Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials

2. MECHANICAL PROPERTIES OF
DENTAL MATERIALS

Dr. Nithin Mathew

3. CONTENTS

+ Introduction + Mechanical Properties based on

+ Force Elastic Deformation
+ Stress + Young's Modulus / Modulus of Elasticity
+ Tensile + Dynamic Young’s Modulus
+ Compressive + Shear Modulus
+ Shear + Flexibility
+ Flexural + Resilience
+ Strain + Poisson's Ratio

+ Elastic & Plastic Deformation
+ Stress - Strain Curve

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

4. + Strength Properties + Mechanical Properties based on
Plastic Deformation

+ Flexural Strength
+ Impact Strength
+ Toughness

+ Proportional Limit

+ Elastic Limit

+ Yield Strength

+ Ultimate tensile strength,

shear strength, compressive + Fracture toughness
strength & flexural strength « Brittleness

+ Ductility

+ Malleability

+ Hardness

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

5. + Hardness tests
+ Brinell
+ Rockwell
+ Vickers
+ Knoops

+ Stress Concentration Effects
+ Methods To Minimize Stress Concentration
+ Conclusion

+ References

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

6. INTRODUCTION

+ In the oral environment restorative materials and dental appliances are exposed to
chemical, thermal and mechanical challenges.

+ These challenges can cause deformation of materials.

+ The mechanical properties of a material define how materials respond to
mechanical challenges.

+ Mechanical properties are defined by the laws of mechanics.

ie It is the physical science that deals with energy, forces and their effects on the
bodies.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

7. + So it is necessary to understand the principles involved in a variety of mechanical
properties to optimise the clinical service of a material.

+ Mechanical properties are measured responses, both elastic (reversible on force
removal) and plastic (irreversible on force removal), of materials under an applied
force or distribution of forces.

+ They are expressed most often in units of stress and strain.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

8. - They can represent measurements of

+ Elastic deformation (reversible)
+ Proportional limit
Resilience
+ Modulus of elasticity

+ Plastic deformation (irreversible)
+ Percentage of elongation

+ Combination of both
+ Toughness
+ Yield strength

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

9. FORCE

* In physics, a force is any influence that causes an object to undergo a certain
change, either concerning its movement, direction, or geometrical construction.

+ A force is defined by 3 characteristics:
+ Point of application
+ Magnitude

+ Direction of application

+ The SI unit of force is Newton (N).

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

10. Occlusal Forces
« Max. occlusal forces: 200 - 3500N

* Occlusal forces between adult teeth are highest in the posterior region closest to the
mandibular hinge axis and decrease from the molar to the incisors.

+ Forces on first and second molars vary from 400 to 800N.

+ Average on bicuspids, cuspids and incisors is about 300, 200 and 150N.

» Increase in force from 235 - 494N in growing children with an average yearly
increase of about 22N.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 10

11. STRESS & STRAIN

Stress

+ The force per unit area acting on millions of atoms or molecules in a given
plane of a material.

+ Force acting per unit area.

+ Unit of measurement is Megapascal (Mpa).

+ Stress is the internal resistance of a material to an external load applied on
that material.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 11

12. Type Of Stress Produced By Examples

Residual Stress caused within the material during the During welding

Stress manufacturing process

Structural Stresses produced in the structure during function. In abutments of fixed partial

Stress Weights they support provide the loadings denture

Pressure Induced in vessels containing pressurized materials In dentures during processing

Stress under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the Molten metal alloy striking the
load walls of the mould during casting

Thermal Material is subjected to internal stress due to different | Materials that undergo thermal

Stress temperatures causing varying expansions in the stress such as inlay wax, soldering
material and welding alloys

Fatigue Stress | Stress caused due to cyclic rotation of a material Rotary instruments undergo

rotational or cyclic fatigue

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

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13. + By means of the direction of force, stresses can be classified as:

+ Tensile stress
+ Compressive stress
+ Shear stress

+ Flexural stress

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 13

14. Tensile Stress

+ Tensile stress occurs when 2 sets of forces are directed away from each other in the
same straight line.

+ Also when one end is constrained and the other end is subjected to a force away
from the constraint.

