Mechanical properties are defined by the laws of mechanics i.e. the physical science dealing with forces that act on bodies and the resultant motion, deformation, or stresses that those bodies experiences. Mechanical properties are usually expressed in units of stress and/or strain.

1. Mechanical Properties
of
Dental Materials
-By
Harshitha M

2. Mechanical Properties ?
• Mechanical properties are defined by the laws of
mechanics i.e. the physical science dealing with
forces that act on bodies and the resultant motion,
deformation, or stresses that those bodies
experiences.
• All the mechanical properties are measure of
resistance of the material to deformation, crack
growth, or fracture under an applied force or
pressure and induced stress.

3. • The failure potential of a dental prosthesis under
applied forces is related to the mechanical
properties and the microstructure of the
prosthetic materials.
• Mechanical are the measured responses, both
elastic (reversible on force reduction) and plastic
(irreversible and nonelastic), of materials under
applied forces, distribution of forces, or pressure.
• Mechanical properties are usually expressed in
units of stress and/or strain.

4. They represent measures of :
a. Elastic Deformation. (Reversible)
– Proportional limit
– Resilience
– Modulus of Elasticity
b. Plastic Deformation.(Irreversible)
– Percentage of elongation.
– Hardness.
c. A combination of elastic and plastic
deformation
– Toughness
– Yield strength

5. Force
• Force, in simple words can be defined as
any influence that causes an object to
undergo certain changes, either
concerning its movement, direction or its
structure.
• It is given by m(mass) x a(acceleration)
• The SI unit of force is Newton (N)

6. Stresses and Stains
• Stress: The force per unit area acting on atoms
or molecules in a given plane of a material. It is
given by force/area (MPa ).
• It’s the internal resistance of a material to an
external load applied on it.
• The stress produced within the solid material is
equal to the applied force divided by the area over
which it acts.
• A tensile force produces a tensile stress,
compressive force produces compressive stress
and shear force produces shear stress.

7. Types of stresses :
• Compressive stress : The resistance to two forces
acting in the same axis but directed towards each
other. It is associated with a compressive strain.
• The load applied tends to shorten the material.
Pushing forces

8. • Tensile stress: The resistance to two forces acting
in a same axis but directed away from each other.
It is associated with tensile strain.
• The load applied tends to stretch or elongate the
material.
Pulling forces

9. • Shearing stress: The resistance to the forces
acting towards each other but in different axis
(the forces are parallel).it is associated with
shearing strain.
• The load applied tends in a twisting motion or one
portion of the body sliding over the other.
Sliding forces

11. • Flexural stress: The resistance to the bending
forces. Flexural forces can produce all three
types of stresses in a material, but in most cases
it results in fracture because of the tensile
component.
• In this situation, the tensile and compressive
stresses are principal axial stresses, whereas the
shear stress represents a combination of tensile
and compressive components.

16. Type of
Stress
Produced by Examples
Residual
Stress
Stress caused within the material during
the manufacturing process
During welding
Structural
Stress
Stresses produced in the structure during
function. Weights they support provide
the loadings
In abutments of fixed
partial denture
Pressure
Stress
Induced in vessels containing pressurized
materials
In dentures during
processing under pressure
and heat
Flow Stress Force of liquid striking against the wall acts
as the load
Molten metal alloy striking
the walk of the mould
during casting
Thermal
Stress
Material is subjected to internal stress due
to different temperatures causing varying
expansions in the material
Materials that undergo
thermal stress such as
inlay wax, soldering and
welding alloys
Fatigue
Stress
Stress caused due to cyclic rotation of a
material
Rotary instruments
undergo rotational or
cyclic fatigue

17. • Stress gradient: when a force is exerted on an
elastic solid, the stress induced near the surface
decreases with distance from the loading point
and increases as the supporting surface is
approached. This pattern is called stress
distribution or stress gradient.
• Stress concentration: It’s the area that exists
because of improper design of a prosthetic
component (such as a notch along the section of
a clasp arm on a partial denture). Under these
conditions a clinical prosthesis may fracture at a
much lower applied force because of the
localized stress that exceeds the material

18. • Strain: The deformation taking place per unit
dimension. This has no units and is expressed
as the ratio or percentage.
• It is change in length per unit length of the
material when stress is applied. It is given by:-
change in length /original length
• There are two types:
1. Elastic strain- due to stress, the material
undergoes reversible change in shape.
2. Plastic strain- due to stress, the material
undergoes irreversible change in shape
(permanent deformation).

