physical and mechanical properties and its application in orthodontics

Physical and Mechanical

Properties and its

application in

orthodontics

Prepared by

Dr.Hardik Lalakiya

Guided by

Dr.Ajay Kubavat

Dr.Chintan Agrawal

Dr.Ketan Mashru

Dr.Bhavik Patel

Dr.Manish Desai

Dr.Vishal Patel

Department of Orthodontics

and Dentofacial Orthopaedics

OUTLINE

Introduction

Crystal structure

and its arrangement

Principal metal

structures and its

arrangement

Classification

Stress and its types

Strain

True Stress strain

curve

Poisson’s ratio

Mechanical properties

based on elastic

deformation

Toughness

Impact strength

Proportional limit

Elastic limit

Yield strength

Permanent Plastic

deformation

Strain hardening

Strength and its

types

Fatigue

Static fatigue

Brittleness

Ductility

Malleability

Physical Properties

Abrasion and abrasion

resistance.

Hardness

Viscosity

Creep and flow

Color and color

perception

Bezold brucke effect

Mechanical properties are defined by the laws of mechanics that is the physical science that deals with the energy and forces and their effects on bodies the discussion centers primarily on the static bodies –those at rest-rather than on dynamic bodies.

Many factors must be taken into account when considering which properties are relevant to the successful performance of the material used in dentistry

The Plantonic Solids

CUBE DODECAHEDRON ICOSAHEDRON

OCTAHEDRON TETRAHEDRON

http://home.teleport.com/~tpgettys/platonic.shtml

Atomic arrangements in crystalline solids

can be described with respect to a network

of lines in three dimensions.

The intersections of the lines are called

“lattice sites” (or lattice points). Each

lattice site has the same environment in the

same direction.

A particular

arrangement of

atoms in a crystal

structure can be

described by

specifying the atom

positions in a

repeating “unit

cell”.

14 Bravais lattices

Principal metal crystal structures

There are three principle crystal

structures for metals:

–(a) Body-centered cubic (BCC)

–(b) Face-centered cubic (FCC)

–(c) Hexagonal close-packed (HCP)

Principal structures

Body centered cubic (BCC)

Face centered cubic (FCC)

Hexagonal closed packed

(HCP)

Classification

Definition: When a force acts on a body

tending to produce deformation . A resistance is developed to this external force application. The INTERNAL reaction is equal in intensity and opposite in direction to the applied external force and is called stress.

Stress = Force/Area

STRESS

•Commonly expressed as Pascal 1Pa = 1N/m2. Itis common to report stress in units ofMegapascals (MPa) where 1 MPa = 106 Pa.

•TYPES OF STRESS :- Tensile Compressive Shear

In english system of measurement ,the stress is usually expressed in pounds per square inch.

3 Types of stress

Tensile

Compressive Stress

Shear stress

Tensile Stress

A tensile Stress is caused by a load that

tends to stretch or elongate a body .

for eg stress developed on the gingival side

of 3 unit bridge bridge

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.

Shear stress

A stress that tends to resist a twisting motion

or sliding of one portion of a body over

another is shearing stress

For eg If a force is applied along the surface

of tooth enamel by a sharp edged instrument

parallel to the interface between the enamel

and an orthodontic bracket may debond by

shear stress failure of the resin luting cement

Complex stress

Complex stress those produced by applied forces that cause flexural or torsional deformation are called

flexural stress

More than two

They are also called as

bending stress.

STRAIN

o A force is applied to a body it undergoes deformation.

o Strain is described as the change in length (Δ L = L – LO) per unit length of the body when it is subjected to a stress.

Strain ( ) = Change in length = L – Lo = ΔL

Original length Lo Lo

Strain has no units of measurement.

It is a Dimensionless quantity.

Reported as an absolute value or as a

percentage.

Facts

The Average max sustainable biting force is

756N (170 pounds) or (77kgs)

The Guiness Book Of World records (1994)

lists the highest biting force as

4337N (975 pounds).

Each type of stress is capable of producing acorresponding deformation in a body.

Tensile stress produces tensile strain.

Compressive stress produces compressivestrain.

Shear stress produces shear strain.

Stress strain curve

Represents energy storage capacity of the

wire so determines amount of work expected

from a particular spring in moving a tooth.

True stress strain curve

A stress strain curve based on stresses

calculated from a Non Constant Cross

sectional area is called a true stress strain

Curve.

