The document discusses the mechanical properties of dental materials. It defines mechanical properties as those defined by the laws of mechanics, including forces and their effects on materials. Mechanical properties need to be considered collectively based on the intended application of the material. The success of any dental restoration depends on the mechanical properties of the material used. Key mechanical properties discussed include stress, strain, strength, elastic modulus, resilience, toughness, ductility and hardness. Various testing methods are used to evaluate these properties.

1. MECHANICAL PROPERTIES
OF
DENTAL MATERIALS
By
Dr Khawaja Rashid Hassan
Assistant Professor
RAWAL INSTITUTE OF HEALTH SCIENCES
RAWAL COLLEGE OF DENTISTRY
ISLAMABAD
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2. MECHANICAL PROPERTIES OF
DENTAL MATERIALS
 Defined by the laws of mechanics.
 The physical science that deals with energy
and forces and their effects on the bodies.
 Mechanical properties need to be
considered collectively.
 Intended application of a material is
important.

3. MECHANICAL PROPERTIES OF
DENTAL MATERIALS
 Failure or success potential of any
prosthesis / restoration is dependent upon
the mechanical properties of the material.
 The material response may be,
1. Elastic …. reversible on force removal.
2. Plastic …… Irreversible / non-elastic.
 Mechanical properties are expressed in
terms of stress and/or strain.

4. MASTICATORY FORCES
Tooth Average
 Occlusal forces force (N)
applied by adult
dentition is greatest in Second 800
posterior region. molar
 In growing children First molar 390
there is an average
annual increase in Bicuspids 288
force of 22 N.
Cupids 208
 Denture wearers only
apply 40% of the Incisors 155
forces given in table.
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5. STRESS
 When a force acts on the body, a resistance is
developed to the external force applied.
 This internal reaction is equal in
magnitude/intensity and opposite in direction
to the applied force and is called as “STRESS”
 Denoted by “S” or “σ”
 Designated as force per unit area (σ=N/m²)
 Pascal = 1 N / m².
 Commonly stress is reported in terms of
megaPascals.

6. STRAIN
 Relative deformation of an object that is
subjected to stress.
 It is change in length per unit length.
 It may be elastic, plastic or both elastic and
plastic.
 It is denoted by “ε”
 Designated as ∆L / L.

7. TYPES OF FORCES APPLIED
 Generally, the force applied may be
1. Axial (tensile or compressive)
2. Shear (sliding, rubbing)
3. Bending (bending movement)
4. Tortional (twisting movement)

8. TYPES OF FORCES APPLIED
 Tension results when a body is subjected to
two sets of forces directed away from each
other in a straight line. Force is directed
away from the objcet.
 Compression results when the body is
subjected to two sets of forces directed
towards each other in a straight line.

9. TYPES OF FORCES APPLIED
TENSION COMPRESSION
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10. TYPES OF FORCES APPLIED
 Shear is a result of two sets of forces
directed parallel to each other , but not
along the same straight line.
 Torsion results from the twisting of the
body.
 Bending results by applying bending
movement.

11. TYPES OF STRESSES
 3 simple types.
1. TENSILE STRESS:
causes the body to stretch or elongate.
Tensile stress is always accompanied by
tensile strain.
2. COMPRESSIVE STRESS:
causes the body to shorten or compress.
Compressive
3. SHEAR STRESS:
resist the sliding or twisting of one portion of
the body over another.

12. TYPES OF FORCES APPLIED
Complex stresses
FLEXURAL STRESS:
 Also called as bending stress.
 Produced by bending forces over the
dental appliance.
 Application of shear force may produce
elastic shear strain or plastic shear strain.

