Good afternoon

1

MECHANICAL

PROPERTIES OF

ORTHODONTIC

BIOMATERIALS

2

content

Introduction

Expression of mechanical properties.

[A] Elastic or reversible deformation –Proportional limit

Resilience

Modulus of elasticity

3

[B]Plastic or irreversible deformation –

Percent elongation

Hardness

[C] Combination of elastic and plastic deformation –

Toughness

Yield strength

conclusion

4

Mechanical properties are defined by law of mechanics

that is the physical science that deals with energy and

forces and their effect on bodies1.

Thus all the mechanical properties are measures of the

resistance of a material to deformation or fracture under

the applied force.

5

Mechanical properties are the measured responses

both elastic (reversible on force removal) and plastic

(irreversible or non elastic) of materials under an

applied forces or pressure.

Mechanical properties are expressed most often in

units of stress and/ or strain.

Keneth j.anusavis,Chiayi shen, H.Ralphs:Phillips’ science of dental materials.Elsevier Inc2013.

6

Expression of mechanical properties:

Expressed in units of stress and strain.

Represents measurement of:

[A] Elastic or reversible deformation –

Proportional limit

Resilience

Modulus of elasticity

Keneth j.anusavis,Chiayi shen, H.Ralphs:Phillips’ science of dental materials.Elsevier Inc2013

7

[B]Plastic or irreversible deformation –

Percent elongation

Hardness

[C] Combination of elastic and plastic deformation –

Toughness

Yield strength

8

To discuss these properties, first we have

to understand the concept of stress and

strain

Based on Newton’s third law of motion (i.e.

for every action there is an equal and opposite

reaction), when an external force act on a solid,

a reaction occurs to oppose this force which is

equal in magnitude but opposite in direction to

the external force. The stress produced within

the solid material is equal to the applied force

divided by the area over which it acts9

Stress : Force per unit area within a structure

subjected to an external force /pressure

For dental applications there are several types of

stress that develop according to the nature of

applied force and object shape.

Nature of force StressTensile force tensile stress

Compressive force compressive stress

Shear /bending force shear stress

10

11

Strain : change in length per unit initial

length

Relative deformation of an object subjected to a stress

Strain may be

[a] Elastic [b] Plastic [c] Elastic and plastic

Reversible deformation Permanent deformation when object deformed past the

elastic limit certain

amount of elastic recovery

Occurs.

12

Force is applied

at the distance

d/2 from

interface [a-b]

Diagram showing elastic shear strain

13

Shear

stress

Force is applied

along interface

Diagram: showing plastic shear strain

plastic

strain

14

Tensile stress:

Ratio of tensile force to the original cross sectional

area perpendicular to the direction of applied

force.

A tensile stress is always accompanied by a tensile

strain, but it is very difficult to generate pure

tensile stress in a body _that is, a stress caused by a

load that tends to stretch or elongate a body.

15

There are few pure tensile stress situations

in dentistry. The deformation of a bridge and

the dimeteral compression of a cylinder can

represent examples of this complex stress

situation.

16

Diagram : showing stresses induced in a three unit bridge

by a flexural force

17

Diagram showing stresses induced in a two unit

cantilever bridge18

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

compressive stress.

Compressive stress is associated with compressive strain.

19

Shear stress : a shear stress tends to resist the

sliding or twisting of one portion of a body over

another .

Example : if a force is applied along the surface of

the tooth enamel and an orthodontic bracket ,the

bracket may debond by shear stress failure of the

resin luting agent .

20

S

f

Plastic

Shear

strain

21

Mechanical properties based on elastic deformation :important

mechanical properties and parameters that are measures of

elastic strain or plastic strain, behaviors of dental materials .

These are :

[a] Elastic modulus / Young’s modulus/ Modulus

of elasticity

[b] Dynamic young’s modulus determined by measurement of ultrasonic wave velocity

[c] Shear modulus

[d] Flexibility

[e] Resilience

[f] Poisson’s ratio 22

Elastic modulus :

Describe the relative stiffness or rigidity of a material

which is measured by slope of the elastic region of

stress -strain graph.

It is independent of the ductility of a material since it

is measured in the linear region of the stress –strain

plot , and it is not a measure of its plasticity or

strength .

23

Conventional stress –strain curve

24

Because the elastic modulus represents the ratio of

elastic stress to the elastic strain, it follows that

the lower the strain for a given stress, the greater

the value of the modulus. For example if one wire

is much more difficult to bend than another of the

same shape and size, considerably higher stress

must be induced before a desired strain or

deformation can be produced in the stiffer wire .

