The document discusses various mechanical properties of dental materials including strain, stress, stress-strain curves, hardness, and strength. It provides definitions and explanations of key terms: 1) Strain and stress occur when forces are applied to materials, causing deformation and internal resisting forces. Stress-strain curves plot these values to compare material properties. 2) Properties like elasticity, strength, and brittleness are determined from the curves. Hardness tests measure material resistance to indentation or scratching. 3) Common tests include Brinell, Knoop, and Rockwell hardness tests as well as transverse strength and diametral compression tests for brittle materials. Understanding material mechanics guides selection of suitable dental materials

1. Mechanical properties of
dental material
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics

2. Strain:
When the external force or load is applied to
a material the phenomenon of strain
occurs – this is a change in dimension of
the material ( the change in length, or
deformation per unit length(
Deformation of length
Strain=
Length

3. Strain

4. Types of strain
1-temporary of elastic strain:
Which disappears on removal of the
external force. The material will return to
its original shape.
2-Permanent or plastic strain:
Which will not disappear on removal of the
external force. The material will not return
to its original shape.

5. Stress:
Associated with strain is the phenomenon of
stress – this is an internal force/unit area
in a material, equal and opposite to the
applied load or force/unit area.
Force
Stress=
Area

6. Stress

7. Types of stress
1(Tensile stress:
Tension results in a body when it is
subjected to two sets of forces directed
away from each other in the same straight
line.

8. 2(Compressive stress:
Compression results when the body is
subjected to two sets of forces directed
towards each other in the same straight
line

9. 3(Shear stress:
Shear is the result of two sets of forces
directed towards each other but not in the
same straight line.

10. 4(Complex stresses:
A single type of stress is extremely difficult
to induce in a structure so in practice the
stresses within a material are complex.
(complex stresses are produced by 3 point
loading(
compression
shear
tension

12. Stress – strain curve
Stress
MPa
ultimate strength
Yield strength
Proportional limit , elastic limit
Strain

13. Stress – strain curve:
A convenient means of comparing the
mechanical properties of materials is to
apply various forces to a material and to
determine the corresponding values of
stress and strain . A plot of the
corresponding values of stress and strain
is referred to as a stress- strain curve.
Such a curve may be obtained in
compression, tension, or shear.

14. From the stress strain curve, the
following properties can be drawn:
1(Proportional limit (P.L(:
It is defined as the maximum stress that a
material will withstand without deviation
from the low of proportionality of stress to
strain (it describes the relation between
stress and strain(

15. 2(Elastic limit (E.L(:
It is defined as the maximum stress that a
material will withstand without permanent
deformation resulting. (it describes the
elastic behavior of the material(
3(Yield strength (Y.S.(:
It is the stress at which the material begins
to function in a plastic manner. (defined as
the stress at which a material exhibits a
specified limiting deviation from
proportionality of stress to strain.

16. 4(Ultimate strength (U.S.(:
If higher and higher forces are applied to a material, a
stress will be reached at witch the material will
fracture. If the fracture occurs from tensile stress,
the property is called the tensile strength, and, if in
compression, the compressive strength.
The ultimate tensile strength is therefore defined as
the maximum stress that a material can withstand
before failure (fracture or rupture( in tension,
whereas the ultimate compressive strength is the
maximum stress a material can withstand in
compression. It is calculated by dividing the load by
the original cross-sectional area.

17. 5(Modulus of elasticity or (Young’s
Modulus( (E(:
It is the constant of proportionality between
stress and strain. It represents the slope of
the elastic portion of the stress – strain
curve. It is a measure of rigidity or stiffness
Materials with higher Young’s modulus value
are said to be stiffer or more rigid than
those of low Young’s modulus values
because they require much more stresses
to produce the same amount of strain.