+ Itis caused by a load that tends to stretch or elongate a body.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 14

15. » In fixed prosthodontics, a sticky candy (Jujube) can be used to
remove crowns by means of a tensile force when the patient
tries to open the mouth after the candy has mechanically
bonded to opposing teeth or gums.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 15

16. Compressive Stress

+ Compressive stress occurs when 2 sets of forces are directed towards each other in
the same straight line.

» Also when one end is constrained and the other end is subjected to a force towards
the constraint.

+ It is caused by a load that tends to compress or shorten a body.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 16

17. Shear Stress

+ Shear stress occurs when 2 sets of forces are directed parallel to each other but not
along the same straight line.

+ Ashear stress tends to resist the sliding of one portion of a body over another.

+ Shear stress can also be produced by a twisting or
torsional action on a material.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 17

18. + Eg: If a force is applied along the surface of tooth enamel by a sharp-edged
instrument parallel to the interface between the enamel and the orthodontic
bracket, the bracket may debond by shear stress failure of the resin luting agent.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 18

19. In the oral cavity, shear failure is unlikely to occur due to:

1. Many of the brittle materials in restored tooth surfaces generally have rough,
curved surfaces.

2. The presence of chamfers, bevels or changes in curvature of a bonded tooth
surface.

3. To produce shear failure, the applied force must be located immediately adjacent
to the interface. The farther away from the interface the load is applied, more likely
that it is a tensile failure.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

19

20. 5. Since the tensile strength of brittle materials is usually well below their shear
strength values, tensile failure is more likely to occur.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 20

21. Flexural Stress (bending)
+» Force per unit area of a material that is subjected to flexural loading (bending).
+ Ashear stress tends to resist the sliding of one portion of a body over another.

+ A flexural force can produce all the three types of stresses in a structure, but in most
cases fracture occurs due to the tensile component.

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22. + Flexural stresses produced in a three-unit fixed
dental prosthesis.

+ Flexural stresses produced in a two-unit cantilever
bridge.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

> Compression ee
Fe he tenance

22

23. + Strength:

Strength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or at
which fracture occurs in several test specimens of the same shape and
size.

=

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 23

24. + Clinical strength of brittle material (ceramics, amalgams, composites) may appear
to be low when large flaws are present or if stress concentration areas exist
because of improper design of a prosthetic component.

+ So under these conditions, such appliances may fracture
at a much lower applied force because the localized
stress exceeds the strength of the material at the critical
location of the flaw or stress concentration.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 24

25. « Elastic stress in ductile material such as gold alloys do
not cause any permanent damage.

« Plastic stresses does cause permanent deformation
and sometimes it may be high enough to produce a
fracture.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

25

26. * Since the stress in structure varies directly with the force ad inversely with area, the
area over which the force acts is an important consideration.

+ This is true when considering dental restorative materials where the area over
which the occlusal forces acts are extremely small such as the cuspal areas of
contact.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 26

27. STRAIN

+ Defined as the change in length per unit original length.

+ Strain of a material is reported as percentage(%).

+ Strain may be either elastic, plastic or a combination of both elastic and plastic.
+ Elastic strain is reversible. ie it disappears when force is removed.

+ Plastic strain represents permanent deformation of the material which never recovers
when the force is removed.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

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28. Eg:
+ A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of

0.000002 m?.

+ If the wire is 0.1m long and if it stretches 0.001m under the load,

+ ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of
0.01%

28

29. ELASTIC AND PLASTIC DEFORMATION

Ser A a-b

interface

Shear
= force

!
| Shear
Force

H "Elastic
>> shear

' strain

Elastic Shear Deformation Plastic Shear Deformation

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 29

30. STRESS - STRAIN CURVE

1800 + PL: Proportional Limit
UTS = 1625

1600

+ YS: Yield Strength

YS = 1536

1400

UTS : Ultimate Tensile Strength
1200
dl + E: Elastic Modulus

1000

YS (0.2%) = 1536 Mpa
UTS = 1625 Mpa
E = 1020/0.053 = 192 GPa

800

Stress (Mpa)

600

400

200

o

0 05 1 15
Strain (%)

thin Mathew

Mechanical Properties of Dental Materials -

31. MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

+ Mechanical properties that are measures of the elastic strain of dental materials
includes:

+ Young's Modulus / Modulus of Elasticity
+ Dynamic Young’s Modulus

+ Shear Modulus

+ Flexibility

Resilience

+ Poisson's Ratio

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

31

32. YOUNG’S MODULUS

Elastic modulus describes the relative stiffness of a material which is measured by
the slope of the elastic region of the stress strain graph.

