Chapter 8. Mechanical Failure - Failure mechanisms

Chapter 8. Mechanical Failure

1. Chapter 8 -1
ISSUES TO ADDRESS...
• How do flaws in a material initiate failure?
• How is fracture resistance quantified; how do different
material classes compare?
• How do we estimate the stress to fracture?
• How do loading rate, loading history, and temperature
affect the failure stress?
Ship-cyclic loading
from waves.
Computer chip-cyclic
thermal loading.
Hip implant-cyclic
loading from walking.
Chapter 8: Mechanical Failure

2. Chapter 8 -
Failure mechanisms
1.Fracture
2.Fatigue
3.Creep
4.Corrosion
5.Buckling
6.Melting
7.Thermal shock
8.Wear
2

3. Chapter 8 -3
1. Fracture mechanisms
•Ductile fracture
–Occurs with plastic deformation
•Brittle fracture
–Little or no plastic deformation
–Suddenly and catastrophic
Fractureis the separation of a body into two or more pieces in
response to an imposed stress that is static (i.e., constant or slowly
changing with time) and at temperatures that are low relative to the
melting temperature of the material.
The applied stress may be tensile, compressive, shear, or torsional;
•Any fracture process involves two steps—crack formation and
propagation—in response to an imposed stress

4. Chapter 8 -4
Ductile vs Brittle Failure
Very
Ductile
Moderately
Ductile
Brittle
Fracture
behavior:
LargeModerate%ARor %EL Small
• Ductile
fracture is usually
desirable!
• Classification:
Ductile:
1.warning before
fracture
2.More strain
energy is required
Brittle:
No
warning
Ductility is a function of
temperature of the material,
the strain rate, and the stress
state. May be quantified in
term of %AR or %EL

5. Chapter 8 -5
• Ductilefailure:
--one piece
--large deformation
Example: Failure of a Pipe
• Brittlefailure:
--many pieces
--small deformation

6. Chapter 8 -6
• Evolution to failure:( stages in the cup-cone fracture)
• Resulting
fracture
surfaces
(steel)
50 mm
particles
serve as void
nucleation
sites.
50 mm
100 mm
Moderately Ductile Failure
necking
s
void
nucleation
void growth
and linkage
shearing
at surface
fracture

7. Chapter 8 -7
Ductile vs. Brittle Failure
cup-and-cone fracture in aluminum brittle fracture in a mild steel
Metal alloys are ductile
Ceramicsare notably brittle
Polymersmay exhibit both types of fracture.

8. Chapter 8 -8
Brittle Failure
Arrows indicate point at which failure originated
Brittle fracture takes place without any appreciable deformation,
and by rapid crack propagation.

9. Chapter 8 -9
• Inter-granular
(the fracture cracks pass
through the grains)
• Intra-granular
(crack propagation is
along grain boundaries)
Al Oxide
(ceramic)
316 S. Steel
(metal)
304 S. Steel
(metal)
Polypropylene
(polymer)
3mm
4mm
160mm
1mm
Brittle Fracture Surfaces

10. Chapter 8 -
Principles of fracture mechanics
1.stress concentration
2.Fracture toughness
3.Design using fracture mechanics
4.Impact fracture testing
10
Thissubjectallowsquantificationoftherelationshipsbetween
materialproperties,stresslevel,thepresenceofcrack-
producingflaws,andcrackpropagationmechanisms.

11. Chapter 8 -11
Flaws are Stress Concentrators!
Results from crack propagation
•Griffith Crack
where

t= radius of curvature
s
o= applied stress=F/A
s
m= stress at crack tipot
/
t
om
K
a
s









ss
21
2

t
Stress Concentration
Flaws: are called stress raiser

12. Chapter 8 -12
Concentration of Stress at Crack Tip
•Stressamplificationisnotrestricted
tothesemicroscopicdefects;itmay
occuratmacroscopicinternal
discontinuities(e.g.,voids),atsharp
corners,andatnotchesinlarge
structures.
•Theeffectofastressraiserismore
significantinbrittlethaninductile
materials.Foraductilematerial,
plasticdeformationensueswhenthe
maximumstressexceedstheyield
strength.Thisleadtoamoreuniform
distributionofstressinthevicinityof
thestressraiser.Suchyieldingand
stressredistributiondonotoccurto
anyappreciableextentaroundflaws
anddiscontinuitiesinbrittle
materials;therefore,essentiallythe
theoreticalstressconcentrationwill
result.