20. Stress and Strain curve
• YS- Yield strength
• PL-Proportional
Limit
• UTS-Ultimate
tensile strength
• E- Elastic Modulus

21. ELASTIC PROPERTIES
Mechanical properties and parameters that are
the measures of the elastic strain or plastic
strain behaviour od dental materials include:
1. Elastic Modulus (Young’s Modulus or
Modulus of Elasticity).
2. Dynamic young’s modulus.
3. Flexibility .
4. Resilience.
5. Poisson’s ratio.

22. Elastic Modulus (Young’s Modulus)
• The term describes the
relative stiffness of the
material. It is measured by
the slope of elastic region of
the stress-strain graph.
• It is the stiffness of the
material that calculated as
the ratio of elastic stress to
elastic strain.
• Higher the modulus of
elasticity, more is the
stiffness of the material ,
and vice versa. (i.e., if the
Young’s modulus of a
material is low, then that
material is flexible).

23. • Young’s modulus of a tensile test material can
be calculated as shown below.
• Its SI units is GPa.
• E.g. impression materials
The impression materials should have low
modulus of elasticity for it to be removed from
the undercut areas in the oral cavity.

24. Dynamic Young’s Modulus
• Young’s Modulus can be measured by a dynamic
method as well as static method.
• The velocity at which sound travels through a solid
can be readily measured by ultrasonic transducers
and receivers
• Hence, the velocity of the sound and the density of
the material can be used to calculate dynamic
young’s modulus and Poisson's ratio.
• This method of determining dynamic elastic moduli
is less complicated compared to the conventional
tests of compressive and tensile strength, but the
values are often found to be higher than those
obtained by static measurements.

25. • However, these values are acceptable.
• If shear stress was induced instead of uniaxial
tensile or compressive stresses, the resulting
shear strain can used to define shear
modulus for the material.
• The shear modulus(G) can be calculated from
the elastic modulus(E) and poisson’s ratio(v).
• A value of 0.25-0.35 for poisson’s ratio is
typical. Thus, the shear modulus is usually
about 38% of the elastic modulus value.

26. Flexibility
• The maximum flexibility is defined as the flexural
strain that occurs when the material is stressed to
its proportional limit.
• Its given : proportional limit/elastic modulus
• For materials used for dental appliances and
restorations, a high value of elastic limit (the
stress above which material will not recover to its
original shape when the force is removed) is a
requirement because the structure is expected
to return to its original shape after it has been
stressed and the force is removed(elastic
recovery).

27. • However, there are cases where a larger
deformation may be needed with slight stress.
• For example, in orthodontic appliances, a
spring is often bent a considerable distance
under the influence of small stress. In such a
case, the structure is said to be flexible and to
posses the property of flexibility.

28. Resilience
• The term is associated with springiness, but it
precisely means the amount of energy absorbed
within a volume of a structure when it is stressed
to its proportional limit.
• It is the amount of energy per unit volume that is
sustained on loading and released on unloading
of any test material.
• For example, when an acrobat falls on a trapeze
net the energy fall is absorbed by the resilience of
the net and when this energy is released the
acrobat is again into the air

30. • Resilience of two or more materials can be compared
by observing the areas under elastic region of their
stress- strain graph.
• The material with larger elastic areas has higher
resilience.
• The area bounded by the elastic region is a measure of
resilience and the total area under the stress-strain
curve is a measure of toughness.

31. • For example : when a dental restoration is
deformed during mastication, it absorbs
energy. If the induced stress is not greater
than proportional limit, the material is not
permanently deformed. It means that only
elastic energy is stored in the material.
• Therefore , a restorative material should have
moderately high elastic modulus and a
relatively low resilience, and hence limiting
the elastic strain.

32. Poisson’s Ratio
• If a cylinder is subjected to tensile and
compressive stress, there will be simultaneous
axial and lateral strain.
• Under tensile load, as the material elongates
there is reduction in the cross section.
• Similarly, under compressive load, there is a n
increase in the cross section.

33. • Within its elastic limit, the ratio of lateral to
the axial strain is called the poisson’s ratio.
• Most rigid materials like enamel, amalgam,
composite etc. exhibit poison's ratio of about
0.3
• More ductile materials such as soft gold alloys
shows a higher degree of reduction in the
cross sectional area and higher poisson’s ratio.