» It is the stiffness of a material that is calculated as
the ratio of the elastic stress to elastic strain.

+ ie. a stiff material will have a high modulus of
elasticity while a flexible material will have a low
modulus of elasticity.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

Stress (Mpa)

UTS = 1625
1600
PL= 1020

00

E = 1020/0.053
=192 GPa

Strain (%)

32

33. + Elastic modulus of a tensile test specimen can be calculated as:

By definition,

Al
Stress (o)=— Strain (e)= ik

o

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 33

34. 1800

1600

1400

1200
PL=1020

1000

800

Stress (Mpa)

600

400

200

UTS = 1625

YS = 1536

E = 1020/0.053 = 192 GPa

0 0.5
Strain (%)

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

Steep Line
Higher modulus and more

rigidity

Flat Line
Lower modulus and less

rigidity

34

35. + Eg: Principle of Elastic Recovery

+ Burnishing of an open metal margin, where a dental abrasive
stone is rotated against the metal margin to close the
marginal gap as a result of elastic and plastic strain.

» After the force is removed, the metal springs back to an
amount equal to the total elastic strain.

» A final clearance of 25um is available for the cement.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

35

36. + Eg: Impression Material

+ The impression materials should have a low modulus
of elasticity to enable it to be removed from the
undercut areas in the mouth.

+ Modulus of elasticity should not be very low that the
material cannot withstand tearing.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 36

37. Hooke's Law

+ According to this law, within the limits of elasticity the strain produced by a stress
(of any one kind) is proportional to the stress.

+ The stress at which a material ceases to obey Hooke's Law is known as the limit of
proportionality.

+ Hooke's law can be expressed by the formula

+ The value of the constant depends on the material and the type of stress.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 37

38. STRESS - STRAIN CURVE : Enamel & Dentin

300

+ Dentin is capable of
sustaining significant plastic
deformation under a

CS = 234 compressive load before it

fractures.

CS = 262

PL=176

+ Enamel - more stiffer and
Epa = 11.7.GPa brittle than dentin.
Epentin = 33.6 GPa

essive Sress ( M

+ But dentin more flexible and
tougher.

Strain
—-Enamel —-Dentin

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

39. POISSON’S RATIO

+ During axial loading in tension or compression, there is a simultaneous strain in the
axial and transverse or lateral directions.

+ Under tensile loading, as a material elongates in the
direction of the load, there is a reduction in cross-
section.

+ Under compressive loading, there is an increase in the
cross-section.

Ga= FIA

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 39

40. + Within the elastic range, the ratio of the lateral to the axial strain is called the
Poisson's Ratio.

+ Poisson's ratio is a unit-less value since it is a ratio of 2 strains.

+ Most rigid materials such as enamel, dentin, amalgam, composite, etc. exhibit a
poisson’s ratio of about 0.3

+ More ductile materials such as soft gold alloys show a higher degree of reduction in
cross-sectional area and higher poisson’s ratio.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 40

41. DYNAMIC YOUNG’S MODULUS

+ Elastic modulus can be measured by a dynamic method as well as a static method.

+ The velocity at which the sound travels through a solid can be readily measured by
ultrasonic transducers and receivers.

+ The velocity of the sound wave and the density of the
material can be used to calculate the elastic modulus
and poisson’s ratio.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 4

42. + Ifa shear stress was induced instead of a uniaxial tensile or compressive stress, the
resulting shear strain could be used to define a shear modulus for the material.

+ The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and Poisson’s

Ratio (v) :

+ The value of 0.25 to 0.30 for Poisson’s ratio is typical.

« Therefore the shear modulus is usually 38% of the elastic modulus value.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 42

43. FLEXIBILITY

+ Defined as the flexural strain that occurs when the material
is stressed to its proportional limit.

