1 mechanical properties renjith raj

MECHANICAL PROPERTIES
OF DENTAL MATERIALS
DR.RENJITH RAJ

CONTENTS
• Introduction
• Stress and types
• Strain
• Stress-strain diagram
• Modulus of elasticity
• Poisson’s ratio
• Flexibility
• Resilience

• Strength properties:
Proportional limit
Elastic limit
Yield strength
Ultimate strength
Flexural strength
Fatigue strength
Impact strength
Toughness
Brittleness
Ductility, malleability
Conclusion

• Are defined by the laws of mechanics, that is, the
physical science that deals with energy and forces and
their effects on bodies.
• It primarily centers around static bodies, those at rest.
• Mechanical properties are thus the measured responses
both elastic and plastic, of materials under an applied
force or distribution of forces or pressure.
4
MECHANICAL PROPERTIES

IMPORTANCE
• In understanding and predicting a material’s behaviour
under load.
• Most materials used in dentistry should have some
minimal mechanical properties so as to be useful in
clinical applications.
• Good mechanical properties of any dental restoration
ensure that , it serves :
Effectively
Safely and,
For a reasonable time period.

6
STRESS
• It is the force per unit area within a structure subjected to
an external force or pressure.( Phillips science of dental
materials, eleventh edition.)
• It is the internal reaction to an externally applied force
which is equal in intensity and opposite in direction to the
applied external force(Restorative dental materials, craig)
Stress = Force
Area
• SI UNITS- MPa or psi .
• is represented by the Greek letter, sigma.

TYPES OF STRESSES:-
 COMPRESSIVE
 TENSILE
 SHEAR

• Compressive stress :
if a body is placed under a load that
tends to compress or shorten it, the
internal resistance to such a load is
called a compressive stress
• Tensile stress :
 A tensile stress is caused by a load
that tends to stretch or elongate a
body
 Tensile stresses can be generated
when structures are flexed
8

• Shear stress:
A stress that tends to resist the sliding or twisting of one
portion of a body over another is termed as shear stress.
9

STRAIN
• Is the change in length per unit length of a body
when it is subjected to a stress.(Craig,11th edn)
• It has no unit of measurement ; is often expressed
as a percentage.
• is represented by the Greek letter, epsilon.
• Strain = Change in Length
Original Length
• Strain () = Deformation
original length
10

• Strain can either be elastic or plastic
• Elastic strain is that strain that totally disappears once
the external load that caused it is removed, is reversible.
• Elastic strain is based upon the fact that a net force of
zero exists between two atoms when they are at
equilibrium.
• If a compressive or tensile force is exerted on the atoms,
an opposite force will attempt to move them back to their
equilibrium position.
11

• When the applied force is released, the atoms return to
their original position; therefore, the material is not
permanently deformed.
• Plastic strain is strain that permanently remains once
the external load that caused it is removed.
• It occurs when the force applied to the atoms moves
them so far from their equilibrium position that they do
not return to it once the force is removed.
12

STRESS-STRAIN DIAGRAM
• Is a graphic way of displaying stress and strain.
• Generally, the diagram is produced by
gradually loading a material using an
Instron or similar testing machine.
• The resultant strain values are measured and used
to calculate stress values.
• These are then plotted against strain to produce the
stress-strain diagram for the material.
• Traditionally, stress is plotted on the vertical axis
and strain on the horizontal axis.
• Many of the basic physical properties of dental
materials can be represented on a stress-strain
diagram. 13

14
• The straight part of the line
represents the region of elastic
deformation
• The curved part of the line
represents the region of plastic
deformation
• The slope of the straight part of
the line represents modulus of
elasticity
• The length of the curved part of
the line represents ductility
• The area under the straight
part of the line represents
resilience
• The area under the entire line
represents toughness

MODULUS OF ELASTICITY
(Young’s Modulus,elastic modulus)
• Elastic modulus describes the relative rigidity or stiffness
of the material, which is measured by the slope of the
elastic region of the stress-strain graph.(Phillips’ science
of dental materials,11th edn).
• It is the ratio of stresses to strain up to or less than the
proportional limit.
• Units – MPa or GPa
( 1 GPa = 1000 MPa )
Elastic modulus(E) = Stress
Strain

• On the stress-strain diagram, the elastic modulus is
indicated by the slope of the linear part of the line.
• Therefore, a material with a steep line will have a higher
elastic modulus and be more rigid than a material with a
flatter line. Conversely, material with a flatter line is more
flexible than the material with a steep line.
• Modulus of elasticity is a reflection of the strength of the
inter-atomic or intermolecular bonds.
• The stronger the basic attraction forces, the greater the
values of the elastic modulus and the more rigid or stiff
the material.
16