A true-stress strain curve may be quite

different from an engineering stress-strain

curve at high loads because significant

changes in the area of specimen may occur.

STRESS STRAIN CURVE

Mechanical Properties Based On

Elastic deformation

Elastic Modulus

Shear Modulus

Flexibility

Resilience

Poisson’s ratio.

Elastic modulus

(young’s modulus or Elasticity)

The term elastic modules describes therelative STIFFNESS or RIGIDITY of a materialwhich is measured by the elastic region ofstress – strain diagram.

It is denoted by letter E

o Determined from stress stain curve bycalculating ratio of stress to strain or slope oflinear portion of curve.

Stress 6

Elastic Modulus = =

Strain

Stress strain curve

Modulus of elasticity is independent of the

ductility of a material and it is not a measure of

its strength.

It is an inherent property of a material and

cannot be altered appreciably by heat

treatment, work hardening or any other kind of

conditioning. This property is called

STRUCTURAL INSENSITIVITY.

The Elastic modulus of a tensile test specimen

can be calculated as follows where

E is elastic modulus

P is the applied force or load

A is the cross sectional area of material under

stress

^l is the increase in length

Lo is the original length

Flexibility

The maximum flexibility is defined as the strain

that occurs when the material is stressed to its

proportional limit.

For example in an orthodontic appliance, a spring

is often bent a considerable distance with a small

stress resulting in such a case structure is said to

be flexible.

Resilience

Popularly the term Resilience is associatedwith “springiness”.

Definition: It is defined as the amount ofenergy absorbed by a structure when it isstressed to its proportional limit.

Area bounded by the elastic region is measureof Resilience.

Poisson’s ratio

Any material when subjected to a tensile or compressive stress, there is simultaneous axial and lateral strain.

Within elastic range the ratio of lateral to axial strain is known as poisson’s ratio.

Dental materials have poisson’s ratio in the range of 0.3 to 0.5.

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.

Conventional Tensile Stress Strain

Curve

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.

Impact Strength (continue)..

It is the energy required to fracture a material under force.

A charpey type tester is used. It has a heavy pendulum which swings down to fracture the specimen.

Another instrument

called Izod impact tester can also be used.

Strength properties

Strength is the stress necessary to cause either fracture(ultimate strength) or a specified amount of plastic deformation(yields strength).

The strength of a material can be described by

Proportional limit

Elastic strain

Yield strength

Ultimate tensile strength, shear,compressive and flexural strength.

Proportional limit (PL)

It is defined as the greatest stress that a

material will sustain without a deviation from the

linear proportionality of stress to strain.

Hooke’s Law :- States that stress – strain ratiois constant upto the proportional limit, theconstant in this linear stress-strain relationshipis Modulus of Elasticity.

Below PL no permanent deformation occurs ina structure.

Region of stress stain Curve.

Below PL – ELASTIC REGION

Above PL – PLASTIC REGION

Elastic limit (EL)

Definition: It is defined as maximum stressthat a material can withstand before itundergoes permanent deformation.

For all practical purposes PL and ELrepresent same stress. But they differ infundamental concept :-

PL deals with proportionality of strain to

stress in structure.

EL describe elastic behavior of the material.

EL & PL limits are usually assumed to beidentical although their experimental values maydiffer slightly.

Yield Strength

(yield stress or proof stress)

It is defined as the stress at which a materialexhibits a specified limiting deviation fromproportionality of stress to strain.

Amount of permanent strain is arbitrarily selected for material being examined and may be indicated as 0.1%, 0.2% or 0.5% (0.001, 0.002, 0.005) permanent strain

Amount of permanent strain may be referred to

as PERCENT OFFSET. Many specifications

use 0.2% as convention.

Permanent (Plastic)

deformation

If the material is deformed by a stress at a point

above the proportional limit before fracture,the

removal of applied force will reduce the stress to

zero,but the strain does not decrease to zero

because the plastic deformation has occurred .

Thus the object does not return to its original

dimension when the force is removed.It remains

bent,streched,compressed or otherwise

plastically deformed.

Strain hardening

Strengthening by increase of dislocation density

(Strain Hardening = Work Hardening = Cold Working)

Ductile metals become stronger when they are

deformed plastically at temperatures well below the

melting point.

The reason for strain hardening is the increase of

dislocation density with plastic deformation.

Average distance between dislocations decreases

and dislocations start blocking the motion of each

other.