13. Chapter 8 -13
Engineering Fracture Design
r/h
sharper fillet radius
increasing
w/h
0 0.5 1.0
1.0
1.5
2.0
2.5
Stress Conc. Factor, K
t
s
max
s
o
=
• Avoid sharp corners!
s
r,
fillet
radius
w
h
o
s
max
It is a measure of
the degree to which
an external stress is
amplified at
the tip of a crack.

14. Chapter 8 -14
Crack Propagation
Cracks propagate due to sharpness of crack tip
•A plastic material deforms at the tip, “blunting” the
crack.
deformed
region
brittle
Energy balance on the crack
•Elastic strain energy-
•energy stored in material as it is elastically deformed
•this energy is released when the crack propagates
•creation of new surfaces requires energy
plastic

15. Chapter 8 -15
When Does a Crack Propagate?
Crack propagates if applied stress is above
critical stress s
c( it is the stress required for
crack propagation in a brittle materials)
where
–E = modulus of elasticity
–
s= specific surface energy
–a= one half length of internal crack
–K
c= s
c/s
0
For ductile => replace 
sby 
s+ 
p
where 
pis plastic deformation energy21
2
/
s
c
a
E








s
i.e., s
m>s
c
orK
t> K
c

16. Chapter 8 -
problem
16

17. Chapter 8 -
Design Against Crack Growth
•Relationship between critical stress for
crack propagation (σc) to crack length (a)
17
-Is fracture toughness, a property that is a measure of a
material’s resistance to brittle fracture when a crack is present.
Y -is a dimensionless parameter or function that depends on both
crack andspecimensizesand geometries, as well as the
manner of load application.( Y = 1 –1.1)

18. Chapter 8 -18
Fracture Toughness
Based on data in Table B5,
Callister 7e.
Composite reinforcement geometry is: f
= fibers; sf = short fibers; w = whiskers;
p = particles. Addition data as noted
(vol. fraction of reinforcement):
1. (55vol%) ASM Handbook, Vol. 21, ASM Int.,
Materials Park, OH (2001) p. 606.
2. (55 vol%) Courtesy J. Cornie, MMC, Inc.,
Waltham, MA.
3. (30 vol%) P.F. Becher et al., Fracture
Mechanics of Ceramics, Vol. 7, Plenum Press
(1986). pp. 61-73.
4. Courtesy CoorsTek, Golden, CO.
5. (30 vol%) S.T. Buljan et al., "Development of
Ceramic Matrix Composites for Application in
Technology for Advanced Engines Program",
ORNL/Sub/85-22011/2, ORNL, 1992.
6. (20vol%) F.D. Gace et al., Ceram. Eng. Sci.
Proc., Vol. 7 (1986) pp. 978-82.
Graphite/
Ceramics/
Semicond
Metals/
Alloys
Composites/
fibers
Polymers
5
K
Ic
(MPa · m
0.5
)
1
Mg alloys
Al alloys
Ti alloys
Steels
Si crystal
Glass-soda
Concrete
Si carbide
PC
Glass
6
0.5
0.7
2
4
3
10
20
30
<100>
<111>
Diamond
PVC
PP
Polyester
PS
PET
C-C(|| fibers)
1
0.6
6
7
40
50
60
70
100
Al oxide
Si nitride
C/C( fibers)
1
Al/Al oxide(sf)
2
Al oxid/SiC(w)
3
Al oxid/ZrO2(p)
4
Si nitr/SiC(w)
5
Glass/SiC(w)
6
Y2O3/ZrO2(p)
4
K
IC -plane strain
fracture toughness

19. Chapter 8 -
The modes of crack surface
displacement
19
Thethreemodesofcracksurfacedisplacement.(a)ModeI,
openingortensilemode;(b)modeII,slidingmode;and(c)
modeIII,tearingmode.