More horizontal the slope-----more springiness.

More vertical the slope------ more stiffness.

25

26

RANGE

Distance that the wire will bend elastically before

permanent deformation occurs.

If the wire is deflected beyond this limit, it will

not return to its original shape, but clinically

useful SPRINGBACK will occur unless the failure

point is reached.

Orthodontic wires are often deformed beyond

their elastic limit, so spring back properties are

important in determining clinical performance.

STRENGTH=STIFFNESS × RANGE.

Calculation of elastic modulus

E is the elastic modulus

P is the applied force or load

A is the cross-sectional area of the material under

stress

Δl is the increase in length

Lo is the original length

By definition: stress=P/A=σStrain= Δl/ Lo= ϵThus,E=stress/strain= σ/ ϵ =[ P/A]/ Δl/ Lo

27

Dynamic Young’s modulus

Elastic modulus can be measured by dynamic method

as well.

Since the velocity at which sound travels through a

solid can be readily measured by ultrasonic

longitudinal and transverse wave transducers and

appropriate receivers, the velocity of sound wave

and the density of the material can be used to

calculate the elastic modulus and Poisson’s ratio

values .

28

Flexibility

The maximum flexibility is defined as

the flexural strain that occurs when the

material is stressed to its proportional

limit.

29

There are instances in which a larger strain or

deformation may be needed with moderate or slight

stress. For example, in an orthodontic appliance, a

spring is often bent considerable distance under the

influence of small stress.

In such a case, the structure is said to be flexible and

it possesses the property of flexibility.

30

Resilience:Resilience can be defined as the amount of

energy absorbed within a unit volume of

structure when it is stressed to its

proportional limit.

As the interatomic spacing increases, the internal

energy increases.

As long as the stress is not greater than the

proportional limit, this energy is known as resilience.

31

The area bounded by elastic region is a measure of

resilience, and the total area under the stress-strain is

toughness.

Amount of permanent deformation that a wire can

withstand before failing is FORMABILITY. 32

FORMABILITY

The resilience of two materials can be

compared by observing the areas under the

elastic region of their stress- strain plots,

assuming that they are plotted on the same

scale. The material with the larger elastic

area has the higher resilience.

33

Poisson’s Ratio

when a tensile force is applied to a cylinder or rod, the

object becomes longer and thinner. Conversely, a

compressive force act to make the cylinder or rod

shorter and thicker.

34

If an axial stress σ in the z [long axis ]direction of a

mutually perpendicular xyz coordinate system

produces an elastic tensile strain, and

accompanying elastic contraction in x and y

direction [σ and σ respectively], the ratio of σ /σ

or σ/σ is an engineering property of the material

called Poisson’s Ratio.

35

Strength properties:

Strength is equal to the degree of stress necessary to

cause either fracture(ultimate strength) or a specified

amount of plastic deformation(yield strength).

The strength of a material can be described by one or

more of the following properties

A. Proportional limit

B. Elastic limit

C .Yield strength or proof stress

D. Ultimate tensile stress

36

A. Proportional limit

Proportional limit is the greatest elastic stress

possible in accordance with hooks law, it represents

the maximum stress above which stress is no longer

proportional to strain.

37

B. Elastic limit

The elastic limit of a material is defined as the

greatest stress which the material can be

subjected such that it returns to it original

dimensions when the force is release.

38

C. Yield strength

Yield strength is used in cases where the proportional

limit cannot be determined

with sufficient accuracy.

Yield strength often is a property that represents

the stress value at which a small amount(0.1% or

0.2%) of plastic strain has occurred.

39

A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred

to as percent offset. 40

D. Ultimate tensile strength

It is the maximum stress that a material can withstand

while being stressed or pulled before failing or

breaking.

UTS determines the maximum force the wire can deliver if used

as SPRING.

41

Permanent (plastic) deformation

If a material is deformed by stress at a point above the

Proportional limit before fracture , removal of the

applied force will reduce the stress to zero but the

plastic deformation remains .Thus the object does not

return to its original dimension when the force is

removed.

42

1. Cold working:

When metal alloys have been stressed beyond their

proportional limits, their hardness and strength

increase at the area of deformation, but their

ductility decreases . as dislocation move and pile up

along grain boundaries , further plastic deformation

in these areas become more difficult. As a result ,

repeated plastic deformation of the metal.

