may be due either to a single large overexposure or
continuing low-level overexposure.
Example dosages and resulting symptoms when an individual receives an exposure to
the whole body within a twenty-four hour period.
100 - 200 rem
First Day No definite symptoms
First Week No definite symptoms
Second Week No definite symptoms
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Third Week Loss of appetite, malaise, sore throat and diarrhea
Fourth Week Recovery is likely in a few months unless complications develop
because of poor health
400 - 500 rem
First Day Nausea, vomiting and diarrhea, usually in the first few hours
First Week Symptoms may continue
Second Week Epilation, loss of appetite
Third Week Hemorrhage, nosebleeds, inflammation of mouth and throat,
diarrhea, emaciation
Fourth Week Rapid emaciation and mortality rate around 50%

Radiation Detectors
Instruments used for radiation measurement fall into two
broad categories:
 Rate measuring instruments.
 Personal dose measuring instruments.
Rate measuring instruments measure the rate at which
exposure is received (more commonly called the radiation
intensity). Survey meters, audible alarms and area monitors
fall into this category. These instruments present a radiation
intensity reading relative to time, such as R/hr or mR/hr. An analogy can be made
between these instruments and the speedometer of a car because both are measuring
units relative to time.
Dose measuring instruments are those that measure the total amount of exposure
received during a measuring period. The dose measuring instruments, or dosimeters,
that are commonly used in industrial radiography are small devices which are designed
to be worn by an individual to measure the exposure received by the individual. An
analogy can be made between these instruments and the odometer of a car because
both are measuring accumulated units.

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Survey Meters
The survey meter is the most important resource a radiographer has
to determine the presence and intensity of radiation. There are many
different models of survey meters available to measure radiation in
the field. They all basically consist of a detector and a readout
display. Analog and digital displays are available. Most of the survey
meters used for industrial radiography use a gas filled detector.
Gas filled detectors consists of a gas filled cylinder with two electrodes having a voltage
applied to them. Whenever the device is brought near radioactive substances, the gas
becomes ionized. The electric field created by the potential difference between the
anode and cathode causes the electrons of each ion pair to move to the anode while
the positively charged gas atom is drawn to the cathode. This results in an electrical
signal that is amplified, correlated to exposure and displayed as a value.

Audible Alarm Rate Meters
Audible alarms are devices that emit a short "beep" or "chirp" when a
predetermined exposure has been received. It is required that these
electronic devices be worn by an individual working with gamma
emitters. These devices reduce the likelihood of accidental exposures in
industrial radiography by alerting the radiographer to exposure levels or
dosages of radiation above a preset amount. It is important to note that
audible alarms are not intended to be and should not be used as
replacements for survey meters. Modern survey meters have this alarm
feature already built in.
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or
her exposure to X-rays or gamma rays. As the name implies, they are commonly worn
in the pocket. The principal advantage of a pocket dosimeter is its ability to provide the
wearer an immediate reading of his or her radiation exposure. It also has the
advantage of being reusable. The limited range, inability to provide a permanent
record, and the potential for discharging and reading loss due to dropping or bumping
are a few of the main disadvantages of a pocket dosimeter.
The two types commonly used in industrial radiography are the Direct Read Pocket
Dosimeter and the Digital Electronic Dosimeter.
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Direct Read Pocket Dosimeter
A direct reading pocket ionization dosimeter is generally of the size and shape of a
fountain pen. The accumulated dose value can be read by pointing the instrument at a
light source and observing the internal fiber
through a system of built-in lenses. The fiber is
viewed on a translucent scale which is graduated
in units of exposure. Typical industrial
radiography pocket dosimeters have a full scale
reading of 200 mR but there are designs that will record
higher amounts. During the shift, the dosimeter reading
should be checked frequently. The measured exposure
should be recorded at the end of each shift.
Digital Electronic Dosimeter
These dosimeters measure both dose information and dose rate and
display them in digital form. Also, some Digital Electronic Dosimeters
include an audible alarm feature which emits an audible signal or
chirp with each recorded increment of exposure. Consequently, the
frequency or chirp rate of the alarm is proportional to the radiation
intensity. Some models can also be set to provide a continuous
audible signal when a preset exposure has been reached.

Film Badges
Personnel dosimetry film badges are commonly used to
measure and record radiation exposure due to gamma rays, X-
rays and beta particles. The detector is, as the name implies, a
piece of radiation sensitive film. The film is packaged in a light
proof, vapor proof envelope preventing light, moisture or
chemical vapors from affecting the film. Film badges need to be
worn correctly so that the dose they receive accurately represents the dose the wearer
receives. Whole body badges are worn on the body between the neck and the waist,
often on the belt or a shirt pocket.
The film is contained inside a film holder or badge. The badge incorporates a series of
filters to determine the quality of the radiation. Radiation of a given energy is
attenuated to a different extent by various types of absorbers. Therefore, the same
quantity of radiation incident on the badge will produce a different degree of
darkening under each filter. By comparing these results, the energy of the radiation
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can be determined and the dose can be calculated knowing the
film response for that energy. The badge holder also contains an
open window to determine radiation exposure due to beta
particles (since beta particles are shielded by a thin amount of
material).
The major advantages of a film badge as a personnel monitoring device are that it
provides a permanent record, it is able to distinguish between different energies of
photons, and it can measure doses due to different types of radiation. It is quite
accurate for exposures greater than 100 mR. The major disadvantages are that it must
be developed and read by a processor (which is time consuming) and prolonged heat
exposure can affect the film.

Thermoluminescent Dosimeter (TLD)
Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a
film badge, it is worn for a period of time (usually 3 months or less) and then must be
processed to determine the dose received, if any. TLDs can measure
doses as low as 1 mR and they have a precision of approximately
15% for low doses which improves to approximately 3% for high
doses. TLDs are reusable, which is an advantage over film badges.
However, no permanent record or re-readability is provided and an
immediate, on the job readout is not possible.
A TLD has a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid
crystal structure. When a TLD it is exposed to ionizing radiation at ambient
temperatures, the radiation interacts with the phosphor crystal causing some of the
atoms in the material to produce free electrons and become ionized. The free
electrons are trapped and locked into place in the imperfections in the crystal lattice
structure.
Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons
in the process. Released electrons return to the original ground state, releasing the
captured energy from ionization as light, hence the name thermoluminescent. Instead
of reading the optical density (blackness) of a film, as is done with film badges, the
amount of light released versus the heating of the individual pieces of
thermoluminescent material is measured. The “glow curve” produced by this process is
then related to the radiation exposure. The process can be repeated many times.

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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the
resulting damage to the body is not immediately apparent, a variety of safety controls
are used to limit exposure. The two basic types of radiation safety controls used to
provide a safe working environment are engineered and administrative controls.
Engineered controls include shielding, interlocks, alarms, warning signals, and material
containment. Administrative controls include postings, procedures, dosimetry, and
training.
Engineered controls such as shielding and door interlocks are used to contain the
radiation in a cabinet or a “radiation vault”. Fixed shielding materials are commonly
high density concrete and/or lead. Door interlocks are used to immediately cut the
power to X-ray generating equipment if a door is accidentally
opened when X-rays are being produced. Warning lights are
used to alert workers and the public that radiation is being
used. Sensors and warning alarms are often used to signal that
a predetermined amount of radiation is present. Safety
controls should never be tampered with or bypassed.
When portable radiography is performed, most often it is not practical to place alarms
or warning lights in the exposure area. Ropes (or cordon off tape)
and signs are used to block the entrance to radiation areas and to
alert the public to the presence of radiation. Occasionally,
radiographers will use battery operated flashing lights to alert the
public to the presence of radiation.
Safety regulations classify the areas surrounding the location where ionizing radiation
is present into restricted areas and controlled areas according to the radiation intensity
level:
Restricted areas: Areas with a dose rate higher than 300 mR/h must be secure so that
nobody can enter this area. If anybody accidently enters this area, radiation must be
terminated and the person must be checked. Access is only permitted under specific
conditions and if there is an absolute need for it, the body dose should be calculated
and the personal dose measured.
Control areas: These are areas with dose rates which are equivalent to or higher than
0.75 mR/h. Control areas must be cordoned off and provided with a radiation warning
signs.
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255. Dr SP Tyagi
X-RAY FILM AND ACCESSORIES
Dr. S. P. Tyagi

X-ray film:
The X-ray film is the medium that record the image of part exposed with X-rays. The x-ray film
is somewhat similar to photographic film in its basic composition. However unlike photographic
film, the light (or radiation) sensitive emulsion is usually coated on both sides of the base of X-
ray film so that it can be used with intensifying screens.
The x-ray film is composed of following –
1: Film base: The central portion of the x-ray film is the base which supports the fragile
photographic emulsion on both of its surface. Ideally the base must be flexible as well as quite
strong so that the films can be repeatedly snapped into x-ray illuminators (Viewing boxes).
Secondly, it must withstand any geometric distortion due to the heat of the developing process
and finally, the base must provide a uniform, highly transparent, optical background.
Historically, photographic glass plates were used as the X-ray film base followed by
cellulose nitrate in early 1920’s. Later cellulose triacetate base was developed in 1924 to avoid
the highly flammable nature of cellulose nitrate. Finally, a stronger, thinner, more
dimensionally stable film base made of polyester was developed in 1960 and that has replaced
all above materials for making of film base.

2: Film Emulsion: The X-ray film emulsion is composed of a mixture of gelatin (derived from
cadaver bones) and small silver halide crystals (grains). The gelatin serves as a matrix which
keeps the silver halide grains well dispersed and prevents their clumping. The developing and
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256. Dr SP Tyagi fixing solutions can penetrate the gelatin very rapidly without changing the strength or
permanence of the gelatin. Small crystal grains of silver halide (1.0 to 1.5 microns in diameter)
comprise the light sensitive substance in the emulsion. These grains, known as silver-iodo-
bromide, are typically between 90 and 99% silver bromide and between 1 and 10% silver
iodide.
The atoms in the silver-iodo-bromide crystal are arranged in a cubic lattice and each
crystal contains many point defects, where a silver ion is displaced and is free to move through
the crystal. It is the mobility of these silver ions that contributes to the formation of the latent
image. In its pure form the silver halide crystal has low photographic sensitivity. The emulsion
is sensitized by heating it under controlled conditions with a reducing agent containing sulphur.
This result in the production of silver sulphide at a site on the surface of the crystal referred to
as a sensitivity speck. It is the sensitivity speck that traps electrons to begin formation of the
latent image centres.
In the process of film exposure, the energy from absorbing a photon of light is sufficient
to liberate an electron from a bromide ion in the crystal. The electron travels freely through the
crystal until it is trapped at a site of crystal imperfection such as a dislocation defect or a
sensitivity speck composed of an AgS molecule. A free silver ion is attracted to the negative
charge and combines with the charge (is reduced) to form an atom of metallic silver (which is
optically black). The single atom of silver acts as an electron trap for another electron and then
attracts another atom of silver which is then reduced to metallic silver. This process continues
while the exposure to light continues.
3: Adhesive layer: In general, the emulsion and the base do not adhere to each other. For this
reason, the emulsion must be attached to the film base using a thin layer of suitable adhesive
which is generally a clear thin layer of gelatin only.
4: Protective layer: To protect the emulsion, which would be easily scratched and damaged by
normal handling, a very thin outer protective layer is applied (again usually made of gelatin).
Types of X-ray films:
1. On the basis of photosensitive emulsion layers:
Single coated: In such type of x-ray films the photosensitive emulsion is coated only on
one surface of film base. These films are used with single intensifying screen cassette with the
film placed in front of the screen, i.e. on the side facing the X-ray tube. These are specific
purpose films used when higher spatial resolution of image is desired.
Double coated: These are routine purpose x-ray films having photosensitive coatings on
both sides of base and used with double screen cassette with the film sandwiched between the
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257. Dr SP Tyagi screens. Such films require lesser exposure factors and lesser processing times. For example
the image can be produced in 1/2 the time required to produce an image on the single sided
film.