• As this property is related to the attraction forces within the
material, it is usually same when the material is in tension or
compression.
• Elastic modulus can be calculated as follows:-
• By definition
where
E is the elastic modulus
Thus P is the applied force or load
A is the cross-sectional area
of the material under stress
∆l is the increase in length
l0 is the original length 17
STRESS = P/A =σ
STRAIN = ∆l /lo= €
E = Stress = σ = P/A
Strain € ∆l /lo

CLINICAL SIGNIFICANCE
• Predicts the material’s ability to resist bending or change
in shape and is an important property for dental bridges,
orthodontic wires and many other restorations.
• It is advantageous for an impression material to be
flexible, whereas it is essential for a restorative material
to be rigid.

Material Elastic modulus
GPa
1) Cobalt – chromium partial
denture alloy
218.0
2) Gold (type IV) alloy 99.3
3) Enamel 84.1
4) Fieldspathic porcelain 69.0
5) Amalgam 27.6
6) Dentin 18.3
7) Acrylic denture resin 2.65
8) Silicone rubber (maxillofacial) 0.002
19

Poisson’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 the cross
section.
• Within the elastic range, the
ratio of the lateral to the axial
strain is called Poisson’s
ratio 20

• For an ideal isotropic material of constant volume the
ratio is is said to be 0. 5
• Most engineering material have values of approx 0.3
• In tensile loading, the Poisson’s ratio indicates that the
reduction in cross section is proportional to the
elongation during the elastic deformation.
21

Material Poisson’s ratio
Composite resin 0.24
Amalgam 0.35
Zinc phosphate cement 0.35
Enamel 0.30
22

FLEXIBILITY
• An object under stress will return to its
original form when the stress is removed.
• This happens within the elastic limit.
• If the stress is more than the elastic limit, permanent
deformation takes place.
• The ability of a material to return to its original form
indicates its elasticity, but the strain taking place at elastic
limit is known as flexibility.
23

• Flexibility is basically bending capacity.
• This can be defined as the strain that occurs when the
material is stretched to its proportional limit
• Flexibility with respect to impression material is important
because the impression material should be flexible
enough to come out of undercut areas without causing
permanent change in shape and size of the impression.
• After coming out of the undercut following removal of the
impression the material must go back to its original
position and this is known as elastic recovery.
24

RESILIENCE
• Resilience can be defined as the amount of energy
absorbed within a unit volume of a structure when it is
stressed to its proportional limit.(Phillip’s science of dental
materials,11th edn)
• The property is often described as "springback potential."
• It is quantitatively measured as the modulus of resilience
which is the proportional limit squared divided by 2 times the
modulus of elasticity.
• This quantity is expressed in units of energy per unit
volume.
25

• Resilience is represented graphically by the area under
the linear part of the stress-strain diagram.
26

Clinical application:
• In most of dental restorations, large strains are avoided
because of the proprioceptive response of the receptors
in the periodontal ligament.
• For ex, a proximal inlay might cause the excessive
movement of the adjacent tooth, if large strains are
developed.
• Hence, the restorative material should exhibit a
moderately high elastic modulus and relatively low
resilience.

STRENGTH PROPERTIES
• Strength is the stress that is necessary to cause
fracture(ultimate strength) or a specified amount of plastic
deformation( yield strength) .(Phillips’ science of dental
materials,11th edn)
• Strength of a material can be defined by one or more of the
following properties:
 Proportional limit.
 Elastic limit.
 Yield strength.
 flexural strength,
 fatigue strength
 impact strength
 toughness , brittleness , hardness
28

PROPORTIONAL LIMIT
• The proportional limit is defined as the greatest stress that
a material will sustain without a deviation from the linear
proportionality of stress to strain. (Restorative
dental materials,craig;11th edn)
• Below the proportional limit, no permanent
deformation occurs in a structure.
• can alternatively be defined as the limit of
proportionality of stress to strain.
Is represented on the stress-strain diagram as the point
where the plotting converts from a straight line to a curve.29

• The region of the stress strain curve
before the proportional limit is called the
Elastic region.
• The region of stress strain curve
beyond the proportional limit is called
the Plastic region.
• Below the proportional limit, stress is
proportional to strain.
• Stresses below the proportional limit
cause elastic (non-permanent)
deformation and those above it cause
elastic and plastic (permanent)
deformation.
• A high proportional limit is desirable for
a restorative material.
30