18. Modulus of elasticity or (Young’s
Modulus) (E):
2
stress Kg/cm 2
Elastic modulus = = = Kg/cm
strain Cm/cm

19. Modulus of elasticity or (Young’s
Modulus) (E):

20. 6)Flexibility:
Maximum flexibility is the strain resulting in
the material when the stress reaches the
elastic limit.
This is very important for impression
materials, which often must be severely
deformed to be removed from undercuts,
but must have the ability to spring back
without suffering any permanent change in
shape.

21. 7)poisson’s ratio:
The increase in length of a material under
tension is associated with a decrease in
cross-sectional area. The increase in
length is known as axial strain and
decrease in cross sectional area is know
as lateral strain.
lateral strain
Poisson’s ratio=
axial strain

22. 8)Ductility and malleability:
Ductility is the ability of a material to
withstand plastic deformation under tensile
stress without fracture. Malleability, is the
ability of a material to withstand plastic
deformation compressive stress without
fracture.
In other words, the malleability of a metal is
its ability to be hammered in to thin sheets
without fracturing, while, ductility is its
ability to be drawn into wire without
fracturing .ductility is measured by the
percentage of elongation.

23. A material which has good ductility shows
high elongation before fracturing. The
percentage elongation represents the
maximum amount of permanent
deformation.
increase in length
Percentage elongation = X 100
original length

26. 9)Brittleness:
If a material showed no or very little plastic
deformation on application of load it is
described as being brittle, in other words,
a brittle material fractures at or near its
proportional limit. More over ,brittle
materials are weak in tension; for
example, dental amalgam has
compressive strength which is nearly six
times higher than its tensile strength.

28. Ductile material Brittle material
1)Is the ability of a 1) brittle material
Material to withstand fractures at or near
Plastic deformation its proportional limit.
Under tensile stress
Without fracture.
Fracture occur far Fracture occur at or
Away from P.L near P.L

30. Necking takes place No necking, but
Before fracture crack propagation
takes place till
fracture
Example are gold Examples are
Alloys and nickel – amalgams,
porcelain,
Chromium alloy. And composites.

33. 10)Resilience:
The modulus of resilience is the maximum
amount of energy a material can absorb
without undergoing permanent
deformation. It is represented by the area
under the elastic portion of the stress –
strain curve.
Acrylic resin denture teeth are more resilient
than porcelain teeth and consequently
absorb most masticatory forces and
transmitted less to the underlying bone,
preserving it.

34. Resilience:

35. 11(Toughness:
It is the energy required to stress the
material to the point of fracture. It is
represented by the area under the elastic
and plastic portion of the stress-strain
curve. Therefore toughness of a material
is the ability to absorb energy. The
toughest materials are those which high
proportional limits and good ductility.
However two highly different materials can
have the same toughness.

36. Toughness

37. 12(Fracture toughness:
It is the ability of the material to resist
fracture through its resistance to crack
propagation. In general, high fracture
toughness indicates good resistance to
crack propagation

38. Mechanical test
1(Diameter compression test (indirect
tensile test(
2(Transverse strength test
3(Hardness test

40. Diameter compression test (indirect
tensile test(
The diametral compression test or indirect
tensile test used to measure the tensile
strength of brittle materials. These brittle
materials include dental amalgam,
cements, ceramics and gypsum products.
These materials are much weaker in
tension than in compression thus this
contributes to their failure in service.

41. In this test a disk of the brittle material is
compressed diametrically in a testing
machine until fracture occurs. The
compressive stress applied to the
specimen introduces tensile stress in the
material. 2P
tensile stress= --------
DT
P = load, D = diameter , T = thickness

43. Transverse strength test
In practice, the stresses within the material
are complex. Thus if a beam is in tension,
and the top is in compression. Shear
stresses are also present. The transverse
strength of a material is obtained by
loading a bar which is supported at each
end with the load applied in the middling. It
is often described as the modulus of
rupture or flexure strength.

44. Transverse strength

45. Clinical significance:
1(Denture base materials in which a stress
of this type is applied to the denture during
mastication.
2(Long bridge spans in which the biting
stress may be severe.

47. Hardness and hardness test:
Hardness : is the resistance of the material to
scratching, indentation or penetration.
It is a surface property not related directly to any
other mechanical property i.e. strong or stiff
materials are not necessary hard.
Hardness can’t be seen or calculated from stress-
strain curved but only by using one of the
following: Brinel,Knoop,Vickers,Rockwell,and
shore A hardness test.

48. Brinell hardness test:
A steel ball is pressed into the surface of the
material under a specified load. The load
is divided by the area of the surface of the
indentation. Thus, the smaller the
indentation the larger the hardness
number becomes, and the harder the
material is. This test is used to determine
the hardness of the metallic materials. it is
expressed in B.H.N

49. Brinell hardness test:

50. Brinell hardness test:

51. Disadvantages:
1(It is difficult to measure the indentation
area.
2(Not suitable for measuring hardness of
brittle materials because the steel ball will
fracture it.
3(Not suitable for measuring hardness of
elastic materials because the indentation
is recovered on removal of the steel ball.

52. Rockwell hardness test:
Rockwell hardness test is similar to Brinell test in
that steel ball or cone is used. Instead of
measuring the diameter of the indentation, the
depth is measured directly by a dial gauge on
the instrument.
Advantage : it is a rapid and easy method for
measuring hardness.
Disadvantage: as for the Brinell test, Rockwell test
is not suitable for brittle and elastic materials.

53. Rockwell hardness test

54. Vicker hardness test:
Vicker hardness test a diamond square –
based pyramid (cone) is used. The
Vicker’s hardness number is determined
by dividing the load by the area of
indentation which is square and not round
as in the Brinell test. This test is easy and
suitable for brittle materials but not for
elastic materials. It is expressed in V.H.N.

55. Vicker hardness test

56. Knoop hardness test:
Knoop hardness test uses a diamond cone
designed to give an indentation having a
long and a short diagonal(7 : 1). The load
may be varied over a wide range, from
one gm to more than a Kg, so that values
for both hard and soft materials con be
obtained. It is expressed in K.H.N.

57. Knoop hardness test

58. Knoop hardness test

59. Advantages:
1)Easy measuring of indentation depth.
2)Can test hardness of brittle materials without
fracture.
3)Can test hardness of elastic materials because
when the indentation is made. The stresses are
distributed in such a manner that only the
dimensions of the short axis are subject to
change by relaxation while the dimensions of the
long axis remain unchanged.
4)Hardness for both soft and hard materials can
be measured.

60. Shore hardness test:
The hardness tests described previously cannot be
used to determine the hardness of the rubbers,
since the indentation disappears after the
removal of the load. An instrument called a
Shore is used in the rubber industry to determine
its hardness. The indicator is attached to a scale
that is graduated form 0 to 100. if the indictor
completely penetrates the sample, a reading of
0 is obtained, and if no penetration occurs, a
reading of 100 results.

61. Shore hardness test

62. Clinical significance:
1)Denture – wearing patients must take care not
to be aggressive during the cleaning of their
dentures by using brushes with hard bristles.
2)Hardness is an important property to consider for
model and die materials on which crown and
bridge wax patterns are made, because a soft
surface may become scratched, affecting the
accuracy of the final restoration.

64. Impact strength:
It is describe to know the effects of the application
of a sudden force to a material because under
these condition materials are often more brittle.
Fatigue strength:
The repeated application of small stress (below the
P.L) to an object causes tiny (very small) cracks
to be generated within its structure. These tiny
cracks do not cause failure immediately. With
each application of stress, the cracks grow until
the material breaks. Metal, ceramics can all fail
by fatigue. Fatigue : is the fracture of a material
when subjected to repeated (cyclic) small
stresses below the P.L.

65. Fatigue strength

66. Creep:
Creep is defined as the time dependant
plastic deformation that occurs in an
object subjected to a small load below its
E.L(P.L).