2. On the basis of use with intensifying screens:
Screen films: These films are used along with intensifying screens and are therefore
ultimately exposed by light and not the X-rays. These films require lesser exposure factors and
processing time for development of radiographic image. The emulsion coating of such films is
also thinner. Such films are versatile and used for most general purpose diagnostic
radiography.

Non-screen films: These films are used without intensifying screens and require more
exposure factors and prolonged processing time for production of comparable radiographic
density to that of non-screen films. They have relatively thicker emulsion and therefore
radiographic image formed on such films have excellent details. Such films are used for specific
purposes such as detection of hail-line fracture or any subtle tissue change that remains
unrecognized in traditional routine radiograph.

3. On the basis of types of light sensitive emulsion coating:
Blue light sensitive films:
Green light sensitive Orthochromatic films:
Red light sensitive Panchromatic films:
The spectral sensitivity of the film must be matched to the emission spectrum of the
intensifying screen in order to increase the sensitivity of the system. The principle emission
from traditionally used calcium tungstate intensifying screens is blue light. Therefore, it is
imperative that the films to be used with such intensifying screens must be sensitive more
towards blue light. The photographic emulsion containing silver bromide is coincidently cream
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258. Dr SP Tyagi coloured that absorbs ultraviolet and blue light, but reflects green and red light and therefore
such films have been used without any problem with calcium tungustate intensifying screens.
However many rare earth intensifying screens principally emit greener lights and
therefore, x-ray films to be used with such screens should be made sensitive to greener
spectrum of light as well. For this, suitable dyes are added in their photosensitive emulsion of
the films. (Such green light sensitive orthochromatic films also require suitable change in x-ray
darkroom safe light colour and intensity). Now a day blue light emitting rare earth intensifying
screens are also available.
 “High lite” films from 3M company were more or less not sensitive to room light
(particularly yellow lights) and therefore, allowed all the procedures of dark room in a
yellow lighted room.
4. On the basis of film speed:
Film speed refers to the relative sensitivity of X-ray film to a given amount of radiation.
Faster films require lesser exposure but produce grainy images that lack definition. They
also have narrow film latitude. Speed wise x-ray films may be categorized as following-
Standard or par speed films
Fast speed films
Ultrafast films
Standard speed films are versatile as they have wide film latitude but require greater exposure.
Film Latitude: It refers to the range of exposure factors that produce diagnostically useful range
of radiographic densities.

Handling and storage care of unexposed and exposed x-ray films:
1. Films should be stored in a cool (10-20
0
c) and low humidity (40-60%) environment.
2. Film boxes should be kept vertically without any pressure on them.
3. Films should never be stored near a source of heat, irradiation or water.
4. Films should be loaded and unloaded from a cassette on a dry and clean bench inside
the dark room under a proper safe light.
5. Films should be handled delicately and any accidental splashing of processing solutions
should be avoided.
6. Films should not be used after their expiry period.
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259. Dr SP Tyagi 7. If an x-ray film has been exposed, the cassette should immediately be transferred to the
dark room or in a lead shielded box to avoid inadvertent subsequent exposures
particularly in cases where serial radiography is being done.
8. The wet processed film should be kept upright in a film drier for its drying.
9. The wet films should never be touched with fingers to avoid finger marks over films.

Intensifying screen:
These screens are fitted in x-ray cassettes and interact with x-rays to convert most of
their radiant energy (>95%) in to visible light thereby, exposing the x-ray film finally with light
(and not the x-rays). The amount of light emitted by the intensifying screen is proportional to
the amount of x-radiation passing through it.
Generally the x-ray films are more sensitive to light rays than the x-rays and therefore
the use of intensifying screens allow reduction in the exposure factors without affecting the
general quality of radiograph.
The intensifying screen typically has following components-
1. Base: Provides a strong, smooth, but flexible support for the fluorescent layer. This is
constructed usually from paper, cardboard or polyester with total thickness not exceeding
approximately 0.18 mm.
Ideal properties of an intensifying screen include: Chemically inert, moisture resistant,
no discolouring with age

2. Substratum: It is the bonding layer between the base & the phosphor layer. It may be
reflective, absorptive or transparent in nature.
3. Phosphor (Fluorescent) Layer: This is the “active” layer of the intensifying screen that
consists of fluorescent crystals, which emit light when struck by x-radiation. Examples of
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260. Dr SP Tyagi typical phosphor materials include calcium tungstate & rare earth phosphors. Earlier
barium lead sulphate and zinc cadmium sulphide were also used as phosphor materials.
The rare earth screens may have any of the following types of phosphor material-
Terbium activated gadolinium oxysulphide
Terbium activated lanthanum oxysulphide
Terbium activated yttrium oxysulphide
Thulium activated lanthanum oxybromide
X-ray absorption efficiency and their light conversion ratio of rare earth screens are far
superior to calcium tungustate type films. For example rare earth screen film combination
has 12 times faster speed than par speed tungustate screen film combination and
exposure is reduced by 15-50%.
4. Super-coat: This is a transparent external protective layer which helps in resisting surface
abrasion. It is constructed from cellulose acetate and has anti-static and waterproofing
qualities.
Fluorescence: It is a kind of luminescence where a cold (nonglowing) substance releases
electromagnetic radiation in the form of visible light while absorbing another form of energy,
but ceases to emit the radiation immediately upon the cessation of the input energy. The
emission of light from an intensifying screen during absorption of X-rays is one example of
fluorescence.
However, if the emission is delayed somewhat, it is called phosphorescence (after glow).

Film holders (cassettes):
The material in the cassette box must be as little absorbing as possible. Presently, the
best material for this is carbon fibre, giving a very rigid structure combined with low density
and a low atomic number.

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261. Radiography
Ref:

http://www.ge-mcs.com/download/x-ray/GEIT-30158EN_industrial-radiography-image-forming-techniques.pdf
http://onlineshowcase.tafensw.edu.au/ndt/content/radiographic/task8/accessible.htm
http://radiopaedia.org/articles/pair-production
http://en.wikipedia.org/wiki/Latent_image


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262. Structure of the X-ray film
•An X-ray film, total thickness
approx. 0.5 mm, is made up
of seven layers,(see figure)
•a transparent cellulose
triacetate or polyester base
(d). On both sides of this
base are applied:
•a layer of hardened gelatine
(a) to protect the emulsion
•emulsion layer (b) which is
suspended in gelatine,
sensitive to radiation
•a very thin layer called the
substratum (c) which bonds
the emulsion layer to the
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263. Radiographic image
Latent image
(A latent image is an invisible image produced by the exposure to light of a photosensitive
material such as photographic film.)(Ref: http://en.wikipedia.org/wiki/Latent_image)
When light or X-radiation strikes a sensitive emulsion, the portions
receiving a sufficient quantity of radiation undergo a change;
extremely small particles of silver halide crystals are converted into
metallic silver.
These traces of silver are so minute that the sensitive layer remains
to all appearances unchanged. The number of silver particles
produced is higher in the portions struck by a greater quantity of
radiation and less high where struck by a lesser quantity.
Developing the latent image
Development is the process by which a latent image is converted
into a visible image. This result is obtained by selective reduction
into black metallic silver of the silver halide crystals in the emulsion. Library Study Material
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264. Characteristics of the X-ray film
This is the science which studies the Photographic properties of a
film, and the methods enabling these to be measured.

Density (optical)
•When a photographic film is placed on an illuminated screen for
viewing, it will be observed that the image is made up of areas of
differing brightness, dependent on the local optical densities
(amount of silver particles) of the developed emulsion.
•Density (D) is defined as the logarithm to base 10 of the ratio of the incident light Io and the transmitted light through the film It,
therefore:
D = log (Io/ It) .
Density is measured by a densitometer.
Industrial radiography on conventional film covers a density range
from 0 to 4, a difference corresponding with a factor 10,000. Library Study Material
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265. Contrast
•The contrast of an image is defined as the
relative brightness between an image and the
adjacent background.
•The contrast between two densities D1 and
D2 on an X-ray film is the density difference
between them and is usually termed the
^radiographi ovtrast_. Library Study Material
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266. Characteristic curve (density curve)
•The characteristic or density curve indicates the
relationship between increasing exposures and
resulting density. By exposure (E) is meant the
radiation dose on the film emulsion. It is the
product of radiation intensity (Io) and exposure time
(t), therefore:
E = Io.t
The ratio between different exposures and related
densities is not usually plotted on a linear scale but
on a logarithmic scale; i.e. density D versus log E.
•Density (D) of a photographic emulsion does not
increase linearly with exposure (E) over the entire
density range, but has a shape as in figure. The
lower part of the curve (a- is alled the ^toe_, the
middle part (b- is alled the ^straight live ~livear
portiov_, avd the upper part ~-d) is called the
^shoulder_. Library Study Material
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267. Film speed (sensitivity)
In radiography the relationship between
exposure (in C/kg) and resulting density is
commonly referred to as film speed.
Graininess
When a developed X-ray film is viewed in detail
on an illuminated screen, minute density
variations are visible in a grainy sort of structure.
This ?isual iupressiov is alled ^graivivess_ avd a
measurement of this phenomenon establishes a
degree of ^gravularity_.
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268. Film interpretation
and reference radiographs
The common term for film interpretation is film viewing. Film
viewing in fact means the evaluation of the image quality of a
radiograph for compliance with the code requirements and the
interpretation of details of any possible defect visible on the film.
For this purpose, the film is placed in front of an illuminated screen
of appropriate brightness/luminance. The edges of the film and
areas of low density need to be masked to avoid glare.
The following conditions are important for good film interpretation:
• rightvess of the illuuivated sreev ~luuivave
• devsity of the radiograph
• diffusiov avd e?evvess of the illuuivated sreev
• auievt light iv the ?ie?ivg roou
• filu ?ie?er’s eye-sight
Poor viewing conditions may cause important defect information on
a radiograph to go unseen. Library Study Material
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269. •The ^lassi_ filu av e ?ie?ed
after photochemical treatment (wet
process) on a film viewing screen.
Defects or irregularities in the
object cause variations in film
density (brightness or
transparency). The parts of the
films which have received more
radiation during exposure – the
regions under cavities, for example
– appear darker, that is, the film
density is higher.
•Digital radiography gives the same
shades of black and white images,
but viewing and interpretation is
done on a computer screen (VDU). Library Study Material
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270. Library Study Material
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271. X-ray Videos

x-ray inspection of CAN:
https://www.youtube.com/watch?v=acljU7PoDS4
case study of x-ray
https://www.youtube.com/watch?v=ksv1JEOWlvw&index
=2&list=PLML1Ygt3lPBV0f9qL3sHn4qyAb9x5RAAY
what is x-ray machine:
https://www.youtube.com/watch?v=eLYLzBpprYM
having an x-ray
https://www.youtube.com/watch?v=dybncisweZE Library Study Material
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272. Introduction to Non-Destructive Testing Techniques
Ultrasonic Testing Page 1 of 36
Ultrasonic Testing
Ultrasonic Testing (UT) uses high frequency sound waves (typically in
the range between 0.5 and 15 MHz) to conduct examinations and
make measurements. Besides its wide use in engineering
applications (such as flaw detection/evaluation, dimensional
measurements, material characterization, etc.), ultrasonics are also
used in the medical field (such as sonography, therapeutic
ultrasound, etc.).
In general, ultrasonic testing is based on the capture and
quantification of either the reflected waves (pulse-echo) or the
transmitted waves (through-transmission). Each of the two types is
used in certain applications, but generally, pulse echo systems are
more useful since they require one-sided access to the object being inspected.

Basic Principles
A typical pulse-echo UT inspection
system consists of several functional
units, such as the pulser/receiver,
transducer, and a display device. A
pulser/receiver is an electronic
device that can produce high voltage
electrical pulses. Driven by the
pulser, the transducer generates
high frequency ultrasonic energy.
The sound energy is introduced and
propagates through the materials in
the form of waves. When there is a
discontinuity (such as a crack) in the wave path, part of the energy will be reflected
back from the flaw surface. The reflected wave signal is transformed into an electrical
signal by the transducer and is displayed on a screen. Knowing the velocity of the
waves, travel time can be directly related to the distance that the signal traveled. From
the signal, information about the reflector location, size, orientation and other
features can sometimes be gained.