20. Chapter 8 -20
• Crack growth condition:
• Largest, most stressedcracks grow first!
Design Against Crack Growth
K≥ Kc= aY
c
s
--Result 1:Max. flaw size
dictates design stress.max
c
design
aY
K

s
s
a
max
no
fracture
fracture
--Result 2:Design stress
dictates max. flaw size.2
1








s

design
c
max
Y
K
a
a
max
s
no
fracture
fracture

21. Chapter 8 -21
Plane strain fracture toughnessck
1 aYK
c
s
1 ck
1
Exist when specimen thickness is much greater than the crack dimensions,Kc
becomes independent of thickness; under these conditions a condition of
plane strain exists. By plane strain we mean that when a load operates on a
crack there is no strain component perpendicular to the front and back faces.
The Kc value for this thick-specimen situation is known as the plane strain
fracture toughness KIc
TheplanestrainfracturetoughnessKIcisafundamentalmaterial
propertythatdependsonmanyfactors,themostinfluentialofwhich
aretemperature,strainrate,andmicrostructure.ThemagnitudeofKIc
diminisheswithincreasingstrainrateanddecreasingtemperature.
increaseswithreductioningrainsize

22. Chapter 8 -22
• Two designs to consider...
Design A
--largest flaw is 9 mm
--failure stress = 112 MPa
Design B
--use same material
--largest flaw is 4 mm
--failure stress = ?
• Key point: Yand Kcare the same in both designs.
Answer:MPa 168)(
Bs
c
• Reducing flaw size pays off!
• Material has Kc= 26 MPa-m
0.5
Design Example: Aircraft Wing
• Use...max
c
c
aY
K

s

s
c
a
max
A
s
c
a
max
B
9 mm112 MPa 4 mm
--Result:

23. Chapter 8 -23
Loading Rate
• Increased loading rate...
--increases syand TS
--decreases %EL
• Why?An increased rate
gives less time for
dislocations to move past
obstacles.
s
e
s
y
s
y
TS
TS
larger
e
smaller
e

24. Chapter 8 -24
Impact Testing
final height initial height
• Impact loading:
--determine the fracture properties of
materials
--determine DBTT or not for materials
(Charpy)
(Izod)

25. Chapter 8 -25
• Ductile-to-Brittle Transition Temperature (DBTT)...
Temperature
BCC metals (e.g., iron at T< 914°C)
Impact Energy
Temperature
High strength materials (s
y> E/150)
polymers
More DuctileBrittle
Ductile-to-brittle
transition temperature
FCC metals (e.g., Cu, Ni)
DBTT–is the material a ductile-to-brittle transition with decreasing
temperature and, if so, the range of temperatures over which it occur
For steel

26. Chapter 8 -26
Structuresconstructedfromalloysthatexhibitthisductile-to-
brittlebehaviorshouldbeusedonlyattemperaturesabovethe
transitiontemperature,toavoidbrittleandcatastrophic
failure.Classicexamplesofthistypeoffailureoccurred,with
disastrousconsequences,duringWorldWarIIwhenanumber
ofweldedtransportships,awayfromcombat,suddenlyand
precipitouslysplitinhalf.Thevesselswereconstructedofa
steelalloythatpossessedadequateductilityaccordingtoroom-
temperaturetensiletests.Thebrittlefracturesoccurredat
relativelylowambienttemperatures,atabout4C(40F),inthe
vicinityofthetransitiontemperatureofthealloy.Eachfracture
crackoriginatedatsomepointofstressconcentration,probably
asharpcornerorfabricationdefect,andthenpropagated
aroundtheentiregirthoftheship

27. Chapter 8 -27
• Pre-WWII: The Titanic • WWII: Liberty ships
• Problem: Used a type of steel with a DBTT ~ Room temp.
Design Strategy:
Stay Above The DBTT!

28. Chapter 8 -28
2. Fatigue
• Fatigue= failure under dynamics and fluctuating stress(cyclic).
• Stress varies with time.
--key parameters are S, sm, and
frequency
s
max
s
min
s
time
s
m
S
• Key points: Fatigue...
--can cause part failure, even though smax< sc.
--causes ~ 90% of mechanical engineering failures.
tension on bottom
compression on top
counter
motor
flex coupling
specimen
bearing bearing

29. Chapter 8 -29
Mean stress
Range of stress
Stress amplitude
Stress ratio

30. Chapter 8 -30
• Fatigue limit, Sfat:
--no fatigue if S< Sfat
Fatigue Design Parameters
S-N curve
S
fat
case for
steel(typ.)
N= Cycles to failure
10
3
10
5
10
7
10
9
unsafe
safe
S= stress amplitude
• Sometimes, the
fatigue limit is zero!
case for
Al(typ.)
N= Cycles to failure
10
3
10
5
10
7
10
9
unsafe
safe
S= stress amplitude