34. STRENGTH PROPERTIES
• Strength of a material can be defined as the
degree of stress necessary for a specific
amount of plastic deformation(yield
strength) or to cause fracture (ultimate
strength).
• When the strength of a material is discussed
its usually referred to as the maximum
stress required to cause fracture of the
material.

35. • Strength of the material can be descried by
one or more of the following properties:
1. Proportional Limit
2. Elastic Limit
3. Yield Strength
4. Ultimate Tensile strength, Shear strength,
Compressive strength and Flexural strength.
• Each of the above is a measure of stress
required to fracture the material.

36. Proportional Limit
• The term is defined as the magnitude of
elastic stress beyond which plastic
deformation takes place.
• Hooke’s law: states that , “The strain in a
solid is proportional to the applied stress
within the elastic limit of that solid.”
• If the materials which obey Hooke’s law, the
elastic stress will be proportional to elastic
strain. For such material, the stress-strain
curve starts from the origin as a straight line.
• Along this line the material behaves elastically,

37. • When a certain stress value
corresponding point A is
exceeded, the line becomes
non linear and stress is no
longer proportional to strain.
• The point A, above which the
curve deviates from the
straight line, is called the
proportional limit.
• Therefore , proportional limit
represents the maximum stress
above which stress is no longer
proportional to strain.

38. • Examples : PL of
enamel-235 MPa ,
dentin-176MPa. Etc.
• The connectors of partial dentures should have high
proportional limit.
• Materials like cobalt/chromium alloys have high proportional
limit and hence are widely used for the fabrication of
connectors because they can withstand high stresses without
permanent deformation.

39. Elastic Limit
• The term defined as the maximum stress that a
material will withstand without permanent
deformation.
• For linearly elastic materials, elastic limit and
proportional limit represents the same stress within
the structure.
• However, they differ in when elastic behaviour of the
material is considered.

40. Yield Strength (Proof Stress)
• It represents a stress value at which a small
amount (0.1% or 0.2%) of plastic strain has
occurred.
• Yield strength is used in cases where the
proportional limit cannot be determined with
sufficient accuracy.
• The amount of deformation occurred is
referred to as percent offset.

41. • A value of either 0.1% or
0.2% of the plastic strain is
often selected as percent
offset.
• To determine yield strength
for a material at 0.2% offset,
a line is drawn parallel to
straight-line region starting
at a value of 0.2% of the
plastic , along the strain axis
and is extended till it
intersects the stress-strain
curve.
• The stress corresponding to
this point is the yield

42. • Proportional limit, elastic limit, and yield
strength are defined differently nut their
values are fairly close to each other in many
cases.
• These values are important because they
represent the stress at which the deformation
begins. If these are exceeded by the
masticatory stresses, the restoration or
prosthesis may no longer function as originally
designed.
• A FPD becomes permanently deformed

43. Ultimate Strength
• Ultimate Tensile strength / stress (UTS) : The
maximum stress a material can withstand
before failure in tension.
• Ultimate Compressive strength/ stress (UCS):
The maximum stress a material can withstand
before failure in compression.
• The ultimate strength is determined by
dividing maximum load in tension (or
compression) by the original cross sectional
area of the test material

44. Permanent Deformation:
• When a material is deformed by stress at a
point above the proportional limit before a
fracture, removal of the applied force will
reduce the stress to zero, but the plastic
deform remains.
• Therefore, the object does not return to its
original dimension when the force is removed.
It remains bent, stretched, compressed, or
otherwise plastically deformed.

45. Cold Working (strain Hardening)
• When the metals are stressed beyond their
proportional limit, their hardness and strength
increases at the area of deformation, but their
ductility decreases.
• Repeated plastic deformation of metal during
bending an orthodontic wire can lead to the
brittleness of the wire at the deformed area of
the wire which may fracture on further
adjustment of the wire.
• To minimize the risk of brittleness, it is advised
to deform the metal in small increments so as

46. Diametral tensile strength
• Tensile strength is generally determined by
subjecting a rod, wire or dumbbell shaped
specimen to tensile load, since such test is
quite difficult to perform for brittle materials
because of alignment and gripping problems,
another test has become popular for
determining this property for brittle dental
material is referred to as “Diametral
compression test”.
• Compressive load is placed against the side of
a short cylindrical (specimens). The vertical

47. • It is calculated by the formula :

49. Flexural strength
• Defined as force per unit area at the instant of
fracture in a test material, subjected to
flexural loading.
• Also called as modulus of rupture / transverse
strength.
• When the load is applied, the specimen
bends, the principal stress on the upper
surface are compressive, where as those on
the lower surface are tensile.
• For a disk shaped specimen, failure stress

50. • Most prosthesis and restoration fractures
develop progressively over may stress cycles
after initiation of a crack from a critical flaw,
and subsequently by the propagation of the
crack until an unexpected, sudden fracture
occurs.
• This phenomenon is called fatigue failure.