» Materials used to fabricate dental appliances and
restorations, a high value for the elastic limit is a necessary
requirement.

+ This is because the structure is expected to return to its original shape after it has
been stressed and the force removed.

+ Maximum flexibility - flexural strain that occurs when the material is stressed to its
proportional limit

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 43

44. + There are instances where a large strain or deformation
may be needed with a moderate or slight stress such as in
an orthodontic appliance.

+ Here a spring is often bent a considerable distance under
the influence of a small stress.

+ In this case, the structure is said to possess the property
of flexibility.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 44

45. RESILIENCE

» It is the amount of energy per unit volume that is sustained on loading and released
upon unloading of a test specimen.

+ Term resilience is associated with springiness of a material but
it means precisely the amount of energy absorbed within a unit
volume of a structure when it is stressed to its proportional
limit.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

45

46. + Resilience of 2 or more materials can be compared by observing the areas under the

elastic region of their stress-strain graph.

+ ie The material with the longer elastic area has the higher resilience.

=
£
&
Ë
ES

PL= 1020

Strain (%)

UTS = 1625

47. + When a dental restoration is deformed during mastication, it absorbs energy.

+» If induced stress is not greater than proportional limit, the restoration is not
permanently deformed.

+ ie only elastic energy is stored in it.

+ So restorative material should exhibit a moderately high elastic modulus and
relatively low resilience - limiting the elastic strain produced.

47

48. + Chewing force on dental restoration causes Deformation
(determined by the magnitude of the induced stress)

+ Large deformation do not occur due to the proprioceptive
receptors in the periodontal ligament.

+ The pain stimulus causes the force to be decreased and
induced stress to be reduced.

+ This prevent damage to teeth or restoration.

49. STRENGTH PROPERTIES

+» Strength can be defined as the

+ Maximum stress that a structure can withstand without
sustaining a specific amount of plastic strain (yield
strength).

OR

+ Stress at the point of fracture (ultimate strength).

+ When we describe the strength of a material, we are most often referring to the
maximum stress that is required to cause fracture of the material.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 49

50. » Strength of a material can be described by one or more of the following properties:

+ Proportional Limit
+ Elastic Limit
+ Yield Strength

+ Ultimate tensile strength, shear strength, compressive strength & flexural

strength

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 50

51. PROPORTIONAL LIMIT

» Defined as the magnitude of elastic stress above which plastic deformation occurs.

100
B Cc D

90

80
+ As stress is increased, the strain is also
increased.

60

Stress (Mpa)

» Initial 0 - A portion shows that stress is
linearly proportional to strain.

+ As strain is doubled, stress is also doubled. Strain

51

52. + After the point A, stress is no longer linearly proportional to strain.

» Here, the value of stress at A is known as the proportional limit.

+ So, it can also be defined as the highest
stress at which the stress-strain curve is a
straight line.

* ie stress is linearly proportional to strain

+ Below the proportional limit, there is no
permanent deformation in a structure, ie
the object will return to its original
dimension when force is removed. Strain

52

53. + The material is elastic in nature below the proportional limit.

100
90

80

+ The region of the stress - strain curve
before the proportional limit is called the
elastic region and the region beyond is
called the plastic region.

Stress (Mpa)

10

Strain

53

54. + When a material is said to have high value of proportional limit, it indicates that the
material is more likely to withstand applied stress without permanent deformation.

° The connectors of partial dentures should have high
proportional limit.

+ Materials like Cobalt/Chromium (alloy) which has high
proportional limit is widely used for the fabrication of
connectors because they can withstand high stresses
without permanent deformation.

54

55. ELASTIC LIMIT

» Defined as the maximum stress that a material will withstand without permanent
deformation.

» For linearly elastic materials, the proportional limit and the elastic limit represents
the same stress within the structure.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 55

56. YIELD STRENGTH

» Defined as the stress at which a test specimen exhibits a specific amount of plastic
strain.

+» Itis a property that represents the stress value at which a small amount of (0.1% -
0.2%) plastic strain has occurred.

» It is a property often used to describe the stress at which the material begins to
function in a plastic manner.