31. Chapter 8 -31
The important parameters that characterize a material’s fatigue
behavior are
1.fatigue limit: This fatigue limit represents the largest value
of fluctuating stress that will not cause failure for essentially
an infinite number of cycles(below which fatigue failure will
not occur.( most nonferrous alloys (Al,Cu,…)don’t have FL
2.fatigue lifeNf. It is the number of cycles to cause failure at a
specified stress level, as taken from the S–N plot

32. Chapter 8 -
Fatigue Mechanism
•Theprocessoffatiguefailureischaracterizedbythree
distinctsteps:(1)crackinitiation,whereinasmallcrack
formsatsomepointofhighstressconcentration;
(2)crackpropagation,duringwhichthiscrackadvances
incrementallywitheachstresscycle;and(3)finalfailure,
whichoccursveryrapidlyoncetheadvancingcrackhas
reachedacriticalsize.
•Cracksassociatedwithfatiguefailurealmostalways
initiate(ornucleate)onthesurfaceofacomponentatsome
pointofstressconcentration.Cracknucleationsites
includesurfacescratches,sharpfillets,keyways,threads,
dents,andthelike.Inaddition,cyclicloadingcanproduce
microscopicsurfacediscontinuitiesresultingfrom
dislocationslipstepswhichmayalsoactasstressraisers,
andthereforeascrackinitiationsites.
32

33. Chapter 8 -33
• Crack grows incrementally
typ. 1 to 6( constant)a~s
increase in crack length per loading cycle
• Failed rotating shaft
--crack grew even though
Kmax< Kc
--crack grows faster as
• sincreases
• crack gets longer
• loading freq. increases.
crack origin
Fatigue Mechanism
m
K
dN
da


34. Chapter 8 -
Factors that affect fatigue life
1.Mean stress
2.Surface effects
3.Environmental effects
34

35. Chapter 8 -35
1. Mean stress
Thedependenceoffatiguelifeonstressamplitudeis
representedontheS–Nplot.increasingthemeanstress
levelleadstoadecreaseinfatiguelife

36. Chapter 8 -36
2. Surface effect
Formanycommonloadingsituations,themaximum
stresswithinacomponentorstructureoccursatitssurface.
consequently,mostcracksleadingtofatiguefailureoriginate
atsurfacepositions,specificallyatstressamplificationsites
•Designfactor:Anynotchorgeometricaldiscontinuitycanact
asastressraiserandfatiguecrackinitiationsite;thesedesign
featuresincludegrooves,holes,keyways,threadsandsoon.
•Surfacetreatment:1.improvingthesurfacefinishby
polishingwillenhancefatiguelifesignificantly2.imposing
residualcompressivestresseswithinathinoutersurfacelayer
•Case(layer)hardening:isatechniquewherebybothsurface
hardnessandfatiguelifeareenhancedforsteelalloys.Thisis
accomplishedbyacarburizingornitridingprocesswherebya
componentisexposedtoacarbonaceousornitrogenous
atmosphereatanelevatedtemperature.

37. Chapter 8 -
Environmental effects
1.thermalfatigue:isnormallyinducedatelevated
temperaturesbyfluctuatingthermalstresses;mechanical
stressesfromanexternalsourceneednotbepresent.Theoriginof
thesethermalstressesistherestrainttothedimensional
expansionand/orcontractionthatwouldnormallyoccurina
structuralmemberwithvariationsintemperature.The
magnitudeofathermalstressdevelopedbyatemperature
changeTisdependentonthecoefficientofthermalexpansionl
andthemodulusofelasticityEaccordingto
•
37
Influence of stress and temperature T on creep behavior.

38. Chapter 8 -38
2.Corrosion fatigue: Failure that occurs by the
simultaneous action of a cyclic stress and chemical attack
•Small pits may form as a result of chemical reactions between
the environment and material, which serve as points of stress
concentration, and therefore as crack nucleation sites.
•Severalapproaches to corrosion fatigue prevention
exist:
-apply protective surface coatings,
-select a more corrosion resistant material
-reduce the corrosiveness of the environment.