51. Biaxial Flexural Test
• For a typical biaxial flexural tests, disk shaped
specimens 12mm–diameter and 1.2mm-
thickness are used.
• The test is based on a piston-on-three-ball
design. The load is applied by means of a
piston with a slightly curved contact surface
and the disk is supported by steel balls.

55. • The flexural strength can be calculated :
• One of the limitations of a 3 point flexural test
is that if the specimen does not fracture at
the midpoint directly under the applied force,
a correction must be made to calculate the
fracture stress at the actual point of fracture.

56. • The four point flexural test is preferred over
the 3 point test because the stress within the
central loading span is constant. Thus, no
correction is required for specimen that
fracture within the central loading span but do
not fracture precisely at the midpoint .
• The same formula can used here as well. The
only changed factor will be the load applied.

59. Impact Strength
• Defined as the energy required to fracture a
material under an impact force.
• The term Impact is used to describe the
describe the reaction of a stationary object to
a collision with a moving object.
• A Charpy-impact tester is usually used to
measure the impact strength. A pendulum is
released which swings down and to fracture
the centre of the material, supported at both
ends. The energy units is joules.

60. CHARPY IMPACT TESTER

61. (Source: https://www.zwickroell.com/en/news/en_gb/charpy-pendulum-impact-test)

62. • In Izod impact tester, the specimen is clamped
vertically at one end. The blow is delivered at
a certain distance above the clamped end.

64. • It the struck object is not permanently deformed, it
stores the energy form the collision in an elastic matter
and this ability of the material corresponds to
resilience.
• Thus, a material with low elastic modulus and high
tensile strength is more resistant to impact forces.

65. • For dental materials of low impact resistance, the
elastic moduli and tensile strengths, are :
Material Elastic Modulus
(GPa)
Tensile Strength
(MPa)
Composite 17 30 - 90
Porcelain 40 50 - 100
Amalgam 21 27 - 55
Alumina Ceramic 350 - 418 120
Acrylic 3.5 60

66. OTHER IMPORTANT PROPERTIES

67. Toughness
• The amount of elastic and plastic deformation
energy required to fracture a material. It
describes how difficult the material would
break.
• It is a measure of resistance to fracture.
• It is measured as the total area under a tensile
stress-strain curve.
• Toughness increases with increase in strength
and ductility.

69. Fracture Toughness
• Also called critical stress intensity .
• It is a property that describes the resistance of
brittle materials to the propagation of flaws
under an applied stress.
• It is given by the symbol K in units of stress
times the square root of crack length,
i.e., MPa x m .
• In general, high fracture toughness indicates
good resistance to crack propagation.
1/2
Ic

70. Material Fracture Toughness
Enamel 0.7 – 1.3
Dentin 3.1
Amalgam 1.3-1.6
Ceramic 1.2 – 3.0
Composite 1.4 – 2.3
Porcelain 0.9 – 1.0

71. Brittleness
• Defined as the relative inability of a material
to sustain plastic deformation before it
fractures.
• E.g. Ceramic, Amalgam, composites,etc. are
brittle at oral temperature.
• If a material shows no or very little plastic
deformation, then the material is said to be
brittle.
• It means that brittle materials fractures at
their proportional limit.
• These materials are weak in tension.

72. Ductility
• Represents the ability of a material to sustain
a large permanent deformation under tensile
load upto a point of fractures.
• Example- a metal that can be drawn readily
into a long thin wire is considered to be
ductile.

73. • Ductility can be measured in 3
methods :
1. Percentage of elongation after fracture
2. The reduction in area of tensile test specimen
3. Cold bend test

74. • Percentage of elongation is the most
commonly used method to measure ductility.
it represents the maximum amount
permanent deformation.
• Done by initially marking two points and
measuring the distance between them on the
test material and then pulling it apart under a
tensile load until it fractures. The fractured
parts are fitted together and the distance
between the marks are again measured.
• The ratio of increase in length to the original
length of the test material, expressed in
percentage is called percentage elongation.