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273. Introduction to Non-Destructive Testing Techniques
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Advantages and Disadvantages
The primary advantages and disadvantages when compared to other NDT methods
are:
Advantages
 It is sensitive to both surface and subsurface discontinuities.
 The depth of penetration for flaw detection or measurement is superior to other
NDT methods.
 Only single-sided access is needed when the pulse-echo technique is used.
 It is highly accurate in determining reflector position and estimating size and
shape.
 Minimal part preparation is required.
 It provides instantaneous results.
 Detailed images can be produced with automated systems.
 It is nonhazardous to operators or nearby personnel and does not affect the
material being tested.
 It has other uses, such as thickness measurement, in addition to flaw detection.
 Its equipment can be highly portable or highly automated.
Disadvantages
 Surface must be accessible to transmit ultrasound.
 Skill and training is more extensive than with some other methods.
 It normally requires a coupling medium to promote the transfer of sound energy
into the test specimen.
 Materials that are rough, irregular in shape, very small, exceptionally thin or not
homogeneous are difficult to inspect.
 Cast iron and other coarse grained materials are difficult to inspect due to low
sound transmission and high signal noise.
 Linear defects oriented parallel to the sound beam may go undetected.
 Reference standards are required for both equipment calibration and the
characterization of flaws.


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274. Introduction to Non-Destructive Testing Techniques
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PHYSICS OF ULTRASOUND
Wave Propagation
Ultrasonic testing is based on the vibration in materials which is generally referred to
as acoustics. All material substances are comprised of atoms, which may be forced into
vibrational motion about their equilibrium positions. Many different patterns of
vibrational motion exist at the atomic level; however, most are irrelevant to acoustics
and ultrasonic testing. Acoustics is focused on particles that contain many atoms that
move in harmony to produce a mechanical wave. When a material is not stressed in
tension or compression beyond its elastic limit, its individual particles perform elastic
oscillations. When the particles of a medium are displaced from their equilibrium
positions, internal restoration forces arise. These elastic restoring forces between
particles, combined with inertia of the particles, lead to the oscillatory motions of the
medium.
In solids, sound waves can propagate in four
principal modes that are based on the way
the particles oscillate. Sound can propagate
as longitudinal waves, shear waves, surface
waves, and in thin materials as plate waves.
Longitudinal and shear waves are the two
modes of propagation most widely used in
ultrasonic testing. The particle movement
responsible for the propagation of
longitudinal and shear waves is illustrated in
the figure.

 In longitudinal waves, the oscillations occur in the longitudinal
direction or the direction of wave propagation. Since
compression and expansion forces are active in these waves,
they are also called pressure or compression waves. They are
also sometimes called density waves because material density
fluctuates as the wave moves. Compression waves can be
generated in gases, liquids, as well as solids because the
energy travels through the atomic structure by a series of
compressions and expansion movements.

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275. Introduction to Non-Destructive Testing Techniques
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 In the transverse or shear waves, particles
oscillate at a right angle or transverse to the
direction of propagation. Shear waves require
an acoustically solid material for effective
propagation, and therefore, are not effectively
propagated in materials such as liquids or
gasses. Shear waves are relatively weak when
compared to longitudinal waves. In fact, shear
waves are usually generated in materials using
some of the energy from longitudinal waves.

Modes of Sound Wave Propagation
In air, sound travels by the compression and rarefaction of air molecules in the
direction of travel. However, in solids, molecules can support vibrations in other
directions. Hence, a number of different types of sound waves are possible. Waves can
be characterized by oscillatory patterns that are capable of maintaining their shape
and propagating in a stable manner. The propagation of waves is often described in
terms of what are called “wave modes”.
As mentioned previously, longitudinal and transverse (shear) waves are most often
used in ultrasonic inspection. However, at surfaces and interfaces, various types of
elliptical or complex vibrations of the particles make other waves possible. Some of
these wave modes such as Rayleigh and Lamb waves are also useful for ultrasonic
inspection.
Though there are many different modes of wave propagation, the table summarizes
the four types of waves that are commonly used in NDT.
Wave Type Particle Vibration
Longitudinal (Compression) Parallel to wave direction
Transverse (Shear) Perpendicular to wave direction
Surface - Rayleigh Elliptical orbit - symmetrical mode
Plate Wave - Lamb Component perpendicular to surface

Since longitudinal and transverse waves were discussed previously, surface and plate
waves are introduced here.
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276. Introduction to Non-Destructive Testing Techniques
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 Surface (or Rayleigh) waves travel at the surface of a relatively thick solid material
penetrating to a depth of one wavelength. A surface wave is a combination of
both a longitudinal and transverse motion which results in an elliptical motion as
shown in the image. The major axis of the ellipse is perpendicular to the surface of
the solid. As the depth of an individual atom from the
surface increases, the width of its elliptical motion
decreases. Surface waves are generated when a
longitudinal wave intersects a surface slightly larger
than the second critical angle and they travel at a
velocity between .87 and .95 of a shear wave.
Rayleigh waves are useful because they are very
sensitive to surface defects (and other surface
features) and they follow the surface around curves.
Because of this, Rayleigh waves can be used to inspect
areas that other waves might have difficulty reaching.

 Plate (or Lamb) waves are similar to surface waves except they can only be
generated in materials a few wavelengths thick (thin plates). Lamb waves are
complex vibrational waves that propagate parallel to the test surface throughout
the thickness of the material. They are influenced a great deal by the test wave
frequency and material thickness. Lamb waves are generated when a wave hits a
surface at an incident angle such that the parallel component of the velocity of
the wave (in the source) is equal to the velocity of the wave in the test material.
Lamb waves will travel several meters in steel and so are useful to scan plate,
wire, and tubes.
o With Lamb waves, a number of modes of particle vibration are possible, but
the two most common are symmetrical and asymmetrical. The complex
motion of the particles is similar to the elliptical orbits for surface waves.
Symmetrical Lamb waves move in a symmetrical
fashion about the median plane of the plate. This is
sometimes called the “extensional mode” because
the wave is stretching and compressing the plate in
the wave motion direction.
The asymmetrical Lamb wave mode is often called
the “flexural mode” because a large portion of the
motion is in a normal direction to the plate, and a
little motion occurs in the direction parallel to the plate. In this mode, the
body of the plate bends as the two surfaces move in the same direction.
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277. Introduction to Non-Destructive Testing Techniques
Ultrasonic Testing Page 6 of 36
Properties of Acoustic Waves
Among the properties of waves propagating in isotropic solid materials are
wavelength, frequency, and velocity. The wavelength is directly proportional to the
velocity of the wave and inversely proportional to the frequency of the wave. This
relationship is shown by the following equation:




Where;
: wavelength (m)
: velocity (m/s)
: frequency (Hz)

The velocity of sound waves in a certain medium is fixed where it is a characteristic of
that medium. As can be noted from the equation, an increase in frequency will result
in a decrease in wavelength. For instance, the velocity of longitudinal waves in steel is
5850 m/s and that results in a wavelength of 5.85 mm when the frequency is 1 MHz.

Wavelength and Defect Detection
In ultrasonic testing, the inspector must make a decision about the frequency of the
transducer that will be used in order to control the wavelength. The wavelength of the
ultrasound used has a significant effect on the probability of detecting a discontinuity.
A general rule of thumb is that a discontinuity must be larger than one-half the
wavelength to stand a reasonable chance of being detected.
Sensitivity and resolution are two terms that are often used in ultrasonic inspection to
describe a technique's ability to locate flaws. Sensitivity is the ability to locate small
discontinuities. Sensitivity generally increases with higher frequency (shorter
wavelengths). Resolution is the ability of the system to locate discontinuities that are
close together within the material or located near the part surface. Resolution also
generally increases as the frequency increases.
The wave frequency can also affect the capability of an inspection in adverse ways.
Therefore, selecting the optimal inspection frequency often involves maintaining a
balance between the favorable and unfavorable results of the selection. Before
selecting an inspection frequency, the material's grain structure and thickness, and the
discontinuity's type, size, and probable location should be considered. As frequency
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278. Introduction to Non-Destructive Testing Techniques
Ultrasonic Testing Page 7 of 36
increases, sound tends to scatter from large or course grain structure and from small
imperfections within a material. Cast materials often have coarse grains and thus
require lower frequencies to be used for evaluations of these products. Wrought and
forged products with directional and refined grain structure can usually be inspected
with higher frequency transducers.
Since more things in a material are likely to scatter a portion of the sound energy at
higher frequencies, the penetration depth (the maximum depth in a material that
flaws can be located) is also reduced. Frequency also has an effect on the shape of the
ultrasonic beam. Beam spread, or the divergence of the beam from the center axis of
the transducer, and how it is affected by frequency will be discussed later.
It should be mentioned, so as not to be misleading, that a number of other variables
will also affect the ability of ultrasound to locate defects. These include the pulse
length, type and voltage applied to the crystal, properties of the crystal, backing
material, transducer diameter, and the receiver circuitry of the instrument. These are
discussed in more detail in a later section.

Sound Propagation in Elastic Materials
It was mentioned previously that sound waves propagate due to the
vibrations or oscillatory motions of particles within a material. An
ultrasonic wave may be visualized as an infinite number of oscillating
masses or particles connected by means of elastic springs. Each
individual particle is influenced by the motion of its nearest neighbor
and both inertial and elastic restoring forces act upon each particle.
A mass on a spring has a single resonant frequency (natural frequency) determined by
its spring constant k and its mass m. Within the elastic limit of any material, there is a
linear relationship between the displacement of a particle and the
force attempting to restore the particle to its equilibrium position.
This linear dependency is described by Hooke's Law. In terms of the
spring model, the relation between force and displacement is written
as F = k x.

The Speed of Sound
Hooke's Law, when used along with Newton's Second Law, can explain a few things
about the speed of sound. The speed of sound within a material is a function of the
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279. Introduction to Non-Destructive Testing Techniques
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properties of the material and is independent of the amplitude of the sound wave.
Newton's Second Law says that the force applied to a particle will be balanced by the
particle's mass and the acceleration of the particle. Mathematically, Newton's Second
Law is written as F = m a. Hooke's Law then says that this force will be balanced by a
force in the opposite direction that is dependent on the amount of displacement and
the spring constant. Therefore, since the applied force and the restoring force are
equal, m a = k x can be written.
Since the mass m and the spring constant k are constants for any given material, it can
be seen that the acceleration a and the displacement x are the only variables. It can
also be seen that they are directly proportional. For instance, if the displacement of
the particle increases, so does its acceleration. It turns out that the time that it takes a
particle to move and return to its equilibrium position is independent of the force
applied. So, within a given material, sound always travels at the same speed no matter
how much force is applied when other variables, such as temperature, are held
constant.