39. Chapter 8 -39
Improving Fatigue Life
1. Impose a compressive
surface stress
(to suppress surface
cracks from growing)
N = Cycles to failure
moderate tensiles
m
Larger tensiles
m
S = stress amplitude
near zero or compressive s
m
Increasing
s
m
--Method 1: shot peening
put
surface
into
compression
shot
--Method 2: carburizing
C-rich gas
2. Remove stress
concentrators.
bad
bad
better
better

40. Chapter 8 -
3. Creep
•The time-dependent permanent deformation
that occurs when material are subjected to a
constant load or stress; for most materials it
is important only at elevated temperatures.
•For metals it becomes important only for
temperatures greater than about 0.4Tm (Tm
absolute melting temperature).
40

41. Chapter 8 -41
Creep
Sample deformation at a constant stress (s) vs. time
Primary Creep: slope (creep rate)
decreases with time.
Secondary Creep: steady-state
i.e., constant slope.
Tertiary Creep: slope (creep rate)
increases with time,i.e.acceleration of rate.
s
s,e
0 t

42. Chapter 8 -42
• Occurs at elevated temperature, T> 0.4 Tm
Creep
elastic
primary
secondary
tertiary

43. Chapter 8 -43

44. Chapter 8 -44
• Strain rate is constant at a given T, s
--strain hardening is balanced by recovery
stress exponent (material constant)
strain rate
activation energy for creep
(material constant)
applied stressmaterial const.





se
RT
Q
K
cn
s exp
2

Secondary Creep
With either increasingstressor temperature, the following
will be noted:
(1)the instantaneous strain at the time of stress
application increases;
(2)the steady-state creep rate is increased;
(3) the rupture lifetime is diminished.

45. Chapter 8 -45
• Strain rate
increases
for higher T, s
10
20
40
100
200
10
-2
10
-1
1
Steady state creep rate (%/1000hr)e
s
Stress (MPa)
427°C
538°C
649°C
Stress (logarithmic scale) versus steady-state creep rate
(logarithmic scale) for a low carbon–nickel alloy at three temperatures.
Stress (logarithmic scale) versus
rupture lifetime (logarithmic
scale) for a low carbon–nickel alloy
at three temperatures

46. Chapter 8 -46
Creep Failure
• EX. Estimate rupture time
S-590 Iron, T= 800°C, s= 20 ksi
• Failure:
along grain boundaries.
time to failure (rupture)
function of
applied stress
temperatureL)t(T
rlog20
applied
stress
g.b. cavities
• Time to rupture, tr
From V.J. Colangelo and F.A. Heiser, Analysis of
Metallurgical Failures(2nd ed.), Fig. 4.32, p. 87, John
Wiley and Sons, Inc., 1987. (Orig. source: Pergamon
Press, Inc.)L)t(T
rlog20
1073K
Ans: tr= 233 hr
24x10
3K-log hrL(10
3
K-log hr)
Stress, ksi
100
10
1
12 20242816
data for
S-590 Iron
20

47. Chapter 8 -
ALLOYS FOR HIGH-TEMPERATURE
USE
47
Thereaseveralfactorsthataffectthecreepcharacteristicsof
metals.Theseincludemeltingtemperature,elasticmodulus,and
grainsize.Ingeneral,thehigherthemeltingtemperature,the
greatertheelasticmodulus,andthelargerthegrainsize,thebetter
isamaterial’sresistancetocreep.
Stainlesssteels,therefractorymetalsandthesuperalloysare
especiallyresilienttocreepandarecommonlyemployedinhigh-
temperatureserviceapplications.Thecreepresistanceofthecobalt
andnickelsuperalloysisenhancedbysolid-solutionalloying,
andalsobytheadditionofadispersedphasewhichisvirtually
insolubleinthematrix.Inaddition,advancedprocessing
techniqueshavebeenutilized;onesuchtechniqueisdirectional
solidification,whichproduceseitherhighlyelongatedgrainsor
single-crystal

48. Chapter 8 -48

49. Chapter 8 -
4. Corrosion
•Corrosionisbreakingdown!ofessential
propertiesinamaterialduetoreactions
withitssurroundings.Inthemost
commonuseoftheword,thismeansa
lossofanelectronofmetalsreactingwith
waterandoxygen
•Weakeningofironduetooxidationofthe
ironatomsisawell-knownexampleof
electrochemicalcorrosion.Thisis
commonlyknownasrustThistypeof
damageusuallyaffectsmetallicmaterials,
andtypicallyproducesoxide(s)and/or
salt(s)oftheoriginalmetal 49

