mechanical_properties

8/7/2019 MECHANICAL_PROPERTIES

1/42

MECHANICAL PROPERTIESOF

DENTAL MATERIALS

DR. VINAMRA DHARIWAL, MDS

23-10-07DEPARTMET OF PROSTHETICS @ Chettinad Hospital & Research Institute

Dental material lecture


2/42

STRESS-STRAIN CURVE

Stress is the force / unit area acting on millions of atom

or molecules in a given plane of a material. Strain is the change in length / unit length. It is the

relative deformation of an object subjected to a stress.

Tensile stress: is caused b a load that tends to stretch

DEPARTMET OF PROSTHETICS @ Chettinad Hospital & Research Institute

or elongate a body. A tensile stress is alwaysaccompanied by tensile strain.

Shear stress: tends to resist the sliding or twisting of

one portion of a body over another.

DR. VINAMRA DHARIWAL


3/42

STRESS-STRAIN CURVE

Compressive stress: if a body is placed under a load that

tends to compress or shorten it, the internal resistanceto such a load is called compressive stress

Elastic strain is reversible i.e. the object fully recovers

its ori inal sha e when the force is removed.


Plastic strain represents a permanent deformation ofthe materials that does not decrease when the force is

removed.



4/42

A tensile load is applied

to a wire in smallincrements until it

breaks.

If each stress is lotted on

STRESS-STRAIN CURVE (Contd..)


a vertical co-ordinate andthe corresponding strain

is plotted on the

horizontal co-ordinate a

curve is obtained.



5/42

This is called stress-strain

curve. It is useful to study some

of the mechanical

ro erties.

STRESS-STRAIN CURVE (Contd..)




6/42

PROPORTIONAL LIMIT

Hookes Law: Within

elastic limit strain is

proportional to stress.

The stress-strain curve is a

strait line. Along this line

ma er a e aveselastically and springs back

to its original size and

shape the instant force is

removed. Beyond point Pthe line becomes non-

linear, stress is no longer

proportional to strain.



7/42

ELASTIC LIMIT

The point Pabove which the curve degresses from a

strait line , is the maximum elastic strain a material canundergo before under going permanent deformation and

this greatest elastic stress is known as Elastic limit.Elastic limit.

For ractical ur ose the elastic limit and ro ortional


limit represent the same stress. However , thefundamental concept is different, one describes the

elastic behavior of the material whereas other deals

with proportionality of stress to strain.



8/42

STRENGTH

It is the maximal stress required to fracture a structure.

Strength is not a measure of individual atom to atomattraction or repulsion , but rather it is a measure of the

interatomic forces collectively over the material which

is stressed.


STRENGTH IS BASICALLY OF FOUR TYPES:

Tensile

Compressive

Shear Flexure



9/42

TENSILE STRENGTH

Tensile Strength is

determined by subjectinga rod , wire or adumbbell shapedspecimen to a tensile


. It is defined as the

maximal stress thestructure will withstandbefore rupture.



10/42

DIAMETRAL TENSILE STRENGTH

Brittle material an

indirect tensile testcalled Diametral

compression test or

Brazillian test is used .


A compressive load isplaced on the diameter

of a short cylindrical

material .

Stress = 2(load) diathickness



11/42

COMPRESSIVE STRENGTH

Crushing srength isdetermined by subjectinga cylindrical specimen toa compressive load.

The strength is obtained


rom e cross sec onaarea and force applied.

Complex failure



12/42

SHEAR SRENGTH

Maximum stress a

material can withstandbefore failure in a shear

mode of loading. It is

tested usin unch or


pushout method. Shear strength = Force/

punch dia * thickness



13/42

TRANSVERSE SRENGTH

Transverse strength or

modulus of rupture orflexure strength

Obtained using a beam

su orted at each end


and load applied in themiddle.

Also called three point

bending test.

Used in long span bridges. Neutral Axis



14/42

POISSONS RATIO

If a cylinder is subjected to tensile stress or compressive

stress, there is simultaneous axial and lateral strain.Within elastic range the ratio of lateral to the axial

strain is called Poisson's ratio.

It is related to nature and s mmetr of inter-atomic


bonding forces. Dental materials have Poisson's ratio value in range of

0.3 to 0.5.



15/42

ELASTIC MODULUS

SYNONYM: Youngs modulus or

Modulus of elasticity

DEFINITION:


elastic range. It is the ratio of elastic stress to elasticstrain. It has a constant value as determined from a

stress strain graph.

ELASTIC STRAIN:

Deformation that is recovered upon removal of an

externally applied force or pressure.



16/42

ELASTIC MODULUS

Because Elastic modulus represents the ratio of Elastic

stress to the Elastic strain, it follows that the lower thestrain for a given stress, the greater the value of the

Modulus and stiffer is the material.

It is not a measure of stren th of a material and is


independent of ductility of material. It is measured in the linear region of the stress-strain

graph.



17/42

This property is indirectly related to other mechanical

properties. Modulus of elasticity is given in units of force per unit

area ,typically giga Newton per square meter (GN/m2 )

or i a Pascals GPa .

ELASTIC MODULUS (Contd..)


E= Stress / Strain = ( P/A) / (l/ lo)

E = Elastic Modulus

P = Applied force or loadA= cross sectional area of the material under stress

l= increase in length

lo= original length



18/42

STRESS STRAIN CURVE -MODULUS OF ELASTICITY


Slope of s-s curve



19/42

APPLICATION

The metal frame of a metal-ceramic bridge should have

a high stiffness. If the metal flexes, the porcelainveneer on it might crack or separate.