+ The amount of permanent strain is referred to as the percent offset.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 56

57. » A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred

to as the percent offset.

+ The point at which at the parallel line
intersect the stress-strain curve is the yield
strength.

+» Elastic limit, proportional limit & yield
strength are defined differently but their
values are fairly close to each other in
many cases.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

Stress (Mpa)

1800

1600

1400

1200

1000

800

600

400

200

0

UTS=1625

YS = 1536

Strain (%)

57

58. + These values are important in the evaluation of dental materials because they
represent the stress at which permanent deformation begins.

« If these values are exceeded by the masticatory stresses, the restoration or
appliance may no longer function as originally designed.

+ Also, A fixed partial denture becomes permanently
deformed through the application of excessive
occlusal forces since a stress equal to or greater than
yield strength is developed.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 58

59. + In the process of shaping an orthodontic appliance or adjusting the clasp of on a
removable partial denture it is necessary to apply a stress into the structure in
excess of yield strength if the material is to be permanently bent or adapted.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 59

60. ULTIMATE STRENGTH

+ Ultimate tensile strength/stress (UTS) is defined as the maximum stress that a
material can withstand before failure in tension.

+ Ultimate compressive strength/stress (UCS) is the maximum stress that a material
can withstand in compression.

+ The ultimate strength / stress is determined by dividing the maximum load in
tension (or compression) by the original cross-sectional area of the test specimen.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 60

61. MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

+ Ifa material is deformed by the stress at a point above the proportional limit before
fracture, and upon removal of the applied force, the stress will reduce to 0, but the
plastic strain (deformation) remains.

+ Thus the object will not return to its original shape when the force is removed.

+ It remains bent, stretched or compressed, ie it becomes plastically deformed.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 61

62. COLD WORKING (Strain Hardening/Work Hardening)

+ When metals are stretched beyond the proportional limit, hardness and strength
increases at the area of deformation but their ductility decreases.

+ Repeated plastic deformation of the metal during bending of orthodontic wire can
lead to brittleness of the deformed area of the wire which may fracture on further
adjustment.

| + To minimize the risk of brittleness, is to
deform the metal in small increments so as
not to plastically deform the metal
excessively.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 62

63. FLEXURAL STRENGTH

+ Defined as the force per unit area at the instant of fracture in a test specimen
subjected to flexural loading.

« Also known as modulus of rupture.

+» Itis a strength test of a bar supported at each end
under a static load.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 63

64. FLEXURAL STRENGTH

+» For a bar with a rectangular cross-section subjected to a 3 point flexure, the flexural
strength can be calculated as

5: Max. flexural stress (Mpa)

P : Load at fracture (N)

L : Distance btw 2 supports (mm)
w : Width of specimen (mm)

t : Thickness of specimen (mm)

Loading Force
pin

Specimen

Supporting pins

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 64

65. + Most prosthesis & restoration fractures develop progressively over many stress
cycles after initiation of a crack from a critical flaw, and subsequently by
propagation of the crack until a sudden, unexpected fracture occurs.

+ This phenomenon is called fatigue failure.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 65

66. IMPACT STRENGTH
» Defined as the energy required to fracture a material under an impact force.

+ Measured using a Charpy Impact tester, where a

pendulum is released that swings down to fracture
the center of a specimen supported at both ends.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 66

67. + A moving object possesses a known kinetic energy.

» If the struck object is not permanently deformed, it stores
the energy of collision in an elastic manner.

+ This ability is due to the resiliency of the material and is
measured by the area under the elastic region of the stress-
strain curve.

+ Thus a material with low elastic modulus and high tensile strength is more
resistant to impact forces.

+ A material with low elastic modulus and low tensile strength has low impact
resistance.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 67

68. Material Elastic Modulus Tensile Strength
(Gpa) (Mpa)
Composite 117 30-90
Porcelain 40 50-100
Amalgam al 27-55
Alumina ceramic 350 - 418 120
Acrylic 3.5 60

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

68

69. TOUGHNESS

+ Itis the ability of a material to absorb elastic energy and
to deform plastically before fracturing.

+ Measured as the total area under a plot of the tensile stress
v/s strain.

urs=1625

+ It can be defined as the amount of elastic and plastic
deformation energy required to fracture a material.