ELASTIC LIMIT
• The elastic limit is defined as the
maximum stress that a material will
withstand without permanent
deformation
• For all practical purposes, the same
as the proportional limit.
• Greatest stress to which a material
can be subjected such that it returns
to its original dimensions when the
force is released.
31

YIELD STRENGTH
• The yield strength is defined as the stress at which a
material exhibits a specified limiting deviation from
proportionality of stress to strain.
• The yield strength of a material is used to describe the
stress at which the material begins to function in a plastic
manner.
• At this stress, a limited permanent strain has occurred in
the material.
32

• Is the amount of stress required to
produce a predetermined amount of
permanent strain usually 0.1% or
0.2% which is called the Percent
Offset
• It is a useful property because it is
easier to measure than the
proportional limit. It is measured
using the stress-strain diagram by
locating the point 0.1% or 0.2% out
on the strain axis and drawing a line
up to the curve which is parallel to
the line found in the elastic region.
33

• Although the term elastic limit, proportional limit and yield
strength are defined differently they have nearly same
magnitudes and can be used interchangeably for all
practical purposes.
• These values are important in the evaluation of dental
materials since they represent the stress at which
permanent deformation begins.
• If they are exceeded by masticatory stresses, the
restoration / appliance may no longer fit as originally
designed.
34

ULTIMATE STRENGTH
• The ultimate tensile strength or
stress is defined as the maximum
stress that a material can withstand
before failure in tension, whereas
the ultimate compressive
strength or stress is the maximum
stress that a material can withstand
in compression.
• Determined by dividing the
maximum load in tension or
compression by the original cross-
sectional area of the test specimen.

• The ultimate strength of an alloy is used in
dentistry to give an indication of the size or cross
section required for a given restoration.
• An alloy that has been stressed to near the
ultimate strength will be permanently deformed,
so a restoration receiving that amount of stress
during function would be useless.

FLEXURAL STRENGTH
(TRANSVERSE STRENGTH OR
MODULUS OF RUPTURE)
• The flexural strength of a material is its ability to bend
before it breaks.
• It is a measure of how a material behaves when under
multiple stresses.
• It is measured by subjecting a beam of the material to
three- or four-point loading which results in the
development of multiple stresses.
37

• This type of test is reserved for
materials like denture base resins
that experience these types of
multiple stresses during function.
• To evaluate flexural strength of a
dental material, generally bar-
shaped specimens with dimension
of 25 mm in length X 2 mm in width
X 2 mm in height are used.
• Specimens are placed on two
supports and a load is applied at
the center.
• This test is known as three-point
bending test . 38

FATIGUE STRENGTH
• A structure that has been subjected to a stress below the
yield stress and subsequently relieved of this stress
should return to its original form without any change in its
internal structure or properties.
• A few such applications of stress do not appreciably
affect a material.
• But when this stress is repeated a great number of
times, the strength of the material may be drastically
reduced and ultimately cause failure.
39

• Fatigue is defined as a progressive fracture under
repeated loading.
• The Fatigue strength is the stress at which a material
fails under repeated loading.
• Failure under repeated or cyclic loading is therefore
dependent on the magnitude of the load and the number
of loading repetition.
• For some materials stress can be loaded infinite number
of times without failure.
• This stress is called the Endurance Limit. (maximum
stress that can be maintained without failure over an
infinite number of cycles). 40

• Any environmental agent that can degrade a material
will reduce fatigue strength.
• High temperature, humidity, aqueous media, saline
environment with proteins and fluctuating pH(deviations
away from neutral), all tend to reduce fatigue strength.
• If the surface is rough, endurance limit is low.
• A rough brittle material would fail in fewer cycles of
stress.
41
Smooth surface
Rough surface
Cycles to fracture
Failurestress

• Endurance limit is the maximum applied stress that a
material can withstand and still have an unlimited
number of cycles to failure.
• Fatigue leads to failure of materials because it promotes
crack propagation.
• The imperfections lead first to the development of micro-
cracks, which coalesce and ultimately lead to a
macroscopic crack and failure.
• Factors that influence fatigue are grain size and shape,
composition, texture, surface chemistry and roughness
42

IMPACT STRENGTH
• It may be defined as the energy required to fracture a
material under an impact force.
• The term impact is used to describe the reaction of a
stationary object to a collision with a moving object.
• A Charpy -Type impact tester is usually used to
measure impact strength.
43

• A pendulum is released that swings down
to fracture the centre of a bar specimen that
is supported at both ends.
• The energy lost by the pendulum during the
fracture of the specimen can be determined
by the comparison of the length of the swing
after the impact with that of its free swing
when no impact occurs.
• The dimensions, shape and design of the
specimen to be tested should be identical
for uniform results.
• Another impact device called the IZOD
IMPACT TESTER,
44
Charpy impact tester