The percent cold work (% CW) is often used to

express the degree of plastic deformation:

%CW is just another measure of the degree of

plastic deformation, in addition to strain.

Strength

It is the maximal stress required to fracture a structure.

Strength is not a measure of individual atom to atom

attraction 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

Tensile strength

Tensile Strength is

determined by subjecting a rod , wire or a dumbbell shaped specimen to a tensile loading.

It is defined as the maximal stress the structure will withstand before rupture.

Diametral Tensile Strength

Brittle material an

indirect tensile test

called Diametral

compression test or

Brazillian test is used

.

A compressive load

is placed on the

diameter of a short

cylindrical material .

Compressive strength

Crushing strength is

determined by

subjecting a cylindrical

specimen to a

compressive load.

The strength is obtained

from the cross sectional

area and force applied.

Complex failure

SHEAR SRENGTH

Maximum stress a

material can withstand

before failure in a

shear mode of loading.

It is tested using punch

or pushout method.

Shear strength = Force/ Π punch dia *

thickness

FLEXURE STRENGTH

Transverse strength or modulus

of rupture or flexure strength

Obtained using a beam

supported at each end and load

applied in the middle.

Also called three point

bending test.

Used in long span bridges.

Neutral Axis

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 cyclic stress of known value and determining the

number of cycles that are required to produce failure.

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.

Eg ceramic materials.

Brittleness

A brittle material fractures at or near its proportional limit.

It is opposite of toughness.

Brittle material will not bend appreciably without breaking.

Though a brittle material may have a very high

compressive strength. E.g. glass.

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 temperature.

MEASUREMENT OF DUCTILITY

1.Percentage elongation after fracture

Gauge length = 51 mm( STANDARD

GAUGE LENGTH FOR DENTAL

MATERIALS)

2.Measuring reduction in cross sectional areas of

fractured ends in comparison to the original area of the

wire. This is also called as reduction in area method.

3. cold bend test

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 dependent on the strength 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 ranks

third in malleability.

Stress concentration factors

THESE INCLUDES

Surface flaws

Internal voids

air bubbles.

Inclusions of other materials

Hertzian load

Sharp angles

Notches

Thermal mismatch

Some clinical relations with

orthodontic wire

Tension Test Results; UTS and E for

stainless steel and titanium material.

Material Type UTS (MPa) E

(GPa)

Stainless steel 1300 193

titanium 1615 179

Stress-Strain curve of stainless steel

specimen the x-axis the strain in the

specimen

and the y-axis stress (MP/mm2). By wp 300

tensile testing machine

Physical Properties

Abrasion and abrasion resistance

Phenomenon of wearing/ removal process that occurs

whenever surfaces slide against each other

The material which causes wearing is called abrasive

The material which is worn is called substrate.

Hardness is one of the common index of a material to

resist abrasion or wear but not the only index.

Other factor which cause and influence abrasion / abrasion

resistance are

Biting force

Frequency of chewing,

Abrasiveness of diet,

Intra oral liquid, temperature changes,

Surface roughness,

Impurities and irregularities (Pits and grooves)

hardness

Resistance to surface penetration / surface scratching

/ability to resist indentation.

Indentation is produced on the surface of the material

from a applied force of a sharp point or an abrasive

particle.

Most hardness test are based on ability of a surface of a

material to resist penetration by diamond point or a steel

ball under a specified

Common tests are

Barcol

Brinell (BH)

Rockwell (RH)

Shore

Vickers (HV)

Knoop (KH) Microhardness

test

Macrohardness

test

Brinell hardness number (BHN)

Oldest, simplest , convenient &

extensively used

Hardened steel ball pressed

with standard load on polished

surface of material .

Load is divided by the area of

projected surface of

indentation .

Thus for a given load smaller

the indentation, larger is the

number and the harder is the

material

Rockwell hardness number (RHN)

Conical diamond point is

used.

Depth of penetration is

measured directly by the

dial gauge on the

Instrument

RHN and BHN are used

for measuring hardness

of metal and alloys and

they are not suitable for

brittle materials.

Vickers hardness test HV test employs square based

pyramid of 136 Degrees

Method of computation is the

load divided by the projected

area of Indentation.

The length of the diagonals are

measured and averaged.

Can be used for brittle

materials.

also called 136 degree

diamond pyramid

test.

Knoop hardness number (KHN)

Uses diamond tip tool.