50. Chapter 8 -50
Rust, the most familiar
example of corrosion
--Moststructuralalloyscorrodemerelyfromexposureto
moistureintheair,buttheprocesscanbestronglyaffectedby
exposuretocertainsubstances.Corrosioncanbeconcentrated
locallytoformapitorcrack,oritcanextendacrossawide
areatoproducegeneraldeterioration

51. Chapter 8 -
Resistant to corrosion
1.Intrinsic chemistry:
Thematerialsmostresistanttocorrosionarethose
forwhichcorrosionisthermodynamically
unfavorable.Anycorrosionproductsofgoldor
platinumtendtodecomposespontaneouslyinto
puremetal,whichiswhytheseelementscanbe
foundinmetallicformonEarth,andisalarge
partoftheirintrinsicvalue
51
GOLD nuggets do not
corrode, even on a
geological time scale.

52. Chapter 8 -52
2.Passivation:
Giventherightconditions,athinfilmofcorrosionproducts
canformonametal'ssurfacespontaneously,actingasa
barriertofurtheroxidation.Whenthislayerstopsgrowing
atlessthanamicrometrethickundertheconditionsthata
materialwillbeusedin,thephenomenonisknownas
passivation
Passivation in air and water is seen in such materials as
aluminum, stainless steel, titanium, and silicon

53. Chapter 8 -53
3.surface treatment ( coating ):
Plating, painting, and the application of enamelare the
most common anti-corrosion treatments. They work by
providing a barrier of corrosion-resistant material between
the damaging environment and the (often cheaper, tougher,
and/or easier-to-process) structural material
Example: chromium on steel

54. Chapter 8 -
5. Buckling
•Inengineering,bucklingisafailuremode
characterizedbyasuddenfailureofastructural
member subjectedtohighcompressive
stresses,wheretheactualcompressive
stressesatfailurearesmallerthantheultimate
compressivestressesthatthematerialis
capableofwithstanding.Thismodeoffailureis
alsodescribedasfailureduetoelastic
instability
54

55. Chapter 8 -55
Buckling in columns
•A column under a centric axial load exhibiting the
characteristic deformation of buckling
•The eccentricity of the axial force results in a bending moment
acting on the beam element

56. Chapter 8 -56
Euler formula that gives the maximum axial load ( critical load)
that column can carry without buckling
F= maximum or critical force (vertical load on column),
E= modulus of elasticity,
I= area moment of inertia,
l= unsupported length of column,
K= column effective length factor, whose value depends on the
conditions of end support of the column, as follows.
For both ends pinned (hinged, free to rotate), K= 1.0.
For both ends fixed, K= 0.50.
For one end fixed and the other end pinned, K= 0.70.
For one end fixed and the other end free to move laterally, K= 2.0.

57. Chapter 8 -
6. Melting
•Meltingisaprocessthatresultsinthe
phasechangeofasubstancefromasolid
toaliquid.Theinternalenergyofasolid
substanceisincreased(typicallybythe
applicationofheat)toaspecific
temperature(calledthemeltingpoint)at
whichitchangestotheliquidphase.An
objectthathasmeltedcompletelyismolten
•Themeltingpointofasubstanceisequal
toitsfreezingpoint
57

58. Chapter 8 -58
•Molecular vibrations
Whentheinternalenergyofasolidisincreasedbytheapplication
ofanexternalenergysource,themolecularvibrationsofthesubstance
increases.Asthesevibrationsincrease,thesubstancebecomesmore
andmoredisordered
•Constant temperature
Substancesmeltataconstanttemperature,themeltingpoint.
Furtherincreasesintemperature(evenwithcontinuedapplicationof
energy)donotoccuruntilthesubstanceismolten

59. Chapter 8 -
7. Thermal chock
•Thermalshockisthenamegiventocrackingas
aresultofrapidtemperaturechange.Glassand
ceramicobjectsareparticularlyvulnerabletothis
formoffailure,duetotheirlowtoughness,low
thermalconductivity,andhighthermalexpansion
coefficients
•Thermalshockoccurswhenathermalgradient
causesdifferentpartsofanobjecttoexpandby
differentamounts.Thisdifferentialexpansioncan
beunderstoodintermsofstressorofstrain,
equivalently.Atsomepoint,thisstress
overcomesthestrengthofthematerial,causinga
cracktoform.Ifnothingstopsthiscrackfrom
propagatingthroughthematerial,itwillcausethe
object'sstructuretofail
59