PERMANENT( PLASTIC) DEFORMATION
• 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 strain does
not decrease to zero because of plastic deformation.
• Thus if the object does not return to its original dimension
when the force is removed, it remains bent, stretched,
compressed or plastically deformed.
46

TOUGHNESS
• Defined as the amount of elastic and plastic
deformation energy required to fracture a material.
• Toughness can be measured as the total area under
the stress strain curve from zero stress to fracture
stress. Toughness depends on strength and ductility.
• The greater the strength and higher the ductility,
greater the toughness.
• Units-same as units of resilience-
mMN/m3 or mMPa/m.
47

FRACTURE TOUGHNESS
• It is a mechanical property that describes the
resistance of brittle materials to the catastrophic
propagation of flaws under an applied stress.
• Inversely proportional to the square root of the flaw depth
in to the surface.
• Presence of fillers in composites increases fracture
toughness.
48

BRITTLENESS
• Brittleness is the relative inability of a material to sustain
plastic deformation before fracture of a material occurs.
• Example : amalgam ,ceramics and composites are brittle at
oral temperatures( 5-55°C).
They sustain little or no plastic strain before they fracture.
• A brittle material may not necessarily
be weak.
• In other words, a brittle material fractures at
or near its proportional limit.
49

DUCTILITY
• Ductility is the ability of materials to sustain a large
permanent deformation under a tensile load before it
fractures.
• Gold is the most ductile metal,
second is silver and Platinum
comes third in ductility.
• An increase in temperature decreases
ductility because a material's strength
generally decreases with an increase
in temperature.
50

1 mechanical properties renjith raj

MEASUREMENT OF DUCTILITY
There are three common methods for measurement of ductility:
1. Percent elongation after fracture:- compares the increase in
length of a wire after fracture in tension to its original length.
2. Reduction in area of tensile test specimens:- The
percentage of decrease in cross-sectional area of the fractured
end in comparison to the original area of the wire or rod.
3. Maximum number of bends performed in a Cold bend
test:-The material is clamped in a vise and bent around a
mandrel, the number of bends to fracture is counted, and
greater the number, the greater is the ductility.
One way that ductility is used in dentistry is as a measure of
Burnishability of margins of a casting.
53

MALLEABILITY
• Malleability is the ability of a material to sustain
permanent deformation without rupture under
compression as in hammering or rolling into a sheet
• Gold is the most malleable, second is silver and
aluminium third
• An increase in temperature generally results in a
increase in malleability because malleability is
dependent upon dislocation movement, and dislocations
generally move more easily at a higher temperature.
54

Ductility Malleability
Gold Gold
Silver Silver
Platinum Aluminium
Iron Copper
Nickel Tin
Copper Platinum
Aluminium Lead
Zinc Zinc
Tin Iron
Lead Nickel

Conclusion
• It is very important to know the properties of the
materials we use in dentistry, especially , restorative
materials. This will enable us to select a material that will
have properties close to that of natural tooth structure.
• Also we will be able to better understand and select
materials from the wide range that are coming in to the
market.
• Hence, a thorough knowledge of the various properties
of restorative materials is a must for each and every
dentist.

REFERENCES
• PHILLIPS’ SCIENCE OF DENTAL MATERIALS,11TH
EDITION.
• RESTORATIVE DENTAL MATERIALS,
ROBERT.C.CRAIG,11TH EDITION.
• NOTES ON DENTAL MATERIALS, E.C. COMBE.
• DENTAL MATERIALS;CLINICAL APPLICATIONS,
PANKAJ DATTA.

Evaluation of fracture toughness and mechanical properties of
ternary thiol-ene-methacrylate systems as resin matrix for
dental restorative composites. (Dent mater may 2013)
OBJECTIVE:
• Study and evaluation of fracture toughness, flexural and dynamic
mechanical properties, and crosslink density of ternary thiol-ene-
methacrylate systems and comparison with corresponding
conventional methacrylate system were considered in the present
study.
METHODS:
• Urethane tetra allyl ether monomer (UTAE) was synthesized as ene
monomer. Different formulations were prepared based on
combination of UTAE, BisGMA/TEGDMA and a tetrathiol monomer
(PETMP). The photocuring reaction was conducted under visible
light using BD/CQ combination as photoinitiator system. Mechanical
properties were evaluated via measuring flexural strength, flexural
modulus and fracturetoughness).

