Mechanical Properties of dental materials

MECHANICAL PROPERTIES
OF
DENTAL MATERIALS
By
Dr Khawaja Rashid Hassan
Assistant Professor
RAWAL INSTITUTE OF HEALTH SCIENCES
RAWAL COLLEGE OF DENTISTRY
ISLAMABAD
1

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.

MECHANICAL PROPERTIES OF
DENTAL MATERIALS



1.
2.


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

MASTICATORY FORCES
Tooth






4

Occlusal forces
applied by adult
dentition is greatest
in posterior region.
In growing children
there is an average
annual increase in
force of 22 N.
Denture wearers
only apply 40% of
the forces given in
table.

Second
molar
First molar

Average
force (N)
800
390

Bicuspids

288

Cupids

208

Incisors

155

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.

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.

TYPES OF FORCES APPLIED

1.
2.
3.
4.

Generally, the force applied may be
Axial (tensile or compressive)
Shear (sliding, rubbing)
Bending (bending movement)
Tortional (twisting movement)

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.

TYPES OF FORCES APPLIED
TENSION

COMPRESSION

9

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.

TYPES OF STRESSES


1.

2.
3.

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

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.

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.

13

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.
14

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.

15

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.
16

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.
17

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”
18

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.
19

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.
20

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.

21

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.
22

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).
23

FIRST MONTHLY CLASS TEST
THEORY PAPER
 3RD

MAY 2012
(THURSDAY)
LECTURE TIMING

 TOPICS:
1)
2)
3)

VIVA
 4TH

MAY 2012
(FRIDAY)
TUTORIAL TIMINGS

4)
5)

INTRODUCTION TO DENTAL
MATERIALS
SELECTION & EVALUATION
OF DENTAL MATERIALS.
BIOCOMPATIBILITY OF
DENTAL MATERIALS.
PHYSICAL PROPERTIES OF
DENTAL MATERIALS.
MACHANICAL PROPERTIES
OF DENTAL MATERIALS

24

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.

25

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.

26

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.
27

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

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.
29

STRESS STRAIN CURVES

30

STRESS STRAIN CURVES

31

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.
32

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.
33

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)
34

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.
35

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.

36

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.
ROCKWELL HARDNESS NUMBER.
37

TERMS TO REMEMBER
Shapes produced by indentors
On materials

KNOOP HARDNESS
TEST

38

VICKERS
HARDNESS
TEST
BRINELL &
ROCKWELL
HARDNESS TEST

QUESTIONS???
39