If one wire is more difficult to bend than another of the

same size and sha e considerabl hi her stress must be


induced before the desired strain can be produced in astiffer wire. Such a material would possess a

comparative high modulus of Elasticity.

A polyether material have greater stiffness than all

other elastomeric impression materials. thus a greaterforce is needed to remove a impression tray from

undercuts in mouth.



20/42

DYNAMIC ELASTIC MODULUS

Measured when system in dynamic status.

Measured by ultrasonic longitudinal and transverse wavetransducers and appropriate receivers.

Easy to calculate but slightly higher values than static

method within acce table ran e.


If shear stress is induced then we can calculate shearstrain and shear modulus of the material.

Shear modulus is usually about 38 % of the elastic

modulus.



21/42

YIELD STRENGTH

A small amount of permanent strain is tolerable.

The limit of tolerable permanent strain is the Yieldstrength.

Yield strength is defined as the stress at which a

material exhibits a s ecified limitin deviation from


proportionality of stress to strain. It is a property that represents the stress value at which

a small amount (0.1% or 0.2%) of plastic strain has

occurred.

A value of either 0.1% or 0.2% of the plastic strain isoften selected and is referred to as the Percent offset.

Cannot be used for brittle materials.



22/42

STRESS STRAIN CURVE YIELD STRENGTH

DEPARTMET OF PROSTHETICS @ Chettinad Hospital & Research InstituteDR. VINAMRA DHARIWAL


23/42

FLEXIBILITY

Ability of a material to deform or under go larger strain

for relatively less stress. The maximal flexibility is defined as the strain that

occurs when the material is stressed to its proportional

limit.


For e.g. in an orthodontic appliance, a spring is oftenbent to a considerable distance under the influence of a

small stress. Such a material is referred as flexible.



24/42

RESILIENCE

The amount of energy absorbed within a unit volume of

a structure when it is stressed to its proportional limit. It is also described as the relative amount of elastic

energy per unit volume released on unloading of a test

s ecimen. It is a term associated with s rin iness of a

DEPARTMET OF PROSTHETICS Chettinad Hos ital & Research Institute

material. Resilience is measured using the stress-strain graph and

it corresponds to the area under the straight line portion

of curve. It is the area bounded by the elastic region.



25/42

The material with larger

elastic area has thehigher resilience.

Modulus of resilience

R= ro ortional limit 2

RESILIENCE


2 Elastic modulus.



26/42

TOUGHNESS

It is defined as energy required to fracture a material.

It is measured as a total area under stress strain curve. Toughness of the material is dependent on the ductility

and malleability of the material than upon the flexibility

or elastic modulus.




27/42

TOUGHNESS



28/42

IMPACT STRENGTH

IMPACT:

It is the reaction of a stationary object to a collision

with a moving object. Depending on the resilience of

the object , energy is stored in the body without causing

deformation or with deformation.


Impact resistance decreases with increase in stiffness. Resilient material have high impact strength.

Increase in volume leads to increase in impact

resistance.



29/42

IMPACT STRENGTH

It is the energy required

to fracture a materialunder force.

A charpey type tester is

used. It has a heav


pendulum which swingsdown to fracture the

specimen.

Another instrument called

Izod impact tester canalso be used.



30/42

PERMANENT DEFORMATION

Once the Elastic limit of a material is crossed by a specific

amount of stress ,the further increase in strain is calledpermanent deformation.

Im ression materials


Clasp



31/42

FATIGUE

A Structure subjected to repeated or cyclic stress below

its proportional limit can produce abrupt failure of these

structure.

Fatigue behavior is determined by subjecting a material

to a c clic stress of known value and determinin the


number of cycles that are required to produce failure.



32/42




33/42

STATIC FATIGUE

Some material support a static load for a long period of

time and fail abruptly. This type of failure may occur in

wet environment. E.g. ceramic materials.



34/42

BRITTLENESS

A brittle material fractures at or near its proportional

limit.

It is opposite of toughness.

Brittle material will not bend appreciably without

breakin .


Though a brittle material may have a very highcompressive strength. E.g. glass.



35/42

DUCTILITY

Ability of a material to withstand permanent

deformation under a tensile load without rupture.

It is the ability of the metal to be drawn into wires.

Ductility depends on tensile strength.

It decreases with increase in tem erature.




36/42

MEASUREMENT OF DUCTILITY

1.Percentage elongation after fracture

Gauge length = 51mm

1.Measuring reduction in cross sectional areas of fractured

ends in com arison to the ori inal area of the wire. This

DUCTILITY (Contd..)


is also called as reduction in area method.2.Cold bend test.



37/42

MALLEABILITY

It is the ability of a material to withstand rupture under

compression.

It is seen in hammering or rolling of a material into

sheets.

It is not de endent on the stren th of the material


It increases with temperature. Gold is most ductile and malleable and silver stands the

second.

Platinum is third most ductile and copper is third most

malleable.



38/42

COLD WORKING

Strain hardening or work hardening.

When a metal is stressed beyond its proportional limit

the hardness and strength of the material increases at

the area of the formation but the ductility of the metal

decreases and the brittleness of the metal increases.


The key to minimize the risk of embrittlement is todeform the metal in small increments.



39/42

STRESS CONCENTRATION FACTORS

THESE INCLUDES

Surface flaws

Internal voids

air bubbles.

Inclusions of other materials


Hertzian load Sharp angles

Notches

Thermal mismatch



40/42

READING GRAPH



41/42

EXCERSICE



42/42

2007 Chettinad Hospital & Research Institute

mechanical_properties

Documents