75. • The second method that can determine
ductility involves necking or cone shaped
constriction that occurs at the end of a ductile
metal wire after rupture under a tensile load.
The percentage reduction in cross sectional
area of the fractured end in comparison to the
original area of the wire referred to as relative
reduction in area.

76. • The third method is measured by maximum
number of bends performed in a cold bend
test.
• The material is clamped in a vise and bent
around a mandrel of specified radius, the
number of bends to fracture is counted, with
the grater the number, the greater the
number of bends, greater is the ductility of
the material.

77. Malleability
• The ability of a material to sustain
considerable permanent deformation without
rupture under Compression, as in hammering
or rolling into sheets.
• Gold is the most ductile and malleable pure
metal and silver is the second.

78. Hardness
• In mineralogy, hardness is its ability to resist
scratching . But in metallurgy and most other
disciplines, hardness id defined as the ability
to resist indentations .
• Hardness tests, are included in ADA
specifications for dental materials, there are
various scales and tests mostly based on the
ability of the material surface to resist
penetration by a point under a specified load,
these test include Barcol, Brinell, Rockwell,
Shore, Vickers and Knoop.

79. CLASSIFICATION
OF
HARDNESS TEST
Method of
Application
Size of the
Indenter
Amount of load
applied to the
indenter
Static Loading
-slowly applied
Macro-indentation
-Large indenter tip
Macrohardness
- > 1kg load
Dynamic Loading
-rapidly applied
Micro-indentation
-Small indenter tip
Microhardness
- < 1kg load

80. Brinell’s Test
• Used extensively for determination of
hardness of hardness of metals and metallic
materials used in dentistry.
• Related to proportional limit and ultimate
tensile strength.
• The method depends on resistance to the
penetration of a small steel ball, typically
1.6 diameter when subjected to a specific
load.
• The hardened steel ball is pressed onto to the
polished surface of the test material. The load

81. Brinell’s hardness test

82. Rockwell’s Test
• This test was developed as a rapid method of determining
hardness of a material.
• Here, a conical diamond point is used.
• The depth of the indentation is directly read from the gauge
on the instrument.
• The value is Rockwell hardness number (RHN).
• This method is not used for brittle materials.

84. Vicker’s test
• The test uses a square shaped pyramidal indenter.
The impression obtained on the material is a square.
• The load value divided by projected area of
indentation gives vicker’s hardness number(VHN).
• The lengths of the diagonals of the indentations are
measured and averaged.

86. Knoop’s Test
• This method was developed for the micro
indentations tests.
• Suitable for testing thin plastic or metal sheets
or brittle materials where the applied force
doesn’t exceed 35N.
• The test is designed so that varying load can
be applied to the indenting instrument.
Hence, the resulting indentation depends on
the load applied and the nature of the
material used.

88. • The knoop and Vickers tests are classified as
micro hardness test while Brinell and Rock
well are macro-hardness test. Knoop and
Vickers can measure hardness in thin object
too. Material KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460

89. • Other less sophisticated tests are SHORE and
BARCOL to measure hardness of materials like
rubber and plastic types of dental materials.
• These utilize portable indenters and are used
in industry for quality control.
• The principle of these tests is also based on
resistance to indentation. The equipment
generally consists of a spring loaded metal
indenter point and a gauge from which
hardness is read directly.
• The hardness is based on the depth of

91. 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 initiation and
broken bonds.

92. • 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.

93. • Two important aspects of 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, amalgam, etc. are extremely
important in areas subjected to tensile stress.

94. CAUSES FOR AREAS OF HIGH STRESS
CONCENTRATION AND METHODS TO MINIMISE
THEM
• Surface defects such as porosity, grinding
roughness.
• polish surface to reduce the depth of the
defects.
• Interior flaws such as voids.
• Little can be done about the interior flaws
other than to ensure highest quality of the
structure or to increase the size of the object.

95. • Marked changes in contour- sharp internal
line angle at axio-pulpal line angle.
• Design of the prosthesis should vary gradually
than abruptly.
• Notches should avoided.
• All internal line angles should be rounded.

96. • 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.

97. • 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.

99. References:
• Shama Bhat, V., Nandish, B.T. & Jayaprakash,
K. (2019). Science of Dental Materials (3rd ed.,
pp. 17-33). New Delhi: CBS.
• Skinner, E., & Phillips, R. (2014). Phillips'
Science of Dental Materials (1st ed., pp. 50-
70). New Delhi: Elsavier.