Material Properties Affecting the Speed of Sound
Of course, sound does travel at different speeds in different materials. This is because
the mass of the atomic particles and the spring constants are different for different
materials. The mass of the particles is related to the density of the material, and the
spring constant is related to the elastic constants of a material. The general
relationship between the speed of sound in a solid and its density and elastic constants
is given by the following equation:





Where;
: speed of sound (m/s)

: elastic constant “in a given direction” (N/m
2
)
: density (kg/m
3
)

This equation may take a number of different forms depending on the type of wave
(longitudinal or shear) and which of the elastic constants that are used. It must also be
mentioned that the subscript “ ” attached to “ ” in the above equation is used to
indicate the directionality of the elastic constants with respect to the wave type and
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280. Introduction to Non-Destructive Testing Techniques
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direction of wave travel. In isotropic materials, the elastic constants are the same for
all directions within the material. However, most materials are anisotropic and the
elastic constants differ with each direction. For example, in a piece of rolled aluminum
plate, the grains are elongated in one direction and compressed in the others and the
elastic constants for the longitudinal direction differs slightly from those for the
transverse or short transverse directions.
For longitudinal waves, the speed of sound in a solid material can be found as:





Where;

: speed of sound for longitudinal waves (m/s)
: Young’s modulus (N/m
2
)
: Poisson’s ratio

While for shear (transverse) waves, the speed of sound is found as:





Where;

: speed of sound for shear waves (m/s)
: Shear modulus of elasticity (N/m
2
);

From the above equations, it can be found that longitudinal waves travel faster than
shear waves (longitudinal waves are approximately twice as fast as shear waves). The
table below gives examples of the compressional and shear sound velocities in some
metals.
Material Compressional velocity


Shear velocity


Aluminum 6320 3130
Steel (1020) 5890 3240
Cast iron 4800 2400
Copper 4660 2330
Titanium 6070 3310
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Attenuation of Sound Waves
When sound travels through a medium, its intensity diminishes with distance. In
idealized materials, sound pressure (signal amplitude) is reduced due to the spreading
of the wave. In natural materials, however, the sound amplitude is further weakened
due to the scattering and absorption. Scattering is the reflection of the sound in
directions other than its original direction of propagation. Absorption
is the conversion of the sound energy to other forms of energy. The
combined effect of scattering and absorption is called attenuation.
Attenuation is generally proportional to the square of sound frequency.
The amplitude change of a decaying plane wave can be expressed as:




Where;

: initial (unattenuated) amplitude
: attenuation coefficient (Np/m)
: traveled distance (m)





























The Decibel (dB) is a logarithmic unit that describes a ratio of two measurements. The difference
between two measurements X1 and X2 is described in decibels as:
The intensity of sound waves (I) is quantified by measuring the variation in sound pressure using a
transducer, and then the pressure is transferred to a voltage signal. Since the intensity of sound
waves is proportional to the square of the pressure amplitude, the ratio of sound intensity in
decibels can be expressed as:
where;
: the change in sound intensity between two measurements


: are the two different transducer output voltages (or readings)
Use of dB units allows ratios of various sizes to be described using easy to work with numbers.

Np (Neper) is a logarithmic dimensionless
quantity and it can be converted to Decibels
by dividing it by 0.1151.
Decibel is a more common unit when
relating the amplitudes of two signals.
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Attenuation can be determined by evaluating the multiple back-wall reflections seen in
a typical A-scan display (like the one shown in the image in the previous page). The
number of decibels between two adjacent signals is measured and this value is divided
by the time interval between them. This calculation produces an attenuation
coefficient in decibels per unit time. Then knowing the velocity of sound it can be
converted to decibels per unit length.

Acoustic Impedance
Sound travels through materials under the influence of sound pressure. Because
molecules or atoms of a solid are bound elastically to one another, the excess pressure
results in a wave propagating through the solid.
The acoustic impedance ( ) of a material is defined as the product of its density ( ) and
the velocity of sound in that material ( ).

Where;
: acoustic impedance (kg/m
2
s) or (N s/m
3
)
: density (kg/m
3
)
: sound velocity (m/s)

The table gives examples of the acoustic impedances for some materials:

Aluminum Copper Steel Titanium
Water
(20°C)
Air
(20°C)
Acou. Imp.
(kg/m
2
s)
17.1 x 10
6
41.6 x 10
6
46.1 x 10
6
28 x 10
6
1.48 x 10
6
413

Acoustic impedance is important in:
 the determination of acoustic transmission and reflection at the boundary of two
materials having different acoustic impedances.
 the design of ultrasonic transducers.
 assessing absorption of sound in a medium.

Reflection and Transmission Coefficients
Ultrasonic waves are reflected at boundaries where there is a difference in acoustic
impedances ( ) of the materials on each side of the boundary. This difference in is
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commonly referred to as the impedance mismatch. The greater the impedance
mismatch, the greater the percentage of energy that will be reflected at the interface
or boundary between one medium and another.
The fraction of the incident wave intensity that is reflected can be derived based on
the fact that particle velocity and local particle pressures must be continuous across
the boundary. When the acoustic impedances of the materials on both sides of the
boundary are known, the fraction of the incident wave intensity that is reflected (the
reflection coefficient) can be calculated as:







Where

are the acoustic impedances of the two materials at the interface.

Since the amount of reflected energy plus the transmitted energy must equal the total
amount of incident energy, the “transmission coefficient” is calculated by simply
subtracting the reflection coefficient from one ( ).
Taking for example a water steel interface and calculating the reflection and
transmission coefficients (using the acoustic impedance information given in the
previous table), we get = 0.88 and = 0.12. This means that the amount of energy
transmitted into the second material is only 12% while 88% is reflected back at the
interface. If we convert the amounts of reflection and transmission to decibels, we find
that to be -1.1 dB and -18.4 dB respectively. The negative sign indicates that
individually, the amount of reflected and transmitted energy is smaller than the
incident energy.
If reflection and transmission at interfaces is followed through the
component, only a small percentage of the original energy makes it
back to the transducer, even when loss by attenuation is ignored.
For example, consider an immersion inspection of a steel block.
The sound energy leaves the transducer, travels through the water,
encounters the front surface of the steel, encounters the back
surface of the steel and reflects back through the front surface on
its way back to the transducer. At the water steel interface (front
surface), 12% of the energy is transmitted. At the back surface,
88% of the 12% that made it through the front surface is reflected.
This is 10.6% of the intensity of the initial incident wave. As the
wave exits the part back through the front surface, only 12% of
Multiplying the reflection coefficient by
100 yields the amount of energy reflected
as a percentage of the original energy.
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10.6 or 1.3% of the original energy is transmitted back to the transducer.
Note that in such calculation the attenuation of the signal as it travels through the
material is not considered. Should it be considered, the amount of signal received back
by the transducer would be even smaller.
Q: What portion of the signal will be reflected at an Air-Steel interface?
A: 99.996%

Refraction and Snell's Law
When an ultrasonic wave passes through an interface between two materials at an
oblique angle, and the materials have different indices of refraction, both reflected and
refracted waves are produced. This also occurs with light, which is why objects seen
across an interface appear to be shifted relative to where they really are. For example,
if you look straight down at an object at the bottom of a glass of water, it looks closer
than it really is.
Refraction takes place at an interface of two materials due to
the difference in acoustic velocities between the two materials.
The figure shows the case where plane sound waves traveling
in one material enters a second material that has a higher
acoustic velocity. When the wave encounters the interface
between these two materials, the portion of the wave in the
second material is moving faster than the portion of the wave
that is still in the first material. As a result, this causes the wave
to bend and change its direction (this is referred to as
“refraction”).
Snell's Law describes the relationship between the angles and
the velocities of the waves. Snell's law equates the ratio of
material velocities to the ratio of the sine's of incident and
refracted angles, as shown in the following equation:












Where;
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: the longitudinal wave velocities in the first and second materials
respectively


: the angles of incident and refracted waves respectively

Note that in the diagram, there is a reflected longitudinal wave (


) shown. This wave
is reflected at the same angle as the incident wave because the two waves are
traveling in the same material, and hence have the same velocities. This reflected wave
is unimportant in our explanation of Snell's Law, but it should be remembered that
some of the wave energy is reflected at the interface.

Mode Conversion
When sound travels in a solid material, one form of wave energy can be transformed
into another form. For example, when a longitudinal wave hits an interface at an angle,
some of the energy can cause particle movement in
the transverse direction to start a shear wave. Mode
conversion occurs when a wave encounters an
interface between materials of different acoustic
impedances and the incident angle is not normal to
the interface. It should be noted that mode
conversion occurs “every time” a wave encounters
an interface at an angle. This mode conversion
occurs for both the portion of the wave that passes
through the interface and the portion that reflects
off the interface.
In the previous section, it was pointed out that when sound waves pass through an
interface between materials having different acoustic velocities, refraction takes place
at the interface. The larger the difference in acoustic velocities between the two
materials, the more the sound is refracted. However, the converted shear wave is not
refracted as much as the longitudinal wave because shear waves travel slower than
longitudinal waves. Therefore, the velocity difference between the incident
longitudinal wave and the shear wave is not as great as it is between the incident and
refracted longitudinal waves. Also note that when a longitudinal wave is reflected
inside the material, the reflected shear wave is reflected at a smaller angle than the
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reflected longitudinal wave. This is also due to the fact that the shear velocity is less
than the longitudinal velocity within a given material.
Snell's Law holds true for shear waves as well as longitudinal waves and can be written
as follows:
























Where;






: the longitudinal wave velocities in the
first and second materials respectively






: the shear wave velocities in the first
and second materials respectively


: the angles of incident and refracted
longitudinal waves respectively


: the angles of the converted reflected
and refracted shear waves respectively

Critical Angles
When a longitudinal wave moves from a slower to a faster material (and thus the wave
is refracted), there is an incident angle that makes the angle of refraction for the
“longitudinal wave” to become 90°. This is angle is known as “the first critical angle”.
The first critical angle can be found from Snell's law by putting in an angle of 90° for
the angle of the refracted ray. At the critical angle of incidence, much of the acoustic
energy is in the form of an inhomogeneous compression wave, which travels along the
interface and decays exponentially with depth from the interface. This wave is
sometimes referred to as a "creep wave". Because of their inhomogeneous nature and
the fact that they decay rapidly, creep waves are not used as extensively as Rayleigh
surface waves in NDT.
When the incident angle is equal or greater than the first critical angle, only the mode
converted shear wave propagates into the material. For this reason, most angle beam
transducers use a shear wave so that the signal is not complicated by having two
waves present.
In many cases there is also an incident angle that makes the angle of refraction for the
“shear wave” to become 90°. This is known as the “second critical angle” and at this
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point, all of the wave energy is reflected or refracted into a surface following shear
wave or shear creep wave. Slightly beyond the second critical angle, surface (Rayleigh)
waves will be generated.
The incident angle for angle-beam transducers is somewhere between the first and
second critical angles such that a shear wave, at a desired angle, is introduced into the
material being inspected.
The figure shows the mode of waves introduced into a steel surface as a function of
the incident angle of the wave generated by the transducer. It can be seen from the
figure that the incident angle for angle
beam (shear) transducers ranges
between 30° to 55°. But it is important
to remember that, due to refraction,
the angle of the shear wave inside the
material is completely different than
the incident angle.