PL=1020

Stress (Mpa)

+ Toughness increases with increase in strength and ductility.

+ ie. Greater the strength and higher the ductility, the greater
is the toughness.

Strain (%)

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 69

70. FRACTURE TOUGHNESS

+» It is the mechanical property that describes the resistance of brittle materials to the
propagation of flaws under an applied stress.

+ The longer the flaw, the lower is the stress needed to cause fracture. [+7
This is because the stress which would normally be supported by the
material are now concentrated at the tip of the flaw.

+ The ability of a flaw to cause fracture depends on the fracture
toughness of the material.

+ Fracture toughness is a material property and is proportional to the
energy consumed in plastic deformation.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 70

71. Enamel 0.7 - 1.3

Dentin 3.1
Amalgam 13-1.6
Ceramic 1.2-3.0
Composite 14-23
Porcelain 0.9- 1.0

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

71

72. BRITTLENESS

» It is the relative inability of a material to sustain plastic deformation before fracture
of a material occurs.

» Eg: amalgam, ceramics, composites are brittle at oral temperatures.

+ They sustain no/little plastic strain before they fracture.

+» ie a brittle material fractures at or near its proportional limit.

+ Dental materials with low or 0% elongation such as amalgam, composite, ceramics,
etc will have little or no burnishability because they have no plastic deformation
potential.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 72

73. DUCTILITY

+ It is the ability of a material to sustain a large permanent
deformation under a tensile load upto the point of fracture.

* Eg: a metal can be drawn readily into long thin wire is said to
be ductile.

+ Ductility is the relative ability of a material to be stretched
plastically at room temperature without fracturing.

+ Its magnitude can be assessed by the amount of permanent
deformation indicated by the stress-strain curve.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 73

74. Methods to determine ductility are:

+ Percent elongation after fracture
+ Reduction in area of tensile test specimens

+ Maximum number of bends performed in a cold bend test.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 74

75. MALLEABILITY

» It is the ability of a material to sustain considerable permanent deformation without
rupture under compression, as in hammering or rolling into a sheet.

* Gold is the most ductile and malleable pure metal, followed by silver.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 75

76. HARDNESS

+ It is the resistance of a material to plastic deformation which is typically produced
by an indentation force.

+ In mineralogy, the relative hardness of a material is based on its ability to resist
scratching.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

76

77. CLASSIFICATION OF HARDNESS TEST

Static Loading Macro-indentation Macrohardness
- slowly applied - Large indenter tip ->1kg load

Dynamic Loading Micro-indentation Microhardness
-rapidly applied - Small indenter tip -<1kg load

Mechanical Properties of Dental Materials - Dr. Nithin Mathew

GZ.

78. + Various hardness tests include:

+ Brinell Test

+ Rockwell Test
+ Vicker's Test
+ Knoop's Test

» Selection of the test should be done on the basis of the material being tested.

Mechanical Properties of Dental Materials - Dr. Nithin Mathew 78

79. BRINELL TEST

+ Used extensively for determining the hardness of metals and metallic materials used
in dentistry.

+ Related to the proportional limit and ultimate tensile strength.

+ The methods depends on the resistance to the penetration of a small steel ball,
typically 1.6mm diameter when subjected to a specified load.

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80. BRINELL TEST
+ Method:
+ A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested.

+ Load remains in contact with the material for a fixed
time of 30s.

+ After 30s it is then removed and the indentation

(©) measurement of impression

diameter is measured. ameter

+ Load value is then divided by area of projected surface of indentation and the
quotient is referred to as the Brinell Hardness Number (BHN).

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81. * Since the brinell test yields a relatively large indentation area, this test is good for
determining average hardness value and poor for determining very localized values.

+ Thus for a given load, the smaller the indentation, the larger is the number and
harder is the material.

Best suited for testing ductile Hardness of cold-worked materials
materials are difficult to measure

Not suitable for brittle materials

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82. ROCKWELL HARDNESS TEST

+ This test was developed as a rapid method for hardness determinations.

+ Here, instead of a steel ball, a conical diamond point is used.

+ The depth of the penetration is directly measured by a dial gauge on the instrument.