. Scanning electron microscopy (SEM) was utilized to study the morphology of the
fractured specimen's cross section. Viscoelastic properties of the samples were also
determined by dynamic mechanical thermal analysis (DMTA). The same study was
performed on a conventional methacrylate system. The data were analyzed and
compared by ANOVA and Tukey HSD tests (significance level=0.05
RESULTS:
The results showed improvement in fracture toughness of the specimens containing
thiol-ene moieties. DMTA revealed a lower glass transition temperature and more
homogenous structure for thiol-ene containing specimens in comparison to the system
containing merely methacrylate monomer. The flexural modulus and flexural strength of
the specimens with higher thiol-ene content were lower than the neat methacrylate
system. The SEM micrographs of the fractured surface of specimens with higher
methacrylate content were smooth and mirror-like (shiny) which represent brittle
fracture.

The effect of filler loading and morphology on the mechanical properties of
contemporary composites(journal of prosthetic dentistry 2009)
PURPOSE:
The objectives of this study were to: (1) classify commercial composites according to
filler morphology, (2) evaluate the influence of filler morphology on filler loading, and (3)
evaluate the effect of filler morphology and loading on the hardness, flexural strength,
flexural modulus, and fracture toughness of contemporary composites.
MATERIAL AND METHODS:
Field emission scanning electron microscopy/energy dispersive spectroscopy was used
to classify 3 specimens from each of 14 commercial composites into 4 groups according
to filler morphology. The specimens (each 5 x 2.5 x 15 mm) were derived from the
fractured remnants after the fracture toughness test. Filler weight content was
determined by the standard ash method, and the volume content was calculated using
the weight percentage and density of the filler and matrix components. Microhardness
was measured with a Vickers hardness tester, and flexural strength and modulus were
measured with a universal testing machine. A 3-point bending test (ASTM E-399) was
used to determine the fracture toughness of each composite. Data were compared with
analysis of variance followed by Duncan's multiple range test, both at the P<.05 level of
significance.
.

RESULTS:
The composites were classified into 4 categories according to filler morphology:
prepolymerized, irregular-shaped, both prepolymerized and irregular-shaped, and round
particles. Filler loading was influenced by filler morphology. Composites containing
prepolymerized filler particles had the lowest filler content (25% to 51% of filler volume),
whereas composites containing round particles had the highest filler content (59% to
60% of filler volume). The mechanical properties of the composites were related to their
filler content. Composites with the highest filler by volume exhibited the highest flexural
strength (120 to 129 MPa), flexural modulus (12 to 15 GPa), and hardness (101 to 117
VHN). Fracture toughness was also affected by filler volume, but
maximum toughness was found at a threshold level of approximately 55% filler volume

ABSTRACT
The purpose of this laboratory investigation was to evaluate three mechanical properties;
the compressive, diametral tensile and flexural strengths of five different core build-up
materials. In this study, a light-actived Hybrid composite resin (President), resin modified
glass ionomer (RMGIC) (Vitremer), amalgam (Cavex avalloy), glass-ionomer (GIC)
(Logofil) and compomer (Dyract AP) restorative materials were used.
MATERIALS AND METHODS
120 samples were prepared according to American Dental Association specification
No. 27 for testing diametral tensile strength (DTS), compressive strength (CS) and
flexural strength (FS). Forty specimens were prepared in cylindric molds (6 mm in height,
3 mm in diameter) for the CS measurements and forty specimens (3 mm in height, 6 mm
in diameter) for diametral tensile strength (DTS). Forty specimens were prepared
(25X 2 X 2 mm) for the FS measurements. All cores materials were prepared according to
manufacturer’s instruction at a temperature of 23.0 +/- 1.0 degrees C. Haunsfield press
and pull machine was used for compressive and flexural strength and the module were
determined at a crosshead speed of 0.5 mm/min. Diametral testing was carried out at
1 mm/min. material.
COMPARISON OF DIAMETRAL TENSILE, FLEXURAL, AND COMPRESSIVE
STRENGTHS OF FIVE CORE BUILD-UP MATERIALS

The results of this study indicated that the diametral tensile strength, flexural strength
and compressive strength of the resin composite (President) and amalgam material
were significantly higher than the other tested materials (p<0.001). On the other hand,
the diametral tensile strength, flexural strength and compressive strength of glass
ionomer based materials (Logofil, Vitremer) were statistically lower than for resin
composites, compomer and amalgam.
Analysis of variance was used for statistically evaluation. Mean compressive,
diametral tensile and flexural strengths with associated standard deviations were
calculated for each material.
RESULTS

1 mechanical properties renjith raj

More Related Content

What's hot (20)

Similar to 1 mechanical properties renjith raj (20)

Recently uploaded (20)

1 mechanical properties renjith raj

Editor's Notes