Wave Interaction or Interference
The understanding of the interaction or interference of waves is important for
understanding the performance of an ultrasonic transducer. When sound emanates
from an ultrasonic transducer, it does not originate from a single point, but instead
originates from many points along the surface of the piezoelectric element. This results
in a sound field with many waves interacting or interfering with each other.
When waves interact, they superimpose on each other, and the amplitude of the
sound pressure at any point of interaction is the sum of the amplitudes of the two
individual waves. First, let's consider two identical waves that originate from the same
point. When they are in phase (so that the peaks and valleys of one are exactly aligned
with those of the other), they combine to double the pressure of either wave acting
alone. When they are completely
out of phase (so that the peaks of
one wave are exactly aligned with
the valleys of the other wave),
they combine to cancel each
other out. When the two waves
are not completely in phase or
out of phase, the resulting wave
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is the sum of the wave amplitudes for all points along the wave.
When the origins of the two interacting waves are not the
same, it is a little harder to picture the wave interaction, but
the principles are the same. Up until now, we have primarily
looked at waves in the form of a 2D plot of wave amplitude
versus wave position. However, anyone that has dropped
something in a pool of water can picture the waves radiating
out from the source with a circular wave front. If two objects
are dropped a short distance apart into the pool of water,
their waves will radiate out from their sources and interact
with each other. At every point where the waves interact, the amplitude of the particle
displacement is the combined sum of the amplitudes of the particle displacement of
the individual waves.
As stated previously, sound waves originate from multiple points along the face of the
transducer. The image shows what the sound field would look like if the waves
originated from just three points (of course there are more than three points of origin
along the face of a transducer). It can be seen that where the waves interact near the
face of the transducer and as a result there are extensive fluctuations and the sound
field is very uneven. In ultrasonic testing, this is
known as the “near field” or Fresnel zone. The sound
field is more uniform away from the transducer in
the “far field” or Fraunhofer zone. At some distance
from the face of the transducer and central to the
face of the transducer, a uniform and intense wave
field develops.
Wave Diffraction
Diffraction involves a change in direction of waves as they
pass through an opening or around a barrier in their path.
Diffraction of sound waves is commonly observed; we notice
sound diffracting around corners or through door openings,
allowing us to hear others who are speaking to us from adjacent rooms.
In ultrasonic testing of solids, diffraction patterns are usually generated
at the edges of sharp reflectors (or discontinuities) such as cracks.
Usually the tip of a crack behaves as point source spreading waves in all
directions due to the diffraction of the incident wave.
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EQUIPMENT & TRANSDUCERS

Piezoelectric Transducers
The conversion of electrical pulses to mechanical vibrations and the conversion of
returned mechanical vibrations back into electrical energy is the basis for ultrasonic
testing. This conversion is done by the transducer using a piece of piezoelectric
material (a polarized material having some parts of the molecule positively charged,
while other parts of the molecule are negatively charged) with electrodes attached to
two of its opposite faces. When an electric field is applied across the material, the
polarized molecules will align themselves with the
electric field causing the material to change
dimensions. In addition, a permanently-polarized
material such as quartz (SiO2) or barium titanate
(BaTiO3) will produce an electric field when the
material changes dimensions as a result of an
imposed mechanical force. This phenomenon is
known as the piezoelectric effect.
The active element of most acoustic transducers used today is a
piezoelectric ceramic, which can be cut in various ways to produce
different wave modes. A large piezoelectric ceramic element can
be seen in the image of a sectioned low frequency transducer. The
most commonly employed ceramic for making transducers is lead
zirconate titanate.
The thickness of the active element is determined by the desired frequency of the
transducer. A thin wafer element vibrates with a wavelength that is twice its thickness.
Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated
wavelength. The higher the frequency of the transducer, the thinner the active
element.

Characteristics of Piezoelectric Transducers
The function of the transducer is to convert electrical signals into mechanical vibrations
(transmit mode) and mechanical vibrations into electrical signals (receive mode). Many
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factors, including material, mechanical and electrical construction, and the external
mechanical and electrical load conditions, influence the behavior of a transducer.

A cut away of a typical contact transducer is shown in the figure. To get as much
energy out of the transducer as possible, an impedance matching layer is placed
between the active element and the face of the transducer. Optimal impedance
matching is achieved by sizing the matching layer so that its thickness is 1/4 of the
desired wavelength. This keeps waves that are reflected within the matching layer in
phase when they exit the layer. For contact transducers, the matching layer is made
from a material that has an acoustical impedance between the active element and
steel. Immersion transducers have a matching layer with an acoustical impedance
between the active element and water. Contact transducers also incorporate a wear
plate to protect the matching layer and active element from scratching.
The backing material supporting the crystal has a great influence on the damping
characteristics of a transducer. Using a backing material with an impedance similar to
that of the active element will produce the most effective damping. Such a transducer
will have a wider bandwidth resulting in higher sensitivity and higher resolution (i.e.,
the ability to locate defects near the surface or in close proximity in the material). As
the mismatch in impedance between the active element and the backing material
increases, material penetration increases but transducer sensitivity is reduced.
The bandwidth refers to the range of frequencies associated with a transducer. The
frequency noted on a transducer is the central frequency and depends primarily on the
backing material. Highly damped transducers will respond to frequencies above and
below the central frequency. The broad frequency range provides a transducer with
high resolving power. Less damped transducers will exhibit a narrower frequency
range and poorer resolving power, but greater penetration.
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The central frequency will also define the capabilities of a transducer. Lower
frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material,
while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but
greater sensitivity to small discontinuities.

Radiated Fields of Ultrasonic Transducers
The sound that emanates from a piezoelectric transducer does not originate from a
point, but instead originates from most of the surface of the piezoelectric element. The
sound field from a typical piezoelectric transducer is shown in the figure where lighter
colors indicating higher intensity. Since the ultrasound originates from a number of
points along the transducer face, the ultrasound intensity along the beam is affected
by constructive and destructive wave
interference as discussed previously. This
wave interference leads to extensive
fluctuations in the sound intensity near
the source and is known as the “near
field”. Because of acoustic variations
within a near field, it can be extremely
difficult to accurately evaluate flaws in
materials when they are positioned
within this area.
The pressure waves combine to form a relatively uniform front at the end of the near
field. The area beyond the near field where the ultrasonic beam is more uniform is
called the “far field”. The transition between the near field and the far field occurs at a
distance, , and is sometimes referred to as the "natural focus" of a flat (or unfocused)
transducer. Spherical or cylindrical focusing changes the structure of a transducer field
by "pulling" the point nearer the transducer. The area just beyond the near field is
where the sound wave is well behaved and at its maximum strength. Therefore,
optimal detection results will be obtained when flaws occur in this area.
For a round transducer (often referred to as piston source transducer), the near field
distance can be found as:






Where;
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: transducer diameter, : transducer frequency, and : sound longitudinal
velocity in the medium through which waves are transmitted.

Transducer Beam Spread
As the sound waves exits the near field and propegate through the material, the sound
beam continiously spreads out. This phenomenon is usually referred to as beam spread
but sometimes it is also referred to as beam divergence or ultrasonic diffraction. It
should be noted that there is actually a difference between beam spread and beam
divergence. Beam spread is a measure of the
whole angle from side to side of the beam in the
far field. Beam divergence is a measure of the
angle from one side of the sound beam to the
central axis of the beam in the far field. Therefore,
beam spread is twice the beam divergence.
Although beam spread must be considered when performing an ultrasonic inspection,
it is important to note that in the far field, or Fraunhofer zone, the maximum sound
pressure is always found along the acoustic axis (centerline) of the transducer.
Therefore, the strongest reflections are likely to come from the area directly in front of
the transducer.
Beam spread occurs because the vibrating particle of the material (through which the
wave is traveling) do not always transfer all of their energy in the direction of wave
propagation. If the particles are not directly aligned in the direction of wave
propagation, some of the energy will get transferred off at an angle. In the near field,
constructive and destructive wave interference fill the sound field with fluctuation. At
the start of the far field, however, the beam strength is always greatest at the center of
the beam and diminishes as it spreads outward.
The beam spread is largely influenced by the frequency and diameter of the
transducer. For a flat piston source transducer, an approximation of the beam
divergence angle at which the sound pressure has decreased by one half (-6 dB) as
compared to its value at the centerline axis can be caculated as:




Where;
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: the beam divergence angle from centerline to point where signal is at half
strength
: sound velocity in the material
: diameter of the transducer
: frequency of the transducer

Transducer Types
Ultrasonic transducers are manufactured for a variety of applications and can be
custom fabricated when necessary. Careful attention must be paid to selecting the
proper transducer for the application. It is important to choose transducers that have
the desired frequency, bandwidth, and focusing to optimize inspection capability. Most
often the transducer is chosen either to enhance the sensitivity or resolution of the
system.
Transducers are classified into two major groups according to the application.
Contact transducers are used for direct contact inspections, and are generally
hand manipulated. They have elements protected in a rugged casing to
withstand sliding contact with a variety of materials. These transducers
have an ergonomic design so that they are easy to grip and move along
a surface. They often have replaceable wear plates to lengthen their
useful life. Coupling materials of water, grease, oils, or commercial
materials are used to remove the air gap between the transducer and the
component being inspected.
Immersion transducers do not contact the component. These transducers are
designed to operate in a liquid environment and all connections are watertight.
Immersion transducers usually have an impedance matching layer that helps to get
more sound energy into the water and, in turn, into the
component being inspected. Immersion transducers
can be purchased with a planer, cylindrically focused or
spherically focused lens. A focused transducer can
improve the sensitivity and axial resolution by
concentrating the sound energy to a smaller area.
Immersion transducers are typically used inside a
water tank or as part of a squirter or bubbler system in
scanning applications.

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Other Types of Contact Transducers
Contact transducers are available in a variety of configurations to improve their
usefulness for a variety of applications. The flat contact transducer shown above is
used in normal beam inspections of relatively flat surfaces, and where near surface
resolution is not critical. If the surface is curved, a shoe that matches the curvature of
the part may need to be added to the face of the transducer. If near surface resolution
is important or if an angle beam inspection is needed, one of the special contact
transducers described below might be used.
 Dual element transducers contain two independently
operated elements in a single housing. One of the elements
transmits and the other receives the ultrasonic signal. Dual
element transducers are especially well suited for making
measurements in applications where reflectors are very near
the transducer since this design eliminates the ring down
effect that single-element transducers experience (when
single-element transducers are operating in pulse echo mode,
the element cannot start receiving reflected signals until the element has
stopped ringing from its transmit function). Dual element transducers are
very useful when making thickness measurements of thin materials and
when inspecting for near surface defects. The two elements are angled
towards each other to create a crossed-beam sound path in the test material.

 Delay line transducers provide versatility with a variety
of replaceable options. Removable delay line, surface
conforming membrane, and protective wear cap
options can make a single transducer effective for a
wide range of applications. As the name implies, the
primary function of a delay line transducer is to
introduce a time delay between the generation of the sound wave
and the arrival of any reflected waves. This allows the transducer
to complete its "sending" function before it starts its "receiving"
function so that near surface resolution is improved. They are
designed for use in applications such as high precision thickness
gauging of thin materials and delamination checks in composite
materials. They are also useful in high-temperature measurement
applications since the delay line provides some insulation to the piezoelectric
element from the heat.
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 Angle beam transducers and wedges are typically used to
introduce a refracted shear wave into the test material.
Transducers can be purchased in a variety of fixed angles
or in adjustable versions where the user determines the
angles of incidence and refraction. In the fixed angle
versions, the angle of refraction that is marked on the
transducer is only accurate for a particular material, which
is usually steel. The most commonly used refraction angles for
fixed angle transducers are 45°, 60° and 70°. The angled sound
path allows the sound beam to be reflected from the backwall to
improve detectability of flaws in and around welded areas. They are also used to
generate surface waves for use in detecting defects on the surface of a component.

 Normal incidence shear wave transducers are unique because they allow the
introduction of shear waves directly into a test piece without the use of an angle
beam wedge. Careful design has enabled manufacturing of transducers with
minimal longitudinal wave contamination.

 Paint brush transducers are used to scan wide areas. These long and narrow
transducers are made up of an array of small crystals and that make it possible to
scan a larger area more rapidly for discontinuities. Smaller and more sensitive
transducers are often then required to further define the details of a discontinuity.

Couplant
A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic
energy from the transducer into the test specimen. Couplant is generally necessary
because the acoustic impedance mismatch between air and solids is large. Therefore,
nearly all of the energy is reflected and very little is transmitted into the test material.
The couplant displaces the air and makes it possible to get more sound energy into the
test specimen so that a usable ultrasonic signal can be obtained.
In contact ultrasonic testing a thin film of oil, glycerin or water is
typically used between the transducer and the test surface.
When shear waves are to be transmitted, the fluid is generally
selected to have a significant viscosity.
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When scanning over the part, an immersion technique is often
used. In immersion ultrasonic testing both the transducer and
the part are immersed in the couplant, which is typically water.
This method of coupling makes it easier to maintain consistent
coupling while moving and manipulating the transducer and/or
the part.

Electromagnetic Acoustic Transducers (EMATs)
Electromagnetic-acoustic transducers (EMAT) are a modern type of ultrasonic
transducers that work based on a totally different physical principle than piezoelectric
transducers and, most importantly, they do not need
couplant. When a wire is placed near the surface of an
electrically conducting object and is driven by a current
at the desired ultrasonic frequency, eddy currents will
be induced in a near surface region of the object. If a
static magnetic field is also present, these eddy
currents will experience forces called “Lorentz forces”
which will cause pressure waves to be generated at the
surface and propagate through the material.
Different types of sound waves (longitudinal, shear, lamb)
can be generated using EMATs by varying the configuration
of the transducer such that the orientation of the static
magnetic field is changed.
EMATs can be used for thickness measurement, flaw
detection, and material property characterization. The EMATs offer many advantages
based on its non-contact couplant-free operation. These advantages include the ability
to operate in remote environments at elevated speeds and temperatures.

Pulser-Receivers
Ultrasonic pulser-receivers are well suited to general purpose ultrasonic testing. Along
with appropriate transducers and an oscilloscope, they can be used for flaw detection
and thickness gauging in a wide variety of metals, plastics, ceramics, and composites.
Ultrasonic pulser-receivers provide a unique, low-cost ultrasonic measurement
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capability. Specialized portable equipment that are dedicated for ultrasonic inspection
merge the pulser-receiver with the scope display in one small size battery operated
unit.
The pulser section of the instrument generates
short, large amplitude electric pulses of controlled
energy, which are converted into short ultrasonic
pulses when applied to an ultrasonic transducer.
Control functions associated with the pulser circuit
include:
 Pulse length or damping: The amount of time the pulse is applied to the transducer.
 Pulse energy: The voltage applied to the transducer. Typical pulser circuits will apply
from 100 volts to 800 volts to a transducer.
In the receiver section the voltage signals produced by the transducer, which represent
the received ultrasonic pulses, are amplified. The amplified signal is available as an
output for display or capture for signal processing. Control functions associated with
the receiver circuit include:
 Signal rectification: The signal can be viewed as positive half wave, negative half
wave or full wave.
 Filtering to shape and smoothing
 Gain, or signal amplification
 Reject control

Data Presentation
Ultrasonic data can be collected and displayed in a number of different formats. The
three most common formats are known in the NDT world as A-scan, B-scan and C-scan
presentations. Each presentation mode provides a different way of looking at and
evaluating the region of material being inspected. Modern computerized ultrasonic
scanning systems can display data in all three presentation forms simultaneously.

A-Scan Presentation
The A-scan presentation displays the amount of received ultrasonic energy as a
function of time. The relative amount of received energy is plotted along the vertical
axis and the elapsed time (which may be related to the traveled distance within the
material) is displayed along the horizontal axis. Most instruments with an A-scan
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display allow the signal to be displayed in its natural radio frequency form (RF), as a
fully rectified RF signal, or as either the positive or negative half of the RF signal. In the
A-scan presentation, relative discontinuity size can be estimated by comparing the
signal amplitude obtained from an unknown reflector to that from a known reflector.
Reflector depth can be determined by the position of the signal on the horizontal time
axis.





In the illustration of the A-scan presentation shown in the figure, the initial pulse
generated by the transducer is represented by the signal IP, which is near time zero. As
the transducer is scanned along the surface of the part, four other signals are likely to
appear at different times on the screen. When the transducer is in its far left position,
only the IP signal and signal A, the sound energy reflecting from surface A, will be seen
on the trace. As the transducer is scanned to the right, a signal from the backwall BW
will appear later in time, showing that the sound has traveled farther to reach this
surface. When the transducer is over flaw B, signal B will appear at a point on the time
scale that is approximately halfway between the IP signal and the BW signal. Since the
IP signal corresponds to the front surface of the material, this indicates that flaw B is
about halfway between the front and back surfaces of the sample. When the
transducer is moved over flaw C, signal C will appear earlier in time since the sound
travel path is shorter and signal B will disappear since sound will no longer be
reflecting from it.

B-Scan Presentation
The B-scan presentation is a type of presentation that is possible for automated linear
scanning systems where it shows a profile (cross-sectional) view of the test specimen.
In the B-scan, the time-of-flight (travel time) of the sound waves is displayed along the
vertical axis and the linear position of the transducer is displayed along the horizontal
axis. From the B-scan, the depth of the reflector and its approximate linear dimensions
in the scan direction can be determined. The B-scan is typically produced by
establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough
to trigger the gate, a point is produced on the B-scan. The gate is triggered by the
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sound reflected from the backwall of the specimen and by smaller reflectors within the
material. In the B-scan image shown previously, line A is produced as the transducer is
scanned over the reduced thickness portion of the specimen. When the transducer
moves to the right of this section, the backwall line BW is produced. When the
transducer is over flaws B and C, lines that are similar to the length of the flaws and at
similar depths within the material are drawn on the B-scan. It should be noted that a
limitation to this display technique is that reflectors may be masked by larger reflectors
near the surface.

C-Scan Presentation
The C-scan presentation is a type of presentation that is possible
for automated two-dimensional scanning systems that provides
a plan-type view of the location and size of test specimen
features. The plane of the image is parallel to the scan pattern
of the transducer. C-scan presentations are typically produced
with an automated data acquisition system, such as a computer
controlled immersion scanning system. Typically, a data
collection gate is established on the A-scan and the amplitude or
the time-of-flight of the signal is recorded at regular intervals as
the transducer is scanned over the test piece. The relative signal
amplitude or the time-of-flight is displayed as a shade of gray or
a color for each of the positions where data was recorded. The C-scan presentation
provides an image of the features that reflect and scatter the sound within and on the
surfaces of the test piece.
High resolution scans can produce very detailed images. The figure
shows two ultrasonic C-scan images of a US quarter. Both images
were produced using a pulse-echo technique with the transducer
scanned over the head side in an immersion scanning system. For
the C-scan image on the top, the gate was set to capture the
amplitude of the sound reflecting from the front surface of the
quarter. Light areas in the image indicate areas that reflected a
greater amount of energy back to the transducer. In the C-scan
image on the bottom, the gate was moved to record the intensity
of the sound reflecting from the back surface of the coin. The
details on the back surface are clearly visible but front surface
features are also still visible since the sound energy is affected by
these features as it travels through the front surface of the coin.
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MEASUREMENT AND CALIBRATION TECHNIQUES

Normal Beam Inspection
Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a
part or structure by accurately measuring the time required for a short ultrasonic pulse
generated by a transducer to travel through a thickness of material, reflect from the
back or the surface of a discontinuity, and be returned to the transducer. In most
applications, this time interval is a few microseconds or less. The two-way transit time
measured is divided by two to account for the down-and-back travel path and
multiplied by the velocity of sound in the test material. The result is expressed in the
well-known relationship:




Where is the distance from the surface to the discontinuity in
the test piece, is the velocity of sound waves in the material,
and is the measured round-trip transit time.
Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz
and 100 MHz, by means of piezoelectric transducers that generate bursts of sound
waves when excited by electrical pulses. Typically, lower frequencies are used to
optimize penetration when measuring thick, highly attenuating or highly scattering
materials, while higher frequencies will be recommended to optimize resolution in
thinner, non-attenuating, non-scattering materials. It is possible to measure most
engineering materials ultrasonically, including metals, plastic, ceramics, composites,
epoxies, and glass as well as liquid levels and the thickness of certain biological
specimens. On-line or in-process measurement of extruded plastics or rolled metal
often is possible, as is measurements of single layers or coatings in multilayer
materials.

Angle Beam Inspection
Angle beam transducers and wedges are typically used to introduce a refracted shear
wave into the test material. An angled sound path allows the sound beam to come in
from the side, thereby improving detectability of flaws in and around welded areas.
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Angle beam inspection is somehow different than normal beam inspection. In normal
beam inspection, the backwall echo is always present on the scope display and when
the transducer basses over a discontinuity a new echo will appear between the initial
pulse and the backwall echo. However, when scanning a surface using an angle beam
transducer there will be no reflected echo on the scope display unless a properly
oriented discontinuity or reflector comes into the beam path.
If a reflection occurs before the sound waves reach the backwall, the reflection is
usually referred to as “first leg reflection”. The angular distance (Sound Path) to the
reflector can be calculated using the same formula used for normal beam transducers
(but of course using the shear velocity instead of the longitudinal velocity) as:





where
is the shear sound velocity in the material.
Knowing the angle of refraction for the transducer,
the surface distance to the reflector and its depth
can be calculated as:








where
is the angle of refraction.
If a reflector came across the sound beam after it has reached and reflected off the
backwall, the reflection is usually referred to as “second leg reflection”. In this case, the
“Sound Path” (the total sound path
for the two legs) and the “Surface
Distance” can be calculated using
the same equations given above,
however, the “Depth” of the
reflector will be calculated as:






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Inspection of Welded Joints
The most commonly occurring defects in welded joints are porosity, slag inclusions,
lack of side-wall fusion, lack of intermediate-pass fusion, lack of root penetration,
undercutting, and longitudinal or transverse cracks. With the exception of single gas
pores all the listed defects are usually well detectable using ultrasonics.
Ultrasonic weld inspections are typically performed using straight beam transducer in
conjunction with angle beam transducers.
 A straight beam transducer, producing a longitudinal wave at normal incidence into
the test piece, is first used to locate any laminations in or near the heat-affected
zone. This is important because an angle beam transducer may not be able to
provide a return signal from a laminar flaw.

 The second step in the inspection involves using an angle beam transducer to
inspect the actual weld. This inspection may include the root, sidewall, crown, and
heat-affected zones of a weld. The process involves scanning the surface of the
material around the weldment with the transducer. This refracted sound wave will
bounce off a reflector (discontinuity) in the path of the sound beam.
To determine the proper scanning
area for both sides of the weld, the
inspector must calculate the skip
distance of the sound beam using
the refracted angle and material
thickness as:

where is the material thickness.
Based on such calculations, the inspector can identify the transducer locations on the
surface of the material corresponding to the face, sidewall, and root of the weld.
The angle of refraction for the angle beam transducer used for inspection is usually
chosen such that (
). Doing so, the second leg of the
beam will be normal to the side wall of the weldment such that lack of fusion can be
easily detected (the first leg will also be normal to the other wall). However, for
improving the detectability of the different types of weld discontinuities, it is
recommended to repeat the scanning using several transducers having different angles
of refraction.
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Crack Tip Diffraction
When the geometry of the part is relatively uncomplicated and the orientation of a
flaw is well known, the length of a crack can be determined by a technique known as
“crack tip diffraction”.
One common application of the tip diffraction technique is to determine the length of
a crack originating from on the backside of a flat plate as shown below. In this case,
when an angle beam transducer is
scanned over the area of the flaw, an
echo appears on the scope display
because of the reflection of the
sound beam from the base of the
crack (top image). As the transducer
moves, a second, but much weaker,
echo appears due to the diffraction
of the sound waves at the tip of the
crack (bottom image). However,
since the distance traveled by the
diffracted sound wave is less, the
second signal appears earlier in time
on the scope.
Crack height ( ) is a function of the ultrasound shear velocity in the material (
), the
incident angle (
) and the difference in arrival times between the two signal ( ).
Since the beam angle and the thickness of the material is the same in both
measurements, two similar right triangles are formed such that one can be overlayed
on the other. A third similar right triangle is made, which is comprised on the crack, the
length and the angle
. The variable is really the difference in time but can
easily be converted to a distance by dividing the time in half (to get the one-way travel
time) and multiplying this value by the velocity of the sound in the material. Using
trigonometry, we can write:


Therefore, the crack height is found to be:






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If the material is relatively thick or the crack is relatively short, the crack base echo and
the crack tip diffraction echo could appear on the scope display simultaneously (as
seen in the figure). This can be attributed to the divergence
of the sound beam where it becomes wide enough to cover
the entire crack length. In such case, though the angle of the
beam striking the base of the crack is slightly different than
the angle of the beam striking the tip of the crack, the
previous equation still holds reasonably accurate and it can
be used for estimating the crack length.