60. Chapter 8 -60
Thermal shock can be prevented by:
1.Reducing the thermal gradient seen by the object, by
a)changing its temperature more slowly
b)increasing the material's thermal conductivity
2.Reducing the material's coefficient of thermal expansion
3.Increasing its strength
4.Increasing its toughness, by
a)crack tip blunting, i.e., plasticity or phase transformation
b)crack deflection

61. Chapter 8 -61
Example.
BorosilicateglasssuchasPyrexismadetowithstand
thermalshockbetterthanmostotherglassthrougha
combinationofreducedexpansioncoefficientandgreater
strength,thoughfusedquartzoutperformsitinboththese
respects.Someglass-ceramicmaterialsincludeacontrolled
proportionofmaterialwithanegativeexpansion
coefficient,sothattheoverallcoefficientcanbereducedto
almostexactlyzerooverareasonablywiderangeof
temperatures

62. Chapter 8 -
8. wear
•Wear is the erosion of material from a solid
surface by the action of another solid, or
itisaprocessinwhichinteractionofsurface(s)or
boundingface(s)ofasolidwiththeworking
environmentresultsinthedimensionallossofthe
solid,withorwithoutlossofmaterial
•Wearenvironmentincludesloads(typesinclude
unidirectionalsliding,reciprocating,rolling,
impact),speed,temperatures,counter-bodies(solid,
liquid,gas),typesofcontact(singlephaseor
multiphaseinwhichphasesinvolvedcanbeliquid
plussolidparticlesplusgasbubbles)
62

63. Chapter 8 -
principal wear processes
•There are four principal wear processes:
a.Adhesive wear
b.Abrasive wear
c.Corrosive wear
d.Surface fatigue
63

64. Chapter 8 -64
a. Adhesive wear
Adhesivewearisalsoknownasscoring,galling,orseizing.It
occurswhentwosolidsurfacesslideoveroneanotherunder
pressure.Surfaceprojections,orasperities,areplastically
deformedandeventuallyweldedtogetherbythehighlocal
pressure.Asslidingcontinues,thesebondsarebroken,
producingcavitiesonthesurface,projectionsonthesecond
surface,andfrequentlytiny,abrasiveparticles,allofwhich
contributetofuturewearofsurfaces
b. Abrasive wear
Whenmaterialisremovedbycontactwithhardparticles,
abrasivewearoccurs.Theparticleseithermaybepresentatthe
surfaceofasecondmaterialormayexistaslooseparticles
betweentwosurfaces

65. Chapter 8 -65
c. Corrosive wear
Oftenreferredtosimplyas“corrosion”,corrosivewearis
deteriorationofusefulpropertiesinamaterialdueto
reactionswithitsenvironment
d. Surface fatigue
Surfacefatigueisaprocessbywhichthesurfaceofa
materialisweakenedbycyclicloading,whichisonetypeof
generalmaterialfatigue

66. Chapter 8 -66
• Engineering materials don't reach theoretical strength.
• Flawsproduce stress concentrationsthat cause
premature failure.
• Sharp corners produce large stress concentrations
and premature failure.
• Failure type depends on Tand stress:
-for noncyclic sand T< 0.4Tm, failure stress decreases with:
-increased maximum flaw size,
-decreased T,
-increased rate of loading.
-for cyclic s:
-cycles to fail decreases as sincreases.
-for higher T(T > 0.4Tm):
-time to fail decreases as sor Tincreases.
SUMMARY

67. Chapter 8 -67
• Stress-strain behavior (Room T):
Ideal vs Real Materials
TS << TS
engineering
materials
perfect
materials
s
e
E/10
E/100
0.1
perfect mat’l-no flaws
carefully produced glass fiber
typical ceramic typical strengthened metal
typical polymer
• DaVinci (500 yrs ago!) observed...
--the longer the wire, the
smaller the load for failure.
• Reasons:
--flaws cause premature failure.
--Larger samples contain more flaws!