43

2. Diametral tensile strength

Tensile strength can gnarly be determined by

subjecting a rod, wire, or dumbbell-shaped specimen

to tensile loading. Since the test is quite difficult to

perform for brittle materials because of alignment and

griping problem, another test can be used to

determine this property for brittle dental materials .It

is referred to as DIAMETRAL COMPRESSION TEST.

44

45

In this method, a compressive load is placed by a flat

plate against the side of a short cylindrical specimen

(disc). The vertical compressive force along the side

of the disc produces a tensile stress that is

perpendicular to the vertical plane passing through

the center of the disc. Fracture occurs along the

vertical plane (the dashed vertical line on the disc).

In such a situation, the tensile stress is directly

proportional to the compressive load applied.

46

It is calculated by the following formula :

Tensile stress= 2F /πDt

Where F= applied force

D=diameter

t=thickness

47

3 .Flexural strength

Also called transverse strength and modulus of rupture,

is essentially a strength test of a bar supported at each

end or a thin disc supported along a lower support

circle under a static load.

48

4. Impact strength:

This property 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.

49

OTHER IMPORTANT PROPERTIES

A: Toughness Amount of elastic and plastic deformation energy required

to fracture a material. Fracture toughness is a measure of

energy required to propagate critical flaws in the

structure. Measured as the total area under the stress

strain graph.

50

B: Fracture toughness

Fracture toughness, or the critical stress intensity, is

a mechanical property that describes the resistance

of brittle materials to the catastrophic propagation

of flaws under an applied stress.

51

C:Brittleness

Is the relative inability of a material to sustain

plastic deformation before fracture of a material

occurs. In other words, a brittle material fractures at

or near its proportional limit.

52

D: Ductility and malleability

Ductility represents the ability of a material to

sustain a large permanent deformation under a tensile

load up to the point of fracture.

For example, a metal can be drawn readily into a long

thin wire is considered to be ductile.

53

Malleability is the ability of a material to sustain

considerable permanent deformation without

rupture under compression, as in hammering or

rolling into sheet.

Gold is the most ductile and malleable pure metal,

and silver is the second. Platinum ranks third in

ductility, and copper ranks third in malleability.

54

E: Hardness

Resistance to indentation.

In mineralogy the relative hardness of a substance

is based on its ability to resist scratching.

55

HARDNESS TESTS

Hardness tests are included in numerous specifications

for dental materials developed by the American Dental

Association (ADA) and standards promoted by the

International Organization for Standardization(ISO).

There are several types of surface hardness tests. Most

are based on the ability of the surface of a material to

resist penetration by a diamond point or steel ball

under a specified load.

56

The most frequently used tests are:

A : BRINELL TEST

B : ROCKWELL TEST

C : VICKERS TEST

D : KNOOP TEST

57

BRINELL TEST

Used extensively for determining the hardness of metals and

metallic materials used in dentistry.

A hardened steel ball is passed under a specific load into the

polished surface of material.

The load is divided by the area of the projected surface of the

indentation, and quotient is referred to as BRINELL HARDNESS

NUMBER [BHN]

For a given load , smaller the indentation , the larger is the number

and the harder the material.

58

ROCKWELL TEST

Similar to Brinell test in that a steel ball or a conical diamond point is used.

The depth of penetration is measured directly by a dial gauge on the instrument.

The Rockwell hardness number [RHN] is designated according to the particular indenter and load employed.

The convenience of this test , with direct reading of the depth of the indentation.

Also called as BRALE test.

59

VICKERS TEST

136 Diamond pyramid test

Employs the same principle of hardness testing as Brinell

test.

Square based pyramid is used.

The load is divided by the projected area of indentation.

The length of diagonals of the indentation are measured

and averaged.

Employed in the testing of dental casting gold alloy.

60

KNOOP TEST

Employs a diamond-tipped tool that is cut in the

geometric configuration .

The impression is rhombic in outline and the length of the

largest diagonal is measured .

The load is divided by the projected area to give the

KNOOP HARDNESS NUMBER [KHN] or [HK].

61

Brinell and Rockwell hardness tests are classified as

macro hardness test and knoop and Vickers test are

classified as micro hardness test.

Both Knoop and Vickers test employ loads less

than 9.8 N.Resulting indentations are small and

limited to depth of 19 micron meter .

Rockwell and Brinell tests give average hardness

values over much larger areas .

62

The principle of hardness tests is based on

resistance to indentation .

The equipment generally consists of a spring-

loaded metal indenter point and a gauge from

which the hardness is read directly.

63

THANK YOU

64