Rhombohedral pyramid

diamond tip is used of

dimension 130 degree and

172.30 degree

The length of the largest

diagonal is measured .

The projected area is divided

in to the load to give KHN

Can be used for extremely

hard and soft materials.

KHN and HV are called as micro hardness

test.

BHN and RHN are macro hardness test.

Shore and Barcol test are sometimes employed

to measure hardness of rubber and plastic type of

dental materials.

These have spring loaded metal indenter point.

Viscosity

Resistance of a liquid to flow Study of flow character of a material is the basis

for Rheology

Importance of knowing flow:

impressions, Gypsum products, cements,

waxes.

Resistance to flow is controlled by internal

frictional forces. Thus viscosity is the measure of

consistency of a medium and its inability to flow.

Change in Viscosity

Whenever a force is applied to a material it will

deform.

The force / area is called stress.

The calculation of deformation is the strain.

Strain = change in length / initial length.

Unit of viscosity is MPa / second or

CETIPOISE

Viscosity of most liquids decreases with

increase in temperature i.e. its flow increases

To explain viscous nature of some materials ,

shear stress / shear strain rate curve is plotted .

Based on Rheologic

behavior fluids are classified in to four types

Newtonian fluid

Pseudoplastic

Dilatant fluid

Plastics

Newtonian fluid

Ideal fluid which

demonstrates a shear

strain proportional to the

shear stress

The plot on the graph is a

straight line

Newtonian fluids has a

constant viscosity and is

independent of the shear

strain rate.

Pseudoplastic fluid

When the viscosity of

a material decreases

with increasing strain

rate until it reaches

the constant value

such a material is

called

Pseudoplastic

materials or fluid.

Dilatant fluid

These are the liquids that

becomes more rigid as the

rate of deformation

increases.

These liquids show

opposite tendency as

described for

pseudoplastic

Plastic

Some classes of material

behave like a rigid body until

some minimum value of

shear stress is reached (off

set value)

These fluids which exhibits

rigid behavior initially and

then attend constant

viscosity are referred to as

plastic.

Ketchup is a familiar

example .

Thixotrophic material

Viscosity of liquid also depends on previous deformation of liquid

A liquid of this type that becomes less viscous and more fluid under more repeated application of pressure is called as Thixotrophic materials

Examples: Dental polishing paste, plaster of paris,

impression materials, resins and cements

Importance of Viscosity

Properties

Teaches us the best way to manipulate the

materials

Guides as on the best use of the materials

Measure of working time

Thixotropic materials stays on tray but on

applying pressure in the mouth the material

flows

Creep and flow

If the metal is held at the temperature near its

melting point and subjected to constant

applied stress, the resulting strain will

increases over time.

Creep is defined as the time dependant

plastic strain of a materials under static /

constant load.

Sag is same as creep but the load is the mass

of the same material .

Creep and flow (continue…)

A filling material called “Amalgam” has

low melting range. So when in mouth it is

close to the melting point and is subjected to

constant biting forces. It gets get deformed.

Here the biting forces keep changing and

continuous Dyanamic creep.

For waxes term flow rather than creep is used

as it is amorphous. The flow of wax is its

potential to deform under small static load / or

its own mass.

Creep and flow (continue…)

Flow is measured using compressive forces mostly.

Testing flow: A cylinder prescribed dimension is subjected to a given compressive stress for a specified time and temperature.

The creep or flow is measured as percentage decrease in length.

Significance of creep / sag.

Thermophysical properties

Heat transfer through solid substances most

commonly occur by means of conduction.

The conduction of heat through metals occurs

through the interaction with atoms.

Thermal conductivity (k) is the

thermophysical measure of how well heat

is transferred through a material by conductive

flow.

The measurement of thermal conductivity is

performed under steady state conditions.

Thermoconductivity Properties

The Thermal conductivity or coefficient of thermal conductivity is the quantity of heat in calories per second that passes through a specimen 1 cm thick having a cross sectional area of 1cm2 ,when the temperature difference between the surfaces Thermoconductivity Properties perpendicular to the heat flow of the specimen is 10 K.

Materials that have a high thermal conductivity are called conductors, whereas materials of low thermal conductivity are called insulators.

Thermoconductivity Properties

(Cont..)

The international system (SI) unit or measure for thermal conductivity is watt / meter / second /o Kelvin

Increase in thermal conductivity , greater is the ability to transfer thermