mechanical properties of dental materials

1. GOOD MORNING!!

2. Mechanical Properties
of
Dental Materials
Presented by
Dr Alka Shukla
1st yr MDS Student

3. CONTENTS
• Introduction
• Stress and its types
• Strain
• Mechanical properties based on elastic deformation
• Strength properties
• Other mechanical properties
• Conclusion
• References

4. INTRODUCTION:
• Mechanical properties are defined by the laws of mechanics
that is the physical science which deals with energy/ forces
and their effects on bodies.
• These properties are measures of the resistance of a material
to deformation or fracture under an applied force.
• These properties are frequently expressed in terms of stress or
strain.

5. • Mechanical properties can represent measurements of :
– Elastic or reversible deformation
– Plastic or irreversible deformation
• Dental materials : these are biocompatible, long lasting direct-
filling tooth restoratives and indirectly processed prosthetic
materials that can withstand the adverse conditions of the oral
environment and soft tissues.

6. Dental
Materials
METALS
CERAMIC
POLYMERSCements
COMPOSITES
Five main groups of dental materials are:

7. STRESS
• Stress is the force per unit area acting on millions of atom or
molecules in a given plane of a material. It typically decreases
as a function of distance from the area of the applied force or
applied pressure.
• Stress in terms of mathematical equation can be expressed as:
Stress = force /area

8. Types of stresses
stress
simple
tensile Compressive shear
complex
Flexural Torsion diametral

10. Tensile stress
• By definition it is tensile force per unit
area perpendicular to the force direction.
• It is caused by a load that tends to stretch or
elongate a body.
• Under tensile stress body tends to attend smaller
cross sectional area.

11. • Most materials which are quite brittle, are highly susceptible
to crack initiation in the presence of surface flaws when
subjected to tensile stress.
• It is always accompanied by tensile strain.

12. Compressive stress
• If a body is placed under the load that
tends to compress or shorten it, the
internal resistance to such a load is
called a compressive stress.
• It is associated with compressive
strain.

13. • To calculate either tensile stress
or compressive stress, the applied force is divided by the cross-
sectional area perpendicular to the force direction.
• Technically, stress is the internal resistance of the body in
terms of the force per unit area and is equal and opposite in
direction to the force (external) applied.
• Internal resistance to force is difficult to measure therefore
convenient procedure is to measure the external force applied
to the cross-sectional area.

14. Shear stress
• It is the stress that tends to resist
a twisting motion, or a sliding of
one portion of a body over
another. It is calculated by
dividing the force by the area
parallel to the force direction.
• In the oral environment shear
failure is unlikely to occur.

15. • The reasons behind this fact are:
– Many of the brittle materials in restored tooth surfaces
generally have rough , curved surfaces
– The presence of chamfers, bevels or changes in curvature
bonded tooth surface
– To produce shear failure, the applied force must be located
immediately adjacent to the interface which is quite
difficult.
– Because the tensile strength of brittle materials is well
below their shear strength values, tensile failure is more
likely to occur.

16. Flexural stress(bending)
• It is produced by bending forces in dental appliances in one of
two ways:
– By subjecting a structure such as an FPD to three point
loading, where by the endpoints are fixed and a force is
applied between these end points.
– By subjecting a cantilevered structure that is supported at
only one end to a load along any part of the supported
section.

17. STRAIN
• Strain is described as the change in length per original length
of the body when it is subjected to a load.
• Strain = deformation/original length
• Each type of stress is capable of producing a corresponding
deformation in a body.
• Strain is unit less.

18. • The deformation resulting from a tensile or pulling force is an
elongation of a body in the axis of applied force , whereas a
compressive or pushing force cause compressive or shortening
of the body in the axis of loading.
• The amount of strain will differ with each type of materials
and with the magnitude of the load applied.

19. MECHANICAL PROPERTIES BASED ON
ELASTIC DEFORMATION:
• There are several mechanical properties and parameters of
the elastic/plastic strain. These are:
– Elastic modulus
– Dynamic Young’s modulus
– Shear modulus
– Flexibility
– Resilience
– Poison’s ratio

20. Elastic modulus
• It describes the relative stiffness or
rigidity of a material which is
measured by the slope of the
elastic region of the stress-strain
graph.
• It can be determined from stress-
strain curve by calculating the
ratio of stress to strain.

21. • Elastic modulus(E) =stress/strain
• The interatomic or intermolecular forces of the material are
responsible for the property of elasticity.
• The stronger the basic attraction forces, the greater the
values of the elastic modulus and the more rigid or stiff the
material.

22. • This property is generally independent of any heat treatment
or mechanical treatment that a metal or alloy has received,
but is dependent on the composition.
• It measures the elasticity of material.
• Materials such as Elastomers and other polymers have low
values for elastic modulus, whereas many metals and
ceramics have much higher values

23. Elastic modulus for few dental materials
– Enamel = 84 GPa
– Dentin = 17 GPa
– Gold(type IV) = 90-95 GPa
– Amalgam =28 Gpa
– Cobalt-Chromium 218-224
– Porcelain = 69-70 Gpa
– Silicon elastomers = 0.002- 0.003

24. Dynamic young’s modulus:
• It is defined as the ratio of stress to strain for small cyclical
deformations at a given frequency and at a particular point on
the stress-strain curve.
• Elastic modulus can be measured by dynamic methods as well.

25. • Since velocity at which sound travels through a solid can be
readily measured, the velocity of the sound wave & the
density of the material can be used to calculate the elastic
modulus and Poison’s ratio.
• This method is less complicated than conventional methods.

26. Flexibility:
• It is defined as the flexural strain that occurs when the
material is stressed to its proportional limit.

27. Proportional limit
• It is defined as the greatest stress
that a material will sustain without
deviation from the linear
proportionality of stress to strain.
• Below the proportional limit, no
permanent deformation occurs in a
structure when stress is removed,
the structure will return to its
original dimensions.

28. Resilience
• It is the resistance of a material to permanent deformation. It
indicates the amount of energy that is necessary to deform
the material to the proportional limit.
• It is measured as area under
the elastic portion of the
stress-strain curve.

29. • It can also be defined as amount of energy absorbed within a
unit volume of a structure when it is stressed to its
proportional limit.
• Ideally the restorative materials should exhibit a moderately
high elastic modulus and relatively low resilience.

30. Toughness
• It is defined as the amount of energy required to fracture a
material.
• It is a measure of the energy required to propagate critical
flaws in the structure.
• Tough material is generally strong although a strong material
is not necessarily tough.
• Toughness is also known as resistance of material to fracture.

31. • Toughness represents the energy required to stress the material to the
point of fracture

32. Poison’s ratio
• During axial loading in tension or compression
there is a simultaneous axial and lateral strain
under tensile loading, as a material elongates
in the direction of load, there is a reduction in
cross-section. Under compressive loading
there is an increase in cross-section. Within
the elastic range , the ratio of the lateral to
the axial strain is called “Poison’s Ratio”

33. • Most rigid material exhibit Poison’s ratio around 0.3
• Examples are :
– Enamel = 0.33
– Amalgam = 0.33
– Resin composite = 0.30
• Ductile materials show higher Poison’s ratio.
• For an ideal isotropic materials of constant volume, the ratio is 0.5

34. STRENGTH PROPERTIES
• Strength is the stress necessary to cause either fracture or a
specified amount of plastic deformation.
• Strength of a material can be described in terms of:
– Elastic limit
– Yield strength or proof strength
– Ultimate tensile strength
– Shear strength
– Compressive strength
– Flexural strength

35. Elastic limit
• It is defined as the maximum stress that a material will
withstand without permanent deformation.
• Also defined as greatest stress to which a material can be
subjected such that it returns to its original dimensions when
the force is released.
• The region of the stress-strain curve before the proportional
limit is called the elastic region.

36. Stress-strain graph showing elastic limit

37. Yield Strength
• It is also known as yield stress or yield point.
• It is defined as the stress at which a material exhibits a
specified limiting deviation from proportionality.
• It is a property that can be determined readily and is often
used to describe the stress at which the material begins to
function in a plastic manner
• It defines the transition from elastic to plastic behaviour.

39. Permanent deformation[plastic]
• If a material is deformed by a stress at a point above the
proportional limit before fracture, the removal of the applied
force will reduce the stress to zero, but the strain does not
return to its original dimension when the force is removed. It
remains bent, stretched, compressed, or otherwise plastically
deformed.
• In stress-strain graph, the region beyond elastic limit is called
plastic region.

40. Flexural Strength
• It is obtained with a simple beam, simply supported at each end, with a
load applied in the middle, such a test is called a three point bending test
and the maximum stress measured in the test is called flexural strength.
• This test is collective measurement
of tensile, compressive, and
shear stresses simultaneously.

41. • It is also known as Flexural strength/transverse strength or
modulus of rupture.
• Mathematical formula to calculate it is:
– Flexural strength =3PL/2bd2
– P=maximum load at the point fracture
– L=distance between the supports
– B=width of specimen
– D=depth or thickness of specimen.

42. Fatigue strength
• Fatigue is defined as a progressive fracture under repeated
loading.
• Fatigue tests are performed by subjecting a specimen to
alternating stress applications below the yield strength until
fracture occurs.
• It is the stress at which a material fails under repeated
loading. Therefore failure under repeated loading is
dependent on the magnitude of the load and the number of
loading repetitions.

43. • Fatigue data are often represented by an S-N curve, a curve
depicting the stress at which material fractures as a function
of the number of loadings.

44. • When stress is sufficiently high, the specimen will fracture at a
relatively low number of cycles. Cycles required to cause
fracture increases as the level of stress decreases.
• For some materials, a stress at which the specimen can be
loaded an infinite number of times without failing is
eventually approached. This stress is called the “ENDURANCE
LIMIT”

45. • Environment to which a material is subjected is a critical
factor in determining fatigue properties. Elevated temp,
humidity, aqueous media, biological subjected(substance) and
pH deviations away from neutral can all reduce fatigue
properties.

46. Applied aspect
• Dental material can be subjected to moderate stresses
repeated a large number of times, it is important in designing
of a restoration to know what stress it can withstand for a
predetermined number of cycles. Restorations should be
designed so the clinical cyclic stresses are below the fatigue
limit.

47. • Fatigue fractures develop from small cracks and propagates
through the grains of a material therefore the cause of cyclic
failure in a material are inhomogeneity and flaws of the
materials. Area of stress concentration, such as surface
defects and notches can lead to failure of restoration.

48. Impact strength
• It is defined as the energy required to fracture a materials
under an impact force. The term impact is used to describe the
reaction of a stationary object.
• The impact resistance of materials is determined from the
total energy absorbed before fracture when struck by a
sudden blow.
• Some substances offer relatively little resistance to the shock,
whereas other of different composition may not fracture
under the same impact.

49. OTHER MECHANICAL PROPERTIES:

50. Fracture toughness
• It is the ability to be plastically deformed without fracture, or
the amount of energy required for fracture.
• The ability of a flaw to cause fracture depends on the fracture
toughness of the material.
• It is proportional to the energy consumed in plastic
deformation.
• The presence of filler in polymers substantially increases
fracture toughness.

51. • Aging or storage in a simulated oral environment or at
elevated temperature can decrease fracture toughness.
• This property describes the resistance of brittle materials to
the catastrophic propagation of flaws under an applied stress.

52. Brittleness
• It is the relative inability of a material to sustain plastic
deformation before fracture of a material occurs.
• Brittle material is not necessarily weak.
• Brittle material are much weaker in tension than in
compression, which contribute to failure of the materials
• Examples of brittle material are : dental amalgam, cements,
ceramics, plaster, stone.

53. Ductility and Malleability
• When structure is stressed beyond its elastic
limit it becomes permanently deformed. If a material sustains
tensile stress and considerable permanent deformation
without rupture, it is ductile.
• Ductility represents the ability of a material to sustain a large
permanent deformation under a tensile load before it
fractures.

54. • The ability of a material to sustain considerable permanent
deformation without rupture under compression is called
malleability.
• It can be measured using 3 methods.
– Percent elongation
– Reduction in area of tensile test specimen
– Cold bend test.

55. • Gold is the most ductile and malleable metal followed by
silver.

56. Hardness
• It may be broadly defined as the resistance to permanent
surface indentation or penetration.
• It is measure of the resistance to plastic deformation and is
measured as a force per unit area of indentation.
• It is indicative of the case of finishing of structure and its
resistance to in service scratching

57. • Some of the most common methods of testing the hardness of
restorative materials are the:
– Brinell
– Knoop
– Vickers
– Rockwell
– Barcol
– Shore A

58. • These all tests have one common quality, that each depends
on the penetration of small, symmetrically shaped indenter
into the surface of the material being tested

59. Brinell hardness number
• It is one of the oldest tests.
• A hardened steel ball is pressed
under a specific load into the
polished surface of material.
• The diameter is measured.
• It is used to evaluate hardness of
metallic or non metallic restorative
materials.

60. Rockwell hardness test
• It is similar to the Brinell test, in this
round diamond point is used.
Instead measuring diameter of the
impression, the depth of penetration
is measured.
• Brinell test and Rockwell tests are
not suitable for brittle materials.

61. Vickers
• Test employs the same principle of hardness testing, instead of
a steel ball, a square based pyramid is used. The length of
diagonals of the indentations are measured. It is suitable for
brittle material therefore is used to measure hardness of tooth
structure.

63. Knoop
• Employs a diamond tipped tool that is cut in geometric
configuration. The impression is rhombic in outline and the
length of the largest diagonal is measured. The projected
area is divided in the load to give knoop hardness number
knoop & vickers are classified as micro hardness tests,
whereas Brinell and Rockwell tests are classified as macro
hardness tests

64. • Knoop and Vickers employ
load of less than 9.8 N. The
resulting indentation are
small and are limited to a
depth of less than 19
micrometre.

65. • Shore and Barcol tests are employed for measurement of
hardness of rubber and plastic types f dental materials. The
principle of these tests is also based on resistance to
indentation.

66. • Many material which have micro structural constituents or in
case of microfilled composites, fillers are smaller than
dimensions of indenter. To measure hardness of such material
recently “Nano- indentation” technology is been introduced.
• This technique is also helpful in measuring modulus. And for
brittle materials, strength and fracture toughness may be
determined.

67. APPLIED ASPECTS:

68. Stress and Strain
• It is the internal resistance which is developed in response to
external force.
• In oral environment, restorations are subjected to various
types of stresses from mastication action.

69. • In cases of shellac and waxes, care must be taken to ensure
thorough softening prior to moulding, otherwise considerable
stresses are introduced which eventually lead to distortion.
• For crown cementation, cements which can transfer the stress to
tooth structure without undergoing permanent deformation should
be used. Example resin cements, glass ionomer cements and zinc
phosphate cements.
• In case of cast restorations, the twisting or torsional forces are
countered by the presence of grooves, boxes or other features such
as pins and potholes.

70. • Strain is important consideration in dental restorative
material such as clasps or orthodontic wires, in which a large
amount of strain can occur before failure.
• While providing a restoration on occlusal surface, care should
be taken that the occlusal contacts should entirely be placed
on the restoration or on the tooth structure but never at the
interface otherwise it will lead to stress generation at the
margins of restoration.

71. • Compressive strength
DENTAL MATERIAL COMPRESSIVE STRENGTH (MPA)
1. Silicate cement 180
2. Glass Ionomer Cement (Type II) 150
3. Cement 150
4. Hybrid Ionomer 105
5. Unfilled resins 70
6. Traditional composite 250-300
7. Hybrid Composite 350-400
8. Dental Amalgam 300-550

72. • Tensile strength
Dental Material Tensile Strength ( MPa)
Silicate 3.5
GIC (type II ) 6.6
Cement 6.7
Hybrid Inomer 20
Unfilled resins 24
Traditional composite 50-65
Hybrid Composite 75-90
Dental Amalgam 45-65

73. • The tensile strength and transverse strength values of
amalgam are very much lower than the compressive strength.
The material is weak in thin sections and unsupported edges
of amalgam are readily fractured under occlusal loads.
• Therefore care must be paid during cavity preparation.

74. • The material is essentially brittle in nature, requiring adequate
support from surrounding structures.
• For cavo-surface margins, 90 degree joint/butt joint is
recommended.
• In cases of composite restorations, the cavo surface margin is
again not beveled as the tensile strength of composites is
lesser when compared to its compressive strength.

75. • Hardness of different materials
Material VHN
Enamel 350
Dentine 60
Acrylic resin 20
Dental amalgam 100
Porcelain 450
Co/Cr alloys 420
Gold alloy type 4 300

76. • The hardness of amalgam is somewhat lower than that of
enamel, a factor that may be responsible for amalgam
restorations developing surface facets when they make
contact with cusps of opposing teeth.
• Gold wears at a rate similar to the enamel therefore it does
not cause excessive loss of enamel of opposing tooth.
• Harder materials are more difficult to polish by mechanical
means.

77. • Hardness is also used to give an indication of the abrasion
resistance of a material.
• Wear can occur by one or more of a number of mechanisms,
some of which may be considered to be of mechanical origin
and others chemical
• Wear due to intermittent stresses caused by, for example,
tooth-restorative contacts is termed fatigue wear. Harder
materials show less wear.

78. • There is a vast difference in hardness between acrylic resin
and porcelain.
• Acrylic teeth are more likely to suffer abrasion than porcelain
teeth.
• Where as porcelain teeth cause more abrasion of natural
enamel opposing to the restoration.

79. Modulus of elasticity
• In case of impression materials, the properties which are
most important are rigidity and elasticity, since they
determine whether an impression material can be used to
record undercuts.
• When standing teeth are to be recorded, or when the patient
has deep soft-tissue undercuts, the set impression material
must be flexible enough to be withdrawn past the undercuts
and elastic enough to give recovery and an accurate
impression.

80. Based on elastic properties

81. • It is advantageous for an impression material to be flexible
and it is essential for a restorative material to be rigid.
• Implants: Function of implant primarily depends upon the
rigidity of the implant structure. This in turn is related to the
dimensions and the modulus of elasticity of the material from
which the implant is manufactured.
• The use of high modulus materials enables implants of smaller
cross-sectional bulk to be used.

82. Proportional limit
A practical example of a situation in which a high proportional
limit is required is in connectors of partial dentures. Such
connectors should not undergo permanent deformation their
shape. A material such as cobalt–chromium (Co/Cr) alloy which
has a high value of proportional limit is popular for this
application since it can withstand high stresses without being
permanently distorted.

83. Malleability and Ductility
• Gold is the most malleable metal.
• Because of this property, the burnishability of margins of
metallic restoration is possible.
• The malleability of stainless steel is utilized when forming a
denture base by the swaging technique. This involves the
adaptation of a sheet of Stainless steel over a preformed cast.

84. • In high load bearing areas, certain properties of pure gold
such as rigidity and elastic limit are insufficient to resist
distortion.
• Ductility is an important property of orthodontic wires and
clasps provided to dentures.
• Clasps or wires of dentures constructed from ductile alloys
can be altered by bending during alteration of appliances.

85. Resilience
A high value of resilience is one parameter often used to
characterise elastomers. Such materials which may, for example,
be used to apply a cushioned lining to a hard denture base are
able to absorb considerable amounts of energy without
being permanently distorted.

86. Brittleness
• Enamel is brittle when compared to dentin. So during cavity
preparation care should be taken that the enamel has sound
dentin support which is more capable of bearing the stress.
• Porcelain is brittle and hard, so teeth constructed from this
material are more likely to chip and fracture than acrylic
teeth.

87. • As porcelain is brittle material, its tensile strength is very low
hence porcelain restorations need adequate depth or bulk to
withstand the load.
• Therefore for porcelain crowns, shoulder margin preparation
is advised.

88. Plastic deformation
• Amalgam undergoes a certain amount of plastic deformation
or creep when subjected to dynamic intra-oral stresses.
• Creep causes the amalgam to flow, such that unsupported
amalgam protrudes from the margin of the cavity .
• These unsupported edges are weak and may be further
weakened by corrosion. Fracture causes the formation of a
‘ditch’ around the margins of the amalgam restoration.

89. Toughness
• In case of amalgam low copper amalgam is tougher than high
copper amalgam.
• In case of wax: Modelling waxes are tough enough to resist
fracture when withdrawn from shallow undercuts. Some
denture modelling waxes are referred to as toughened by
their manufacturers. Sheets of toughened modelling wax can
typically be bent without fracturing.

90. CONCLUSION
• Three inter-related factors are important in the long-term
function of dental restorative materials
– Material choice
– Component geometry
– Component design
• The mechanical properties of dental materials are important
for the dentist to understand when designing a restoration or
making adjustments to a prosthesis.

91. • The failure potentials of a prosthesis under applied forces is
mainly related to the these properties of the prosthetic
material.
• The goal should be to ensure that the properties of oral
restorations must adequately withstand the stresses of
mastication.

92. • For example:
– In areas of high stress, materials having high elastic moduli
and strength properties should be used if possible.
– Restorations and appliances should be designed so that
resulting forces of mastication are distribute as uniformly
as possible.

93. • Mechanical properties are important in understanding and
predicting a materials behavior under various conditions,
therefore it helps in the determination of clinical failures
(whether they are caused by material deficiencies, human
errors or patient factors). Thus it helps in understanding the
design modifications that will improve the quality of
restoration and render good , efficient, long-lasting
restorations or prosthesis to needy patients.

94. References
• Philips's Science Of Dental Materials-11th Edition.
• Craig’s Restorative Dental Material- 12th Edition
• Sturdevant’s Operative Dentistry- 6th Edition
• Summitt ‘s Operative dentistry- – 2nd Edition
• Charbeneau’s Operative Dentistry- 3rd Edition
• Baum’ s Textbook of Operative Dentistry- 3rd Edition
• Applied Dental Materials- 9th Edition