Calibration Methods
Calibration refers to the act of evaluating and adjusting the precision and accuracy of
measurement equipment. In ultrasonic testing, several forms of calibrations must
occur. First, the electronics of the equipment must be calibrated to ensure that they
are performing as designed. This operation is usually performed by the equipment
manufacturer and will not be discussed further in this material. It is also usually
necessary for the operator to perform a "user calibration" of the equipment. This user
calibration is necessary because most ultrasonic equipment can be reconfigured for
use in a large variety of applications. The user must "calibrate" the system, which
includes the equipment settings, the transducer, and the test setup, to validate that
the desired level of precision and accuracy are achieved.
In ultrasonic testing, reference standards are used to establish a general level of
consistency in measurements and to help interpret and quantify the information
contained in the received signal. The figure shows some of the most commonly used
reference standards for the calibration of ultrasonic equipment. Reference standards
are used to validate that the equipment and the setup provide similar results from one
day to the next and that similar results are produced by different systems. Reference
standards also help the inspector to estimate the size of flaws. In a pulse-echo type
setup, signal strength depends on
both the size of the flaw and the
distance between the flaw and the
transducer. The inspector can use a
reference standard with an artificially
induced flaw of known size and at
approximately the same distance
away for the transducer to produce a
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signal. By comparing the signal from the reference standard to that received from the
actual flaw, the inspector can estimate the flaw size.
The material of the reference standard should be the same as the material being
inspected and the artificially induced flaw should closely resemble that of the actual
flaw. This second requirement is a major limitation of most standard reference
samples. Most use drilled holes and notches that do not closely represent real flaws. In
most cases the artificially induced defects in reference standards are better reflectors
of sound energy (due to their flatter and smoother surfaces) and produce indications
that are larger than those that a similar sized flaw would produce. Producing more
"realistic" defects is cost prohibitive in most cases and, therefore, the inspector can
only make an estimate of the flaw size.
Reference standards are mainly used to calibrate instruments prior to performing the
inspection and, in general, they are also useful for:
 Checking the performance of both angle-beam and normal-beam transducers
(sensitivity, resolution, beam spread, etc.)
 Determining the sound beam exit point of angle-beam transducers
 Determining the refracted angle produced
 Calibrating sound path distance
 Evaluating instrument performance (time base, linearity, etc.)

Introduction to Some of the Common Standards
A wide variety of standard calibration blocks of different designs, sizes and systems of
units (mm or inch) are available. The type of standard calibration block used is
dependent on the NDT application and the form and shape of the object being
evaluated. The most commonly used standard calibration blocks are those of the;
International Institute of Welding (IIW), American Welding Society (AWS) and
American Society of Testing and Materials (ASTM). Only two of the most commonly
used standard calibration blocks are introduced here.
IIW Type US-1 Calibration Block
This block is a general purpose calibration block that can be used for calibrating angle-
beam transducers as well as normal beam transducers. The material from which IIW
blocks are prepared is specified as killed, open hearth or electric furnace, low-carbon
steel in the normalized condition and with a grain size of McQuaid-Ehn No. 8. Official
IIW blocks are dimensioned in the metric system of units.
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The block has several features that facilitate checking
and calibrating many of the parameters and functions of
the transducer as well as the instrument where that
includes; angle-beam exit (index) point, beam angle,
beam spared, time base, linearity, resolution, dead zone,
sensitivity and range setting.

The figure below shows some of the uses of the block.


ASTM - Miniature Angle-Beam Calibration Block (V2)
The miniature angle-beam block is used in a somewhat similar manner as the as the
IIW block, but is smaller and lighter. The miniature angle-beam block is primarily used
in the field for checking the characteristics of angle-beam transducers.
With the miniature block, beam angle and exit point
can be checked for an angle-beam transducer. Both
the 25 and 50 mm radius surfaces provide ways for
checking the location of the exit point of the
transducer and for calibrating the time base of the
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instrument in terms of metal distance. The small hole provides a reflector for checking
beam angle and for setting the instrument gain.

Distance Amplitude Correction (DAC)
Acoustic signals from the same reflecting surface will
have different amplitudes at different distances from
the transducer. A distance amplitude correction (DAC)
curve provides a means of establishing a graphic
“reference level sensitivity” as a function of the
distance to the reflector (i.e., time on the A-scan
display). The use of DAC allows signals reflected from
similar discontinuities to be evaluated where signal
attenuation as a function of depth has been correlated.
DAC will allow for loss in amplitude over material depth (time) to be represented
graphically on the A-scan display. Because near field length and beam spread vary
according to transducer size and frequency, and materials vary in attenuation and
velocity, a DAC curve must be established for each different situation. DAC may be
employed in both longitudinal and shear modes of operation as well as either contact
or immersion inspection techniques.
A DAC curve is constructed from the peak amplitude
responses from reflectors of equal area at different
distances in the same material. Reference standards
which incorporate side drilled holes (SDH), flat
bottom holes (FBH), or notches whereby the
reflectors are located at varying depths are
commonly used. A-scan echoes are displayed at
their non-electronically compensated height and
the peak amplitude of each signal is marked to
construct the DAC curve as shown in the figure. It is
important to recognize that regardless of the type
of reflector used, the size and shape of the reflector
must be constant.
The same method is used for constructing DAC curves for angle beam transducers,
however in that case both the first and second leg reflections can be used for
constructing the DAC curve.
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347. Fundamentals of Resonant Acoustic Method NDT

Gail R Stultz
[email protected]
Richard W Bono
[email protected]
Mark I Schiefer
[email protected]

The Modal Shop, Inc.
3149 East Kemper Rd.
Cincinnati, OH 45241










Abstract


Rapid conversion of machined parts to powdered metal and cast is driving industries, especially
automotive. Due to the high expectations of both primary manufacturers and end consumers,
defects cannot be tolerated even in million piece quantities. There is, in effect, a growing
requirement for zero defect supply chain commitments. To achieve zero defect output,
manufacturers are making the commitment to move to online NDT. This type of online inspection
requires accuracy, reliability, and high throughput. Resonant Acoustic Method NDT (RAM NDT)
provides a proven technique exhibiting these pivotal performance requirements and automates
economically. RAM NDT tests, reports and screens for most common part flaws in a manner
similar to the way NASA tests flight hardware and automotive manufacturers validate their new
car designs. Utilizing structural dynamics and statistical variation, RAM NDT provides mature,
laboratory proven technology in a robust, economical, process-friendly manner.


1. Motivational Example


As with most powdered metal component suppliers, Company ABC is already doing spot
magnetic particle testing on batches of parts from a given production run. The problem starts
when a customer, say an automotive manufacturer, experiences field failures. The result is that
Company ABC is put on parts-hold and has to pay for both containment and 100% field
inspection on the customer site. At the risk of permanently damaging the company’s reputation
and losing both existing and new business, significantly larger part batches are subjected to
magnetic particle inspection, with 100% of the production lots inspected via a 300% visual part
sort – where each part is visually inspected by three separate technicians. Everyone who can be
pulled from another job is pulled in to help out during this time of crisis. To ensure necessary
quality, 100% end-of-line part inspection must be implemented; traditional NDT techniques such
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348. as magnetic particle, liquid penetrant, eddy current and X-ray, or purely visual inspection, are
painstaking, subjective manual processes. As a result, rarely does the 100% inspection continue,
and the cycle of “flawed-parts roulette” continues.

Providing relief and security for the high volume manufacturer, RAM NDT offers reliable
inspection, with quantitative, objective results. This technique is easily automated to eliminate
human error with fast throughput for cost effective 100% inspection, simple and straightforward
with minimal disruption to production. RAM NDT is a volumetric, resonant inspection technique
that measures the structural integrity of each part to detect defects on a component level. With a
large number of successes on the production lines of powdered metal, cast and forged parts, RAM
NDT is the simple, effective solution to this common problem.


2. History


The history of NDT techniques used for quality control testing in part manufacturing dates back
to the beginning of the industrial manufacturing era. Initially, the basic visual inspection of the
operators themselves served as the primary means of monitoring part acceptability. More
sophisticated NDT techniques evolved, and magnetic particle inspection eventually became the
de facto standard for testing ferrous metallic components such as castings, forgings and, more
recently, powdered metals. This subjective and visual technology has remained essentially
unchanged for the past 50+ years, yet continues to be the most common inspection tool for such
parts.

Traditional NDT techniques focus on detecting and diagnosing defects. They use visual
techniques or imaging to scan for any indication of defects. For the case presented in our
motivational example, identifying the type of defect itself is secondary to identifying the
defective parts. While diagnosing specific defects is applicable when evaluating and inspecting
some systems, such as gas pipelines or similar, it is not appropriate for high volume 100%
manufactured part inspection. For these components it is of primary importance to detect if a part
is non-conforming rather than why. Therefore, an end-of-line “go/no go” objective inspection,
such as by RAM NDT, is preferred here to a subjective diagnosis.

Scanning methods include magnetic particle testing (MT), ultrasonic testing (UT), eddy
current/electromagnetic testing (ET), dye penetrant testing (PT), X-ray/radiographic testing (RT)
and visual testing (VT). The fundamental difference between these traditional NDT techniques
and resonant inspection (RI) is this scanning methodology. Scanning methods are manual and
require subjective interpretation by an operator. As a result, the operator requires a certain level
of technical training and/or certification to properly diagnose such indications of defect and infer
the effects on the functionality of a part. Additionally, whenever such a technique requires the
judgment of an operator, overall reliability suffers. In Juran’s Quality Handbook, Juran states
that operators average only 80% reliability – this statistic is a reflection of the human
interpretation factor, not the accuracy of the techniques themselves, see ref 1. None of these
scanning techniques allow for efficient, cost effective or reliable quality control testing of 100%
of manufactured parts of any appreciable volume. It should be noted that in some cases eddy
current techniques can be implemented as a “whole part” test by using an encircling coil, easily
automated with high throughput. However, in these cases the effectiveness of ET’s flaw
detection is reduced, limited to detecting on certain types or configurations of surface flaws.

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349. Resonant inspection, conversely, measures the structural response of a part and evaluates it
against the statistical variation from a control set of good parts to screen defects. Its volumetric
approach tests the whole part, both for external and internal structural flaws or deviations,
providing objective and quantitative results. This structural response is a unique and measurable
signature, defined by a component’s mechanical resonances. These resonances are a function of
part geometry and material properties and are the basis for RI techniques. By measuring the
resonances of a part, one determines the structural characteristics of that part in a single test.
Typical flaws and defects adversely affecting the structural characteristics of a part are given in
Table 1 for powdered metal, cast and forged applications. Many of the traditional NDT
techniques previously discussed can detect these flaws as well, but often only RI can detect all in
a single test, throughout the entire part (including deep sub-surface defects), in an automated and
objective fashion.

Table 1. Typical structural defects detectable by resonant inspection.

Cast Forged Powdered Metal
Cracks Cracks Cracks
Cold shuts Missed or double strikes Chips
Porosity Porosity Voids
Hardness/density Hardness Hardness/density
Inclusions Inclusions Inclusions
Heat treat Heat treat Heat treat
Compressive & residual stress Quenching problems Decarb
Nodularity Laps Oxides
Gross dimensions Gross dime nsions Gross dimensions
Raw material contaminants Raw material
contaminants
Raw material
contaminants
Missed processes/operations Missed
processes/operations
Missed
processes/operations
After defective parts have been sorted with RI, complimentary traditional NDT techniques may
provide a means for subjective diagnosis on the smaller subset of parts. This is useful for
determining a defect’s root cause and ultimately improving the production processes. Table 2
provides a generic NDT selection table stating the capabilities of the various methods. The
ASME has published standards that detail each of the traditional NDT methodologies mentioned
here, see ref 2-8.

Table 2. General overview of common NDT techniques.

ET MT/PT UT RT RAM
Defect Type
Cracks/chips/porosity/voids Yes Yes Yes Yes/No Yes
Missed
processes/operations
Yes/No No Yes/No Yes/No Yes
Material property Yes/No No No No Yes
Structurally significant Yes Yes Yes Yes Yes
Production lot variations Yes/No Yes Yes Yes Yes/No
Defect Location
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350. Surface (external) Yes Yes Yes No Yes
Internal No No Yes Yes Yes
Brazing/bonding/welding No No Yes/No Yes/No Yes
Speed/Training/Cost
Part throughput Medium Low High Low High
Training requirements High High Medium High Low
Overall inspection costs Medium Medium High High Low
Automation Capacity
Quantitative results Yes/No No Yes/No No Yes
Automation requirements Medium N/A Complex Complex Easy
Automation cost Medium N/A High High Low/Medium


3. Theoretical Background


Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure or
system. All structures, even structures such as metal gears or similar parts that are apparently
rigid to the human eye, undergo deformation. These deformations can be described using modal
analysis. Specifically, all structures have mechanical resonances, where the structure itself
amplifies any energy imparted to it at certain frequencies. For example, tuning forks or bells will
vibrate at very specific frequencies, their natural frequencies, for long periods of time with just a
small tap. The sound that is made is directly due to these natural frequencies. In fact, any noise
generated by a structure is done so by vibration, which is simply a pattern of summed sinusoidal
deformations. RAM NDT utilizes this structural dynamic behavior to evaluate the integrity and
consistency of parts.