+ This test is not suitable for testing brittle materials.

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83. The value is the Rockwell Hardness
Number (RHN)

ADVANTAGES

Direct reading of the depth of
indentation

Minor load
FO Minor load+Major load =Total load
FO*F1=F

Minor load
FO

Zero Reference Line

Not suitable for brittle materials

Rapid testing time

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84. VICKER’S HARDNESS TEST

+ This test uses a square based pyramidal
indenter.

+ The impression obtained on the material is
a square.

+ The method is similar to Knoop’s and

Brinell tests. (a) Vickers indentation (b) measurement ot impress
‘agonals

+ The load value divided by the projected area of
indentation gives the Vicker’s Hardness Number
(VHN).

+ The lengths of the diagonals of the indentations are
measured and averaged.

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85. + Used for testing dental casting gold alloys as well as tooth

structure.

+ Suitable for determining the hardness of brittle materials.

ADVANTAGES

Useful for measuring the hardness of
small areas and hard materials

Material requires highly polished
surface

Longer time required to complete
the test

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86. KNOOP’S HARDNESS TEST

+ This test was developed mainly to fulfill the needs of a micro-indentation test
method.

+ Suitable for testing thin plastic or metal sheets or brittle materials where the
applied force does not exceed 35N.

+ This test is designed so that varying loads may be applied to the indenting
instrument.

+ Therefore the resulting indentation varies according to the load applied and the
nature of the material tested.

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87. Operatin
Position,

+ The impression is rhomboid in outline and the length
ofthe largest diagonal is measured.

+ The load value is then divided by the projected area to
get the Knoop’s Hardness Number (KHN)

Useful for very brittle materials or Material requires highly polished
thin sheets surface

Longer time required to complete
the test

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88. Enamel 343

Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460

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89. STRESS CONCENTRATION EFFECTS

+ The cause of strength reduction is the presence of small microscopic flaws or
microstructural defects on the surface or within the internal structure.

+ These flaws are especially critical in brittle materials in areas of tensile stress
because tensile stress tends to open cracks.

» Stress at the tips of these flaws is greatly
increased which leads to crack inititation
and broken bonds.

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90. Brittle or ductile

material

+ When a brittle or a ductile material is subjected to compressive stress,
it tends to close the crack and this stress distribution is more uniform.

+ When a ductile material is subjected to tensile force, it tends to
opening of the flaw and only plastic deformation has occurred.

| Materials - Dr. Nithin Mathew

91. » 2 important aspects of the flaws :

1. Stress intensity increases with the length of the flaw.

2. Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region.

Therefore, the surface of brittle materials such as ceramics, amalgams, etc. are
extremely important in areas subjected to tensile stress.

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92. CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION
AND METHODS TO MINIMIZE THEM

1. Surface defects such as porosity, grinding roughness.
+ Polish surface to reduce the depth of the defects.

2. Interior flaws such as voids.

+ Little can be done about the interior flaws but to
ensure highest quality of the structure or to
increase the size of the object.

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93. 3. Marked changes in contour - sharp internal line angle at
axio-pulpal line angle.

+ Design of the prosthesis should vary gradually than
abruptly.

+ Notches should be avoided
+ All internal line angles should be rounded

4. A large difference in elastic moduli or thermal expansion
coefficient across bonded surface.
+ The elastic moduli of the 2 materials should be closely
matched
+ Materials must be closely matched in their
coefficients of expansion or contraction.

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94. 5. A Hertzian load (load applied to a point on the surface of a brittle material).

+ Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large.

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95. CONCLUSION

+ While designing a dental appliance or a restorative material, it should have
adequate mechanical properties to withstand the stress and strain caused by the
forces of mastication.

+ All the methods must be employed to minimize stress concentration so that the
restorative material or the appliance is in harmony with the different types of forces
occurring in the oral cavity.

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96. REFERENCES

+ Phillip's Science of Dental Materials - 10' & 12t* Edition
+ Craig’s Restorative Dental Materials - 13' Edition
» Dental materials and their selection: William J'O Brien - 3" Edition

+ Materials Used in Dentistry: S. Mahalaxmi - 1* Edition

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