13. Hooke's Law
Hooke's Law states that "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.
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14. Hooke's Law
 Hooke's law can be expressed by the
formula
stress / strain = a constant.
 The value of the constant depends on the
material and the type of stress. For tensile
and compressive forces it is called Young's
modulus, E; for shearing forces, the shear
modulus, S; and, for forces affecting the
volume of the object, the bulk modulus, K.
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15. PROPORTIONAL LIMIT
 It is the maximum stress at which the
stress is equivalent/proportional to strain
and above this limit the plastic
deformation of a material occurs.
 The material may be subjected to any
type of applied force.
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16. STENGTH
Strength is the maximum stress that a
material can withstand without sustaining
a specific amount of plastic strain.
OR
Stress at the point of fracture.
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17. STRENGTH PROPERTIES
ULTIMATE TENSILE STENGTH :
Simply called as TENSILE STRENGTH.
It is defined as the Tensile stress at the
point of fracture.
YIELD STRENGTH :
It is the stress at which a test specimen
exhibits a specific amount of plastic strain.
Used in the conditions when proportional
limit cannot be determined with accuracy.
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18. STRENGTH PROPERTIES
SHEAR STRENGTH:
Maximum shear stress at the point of
fracture.
FLEXURAL STRENGTH:
Defined as “force per unit area at the
point of fracture of a specimen that is
subjected to flexural loading”
Also called as “BENDING STRENGTH” or
“MODULUS OF RUPTURE”
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19. STRENGTH PROPERTIES
FATIGUE STRENGTH:
 Determined by subjecting a material to cyclic
stress of maximum known value and
determining the number of cycles required to
cause failure of the material.
 Maximum service stress (endurance limit) can be
maintained without failure over an infinite
number of cycles.
 Endurance limit is lower for materials with brittle
and rough surface.
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20. STRENGTH PROPERTIES
FATIGUE STRENGTH:
 Dental restorative materials may exhibit static
fatigue failure or dynamic fatigue failure.
 Depends upon the nature of loading or residual
stress situations.
 Failure begins as a flaw that propagates till the
catastrophic fracture occurs.
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21. STRENGTH PROPERTIES
IMPACT STRENGTH:
 Impact is the reaction of a stationary
object to a collusion with a moving body.
 Impact strength is defined as energy
required to fracture a material under an
impact force.
 The energy units are joules.
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22. ELASTIC MODULUS
 Also called as modulus of elasticity or Young’s
modulus.
 It is the relative stiffness or rigidity of a material.
 Measured by the slope of the elastic region of
the stress strain curve.
 If a tensile or compressive stress (below the
proportional limit) is divided by corresponding
strain value, a constant of proportionality will be
obtained.
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23. ELASTIC MODULUS
 Unaffected by the amount of elastic or
plastic stress induced in the material.
 Independent of ductility of a material.
 The lower the strain for a given
stress, greater will be the elastic modulus.
 E.g. two wires of same shape and size.
 Polyether impression materials.
 Unit is Giganewtons/m² (GPa).
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24. FIRST MONTHLY CLASS TEST
THEORY PAPER  TOPICS:
 3RD MAY 2012 1) INTRODUCTION TO DENTAL
(THURSDAY) MATERIALS
2) SELECTION & EVALUATION
LECTURE TIMING OF DENTAL MATERIALS.
3) BIOCOMPATIBILITY OF
DENTAL MATERIALS.
VIVA
4) PHYSICAL PROPERTIES OF
 4TH
MAY 2012 DENTAL MATERIALS.
(FRIDAY) 5) MACHANICAL PROPERTIES
OF DENTAL MATERIALS
TUTORIAL TIMINGS
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25. STRESS-STRAIN CURVE
 For materials in which strain is
independent of the length of time that a
load is applied “ STRESS STRAIN CURVES“
are important.
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26. ANALYSIS FOR A STRESS STRAIN
CURVE
STIFFNESS & FLEXIBILITY
1) If longitudinal portion of the curve is
closer to the long axis the material is stiff
& not flexible.
2) If it is away from the long axis the
material is flexible.
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27. ANALYSIS FOR A STRESS STRAIN
CURVE
TOUGHNESS & BRITTLENESS
1) If material fractures after a long concave
portion of the curve, it donates that the
material is tough & ductile.
2) If elastic portion of the curve is minimal,
it shows the brittleness of the material.
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28. ANALYSIS FOR A STRESS STRAIN
CURVE
STRNGTH & WEAKNESS
 If longitudinal portion of curve is longer,
means that the material is strong.
 If longitudinal portion is short the material
is weak.
HENCE FROM THE ANALYSIS OF THE
STRESS STRAIN CURVE IT IS
POSSIBLE TO HAVE AN IDEA ABOUT
THE PROPERTIES OF A MATERIAL. 28

29. STRAIN TIME CURVES
 For materials in which the strain is
dependent upon the time for which the
load is being applied “STRAIN TIME
CURVES” are mor useful in explaining the
properties of a material than stress strain
curves.
 Examples:
Alginate & rubber base impression
materials, dental amalgam & human
dentin.
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30. STRESS STRAIN CURVES
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31. STRESS STRAIN CURVES
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32. Dynamic Young’s Modulus
 Can be measured by dynamic method.
 Ultrasonic longitudinal and transverse
wave transducers and appropriate
receivers are used.
 The velocity of sound wave and density of
material are used to calculate elastic
modulus.
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33. RESILIENCE
 The amount of elastic energy per unit
volume released when the stress is
removed.
 With increase in interatomic spacing the
internal energy increases.
 Until the stress is lower than proportional
limit, the energy is called as RELILIENCE.
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34. TOUGHNESS
 Amount of elastic and plastic deformation
energy required to fracture a material.
 Measured by the area under the elastic region of
the stress strain curve.
 Toughness increases with increase in strength
and ductility.
 Tough materials are generally strong.
 Resistance of a brittle material to propagation of
flaws under an applied stress (FRACTURE
TOUGHNESS)
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35. DUCTILITY and MALLEABILITY
 DUCTILITY:
Ability of a material to deform plastically
under a tensile stress before fracture. e.g.
metal drawn readily into long thin wires.
 MALLEABILITY:
The ability of a material to sustain plastic
deformation, without fracture under
compression.
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36. DUCTILITY and MALLEABILITY
 Gold is the most ductile and malleable
pure metal, followed by silver.
 Platinum is ranked third in ductility.
 Copper ranks third in malleability.
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37. HARDNESS
 In mineralogy, relative hardness of a
substance is based upon its ability to resist
scratching.
 In metallurgy and mostly in all other
disciplines, hardness is defined as
resistance to indentation.
 Designated as
 KNOOP HARDNESS NUMBER.
 BRINELL HARDNESS NUMBER.
 VICKERS HARDNESS NUMBER. 37
 ROCKWELL HARDNESS NUMBER.

38. TERMS TO REMEMBER
Shapes produced by indentors
On materials
KNOOP HARDNESS VICKERS
TEST HARDNESS
TEST
BRINELL &
ROCKWELL
38 HARDNESS TEST

39. QUESTIONS???
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