For illustrative purposes, consider the single degree-of-freedom (SDOF) mass, spring, damper
system in Figure 1. It has one DOF because its state can be be determined b one quantity (x), the
displacement of the mass. The elements of this simplified model are the mass (m), stiffness (k)
and damping (c). The energy imparted into the system by the excitation force (f) is stored in the
system as kinetic energy of the mass and potential energy of the spring and is dissipated by the
damping. The mathematical representation of the SDOF system, which is called its equations of
motion, is given in Equation (1) below.


mx¨
(t)+cx
.
(t)+kx(t)=f(t)
(1)

The solution to the equation of motion produces an eigenvalue problem which yields the
undamped natural frequency as


>n=
k
m
(2)

Equation (2) reveals the natural frequencies, or resonances, of a system that are determined by its
mass (i.e., volume and density) and stiffness (i.e., Young's modulus and cross-sectional
geometry). While in Equation (2) holds only for an SDOF system, the underlying relationship of
mass and stiffness can be generalized for more complex systems. That is, an increase in stiffness
will increase the natural frequency and an increase in mass will decrease the natural frequency.
For example, consider the strings on a guitar. The larger diameter strings (more mass) produce
lower tones than the smaller strings (less mass). Also, a string has a higher pitch when tightened
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351. (increased stiffness) than when loosened (decreased stiffness). It is these fundamental properties
of the resonances of a system that RAM NDT utilizes to evaluate the integrity and consistency of
parts.




Stiffness Damping
k c


Displacement Mass Excitation Force
x m f




Figure 1. Single Degree of Freedom (SDOF) discrete parameter model

The natural frequencies are global properties of a given structure and the presence of structural
defects causes shifts in these resonances. For example, a crack will change the stiffness in the
region near the crack and a variation in density or the presence of porosity will change the mass.
A crack defect typically reduces the stiffness in the material, thus decreasing the natural
frequency. Similarly, porosity in a cast part reduces mass, thus increasing the natural frequency.
These shifts are measurable if the defect is structurally significant with respect to the either the
size or location of the flaw within a specific resonance mode shape. With some defects, a shift in
resonant frequency can also be noticed audibly, such as a cracked bell that does not ring true.


4. Resonant Acoustic Method (RAM NDT)


An introductory overview of the resonant inspection technique and theoretical background has
previously been presented in Sections 2 and 3, respectively. This portion of the paper discusses
in more detail the specific implementation of resonant inspection and the associated advantages
of the Resonant Acoustic Method.

RI is basically experimental modal analysis simplified for application to high volume production
manufacturing and quality control testing. The generic, step-by-step procedure is as follows:
1. Excite the part with a known and repeatable force input. This force is typically generated
by a controlled impact or actuator providing broadband or sinusoidal energy over the
appropriate frequency range of analysis.
2. Measure the structural response of the part to the applied input force using a dynamic
sensor such as a microphone or accelerometer (vibration pickup) and a high-speed analog
to digital converter (ADC) with appropriate anti-aliasing filters.
3. Process the acquired time data with a Fast Fourier Transform (FFT) for analysis in the
frequency domain.
4. Analyze the consistency of the frequency spectrum from part to part by comparing to a
spectral template created from known good parts. Mechanical resonances are indicated
as peaks in the frequency spectrum of the response. “Good” parts (structurally sound)
have consistent spectral signatures (i.e. the mechanical resonances are the same among
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352. parts) while “bad” parts are different. Generally these templates are setup to evaluate the
consistency of the frequency and amplitude of ten or fewer peaks. Any deviation in (a
range of) peak frequency or amplitude constitutes a structurally significant difference that
provides a quantitative and objective part rejection.

The Resonant Acoustic Method technique performs resonant inspection by impacting a part and
“listening” to its acoustic spectral signature with a microphone. The controlled impact provides
broadband input energy to excite the part and the microphone allows for a non-contact
measurement of the structural response. The part’s mechanical resonances amplify the broadband
input energy at its specific natural frequencies, measured by the microphone above the
background noise in the test environment. An example of such a spectrum from 0 to 40 kHz is
given in Figure 2.


Figure 2. Typical acoustic signature for powdered metal part.

Gross defects can often be distinguished directly by the human ear, but human hearing is
subjective and limited to approximately 20 kHz maximum. By analyzing data beyond 20 kHz, to
upwards of 50 kHz, much smaller defects can be detected, even across production lots given
reasonable process control. Typically, these defects cause frequency shifts as shown in Figure 3.
These shifts are a function of how the specific defect affects the mechanical resonance, which is
dependent upon the specific defect location with respect to the deformation pattern of the
resonance. Fortunately, mechanical resonances are global properties of a structure, and generally
a defect will alter at least one resonant frequency. For this reason, it is good practice to set up
multiple criteria ranges for analysis.



Figure 3. Data showing frequency shift due to structural defect in part.

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353. An additional signal processing tool for improving analysis and sorting of good parts versus
defective parts is implemented with a time delay function. Often times a defect may not cause a
substantial shift in resonant frequency, but instead reduces the structure’s capability to “hold its
tone” over time. By delaying the structural response measurement (many times just milliseconds)
the resonant peak is not measurable from defective parts because the energy decays too rapidly.
The peak in the frequency spectrum disappears, shown in Figure 4. A practical example of this is
a cracked bell – when struck, it does not ring for an extended period of time as a “good” bell
would.


Figure 4. Data showing resonance of good part against defective part, processed using time delay
technique causing the peak to “disappear”.

RAM NDT’s basic measurement procedure allows for easy automation and very high part testing
throughput. There is no part preparation required – no magnetizing, cleaning, immersion, etc.
Expendable costs associated with such preparation, such as chemicals and waste removal, are
eliminated. The single impact and non-contact response measurement (via microphone) can be
made as a part is moving down a conveyor, often as fast as a part per second. The parts do not
need to be physically stopped; nor are they required to be precisely located with expensive
robotics on contact actuators and vibration pickups. Simple guides are typically adequate to
rotate/position the part for impact and allow flexibility to test many different types of parts or
geometries. Given this capacity for automation and throughput, and its quantitative analysis with
objective results, RAM NDT is ideal for plant floor, high volume quality control test applications.

The core system components are shown in Figures 5 and 6. From the rugged microphone and
industrial electric impactor to the NEMA 4 smart digital controller, the packaging is ideally suited
for dirty, plant environments such as ductile iron foundries. A typical conveyor system for fully
automated testing is shown in Figure 7.



Figure 5. Industrial electric impact hammer designed for millions of impacts and rugged
microphone for non-contact response measurement, shown mounted on conveyor section.
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354. Figure 6. Smart digital controller measures signals and processes data, independent of PC, with
internal digital relay outputs.



Figure 7. Fully automated system on 4 ft conveyor section, shown with acoustic chamber for
testing parts.

Successful implementation of RAM NDT depends upon proper setup of the accept/reject criteria
ranges in the part template. Each type and/or geometry of part requires a separate template. Parts
need to be tested in the same manufacturing state. Typically, templates can be setup quickly with
just a few dozen parts in less than a ½ hour. This sample set should include both good parts
(ideally with at least several from different production batches) and parts with the expected
variety of flaws. It is recommended to validate the template and resulting part sort with a larger
statistical data set of a few hundred pieces. Often other NDT techniques, for example magnetic
particle inspection, are complimentary in this regard, or destructive evaluations are commonly
used for correlation as well. Once the specific part’s template is verified for accuracy, large
volumes of parts can be 100% tested quickly and reliably.

System validation can be performed using a controlled set of known parts. Parts of a given type,
both good and defective, are kept as “standards” and run through the automated system for
validation on a regular basis. Across batches over time, signatures often show trends where
mechanical resonances shift due to acceptable variations in material properties (density, etc.) or
process variations (heat treatment, etc.) By investing time upfront with this type of system
validation procedure, process engineers and technicians have a better understanding of their parts
and manufacturing processes and ensure the reliability of their inspection system.


5. Case Study: Powdered Metal Sprocket


The manufacturer of the powdered metal sprocket shown in Figure 8 below needed to automate
inspection, primarily for cracks and flawed teeth. The initial part template was set up using 70
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355. samples. 30 of these were visually inspected as good parts, while the remaining 40 were
determined to have a variety of flaws such as broken, chipped and cracked teeth as shown in
Figure 9. Typical data from several parts is shown in Figure 10, where frequency shifts down
from cracks (two blue traces left of acceptance box) and up from broken teeth (red, pink and olive
traces right of box) are clearly displayed against the two good samples (black/gray traces peak
within box). These physical flaws correlate nicely with the theory presented in Section 2. A
crack is simply a weaker spring (lower stiffness, k, in Eq. 1) and a broken tooth reduces mass
(lower mass, m, in Eq. 1) which affects the resonant frequency accordingly.



Figure 8. Powdered metal sprocket.







Figure 9. View of powdered metal sprocket with chipped, broken and cracked teeth, as indicated,
clockwise from top.




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356. Figure 10. Data at 6700 Hz resonance from 7 samples.

Results of this evaluation showed that the flawed samples could be reliably sorted from the good
parts. Of note, one of the “good” samples had significant shifts in resonances, indicating the
presence of structural defects. (This is common, often related to the inability of visual scanning
techniques such as magnetic particle testing to detect internal flaws.) Given the volumetric,
whole-part testing by RAM, this part was successfully sorted and contained where subjective,
visual inspection failed.

The resulting template configured for this sprocket was implemented successfully in production,
with millions of parts reliably tested per year. Prior to RAM NDT, the facility was scrapping 6-
8% of production parts and still had field failures returned by their customers, all while trying to
keep up with 100% inspection via magnetic particle testing. RAM NDT reduced this scrap rate to
under 2% by eliminating false rejects (for example, a part that has a flaw indication on its surface
yet is structurally sound.) Additionally, and more importantly, RAM NDT has prevented any
defective parts from shipping to customers.


6. Conclusion


The RAM NDT technique serves quality minded manufacturers who are dissatisfied with visual
detection techniques such as magnetic particle, liquid penetrant, or X-ray, which are time
consuming, costly and subjective. RAM NDT allows for simple integration of a turnkey system
that is a reliable, fully automated method for quality control and process improvement. This
rapidly growing technique creates an economical, on-line inspection system that provides for zero
defect product supply. Unlike previous implementation of resonant inspection which are
excessively complicated and costly to automate, RAM NDT is fast, simple and reliable, and
easily re-configurable. As a result, powdered metal and casting manufacturers around the world
have proven the benefits of RAM NDT resonant inspection over their traditional inspection
techniques.


7. References


[1] Juran, Joseph M. and Godfrey, A. Blanton, Fifth Edition, Juran’s Quality Handbook,
McGraw-Hill.
[2] ASTM E1444-01 Standard Practice for Magnetic Particle Examination.
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357. [3] ASTM E309-95 Standard Practice for Eddy-Current Examination.
[4] ASTM B594-02 Standard Practice for Ultrasonic Inspection Examination.
[5] ASTM F1467-99 Standard Guide for Use of an X-Ray Tester.
[6] ASTM E2001-98 Standard Guide for Resonant Ultrasound Spectroscopy for Defect
Detection.
[7] ASTM D4086-92a (1997) e1 Standard Practice for Visual Evaluation.
[8] ASTM E165-02 Standard Test Method for Liquid Penetrant Examination.
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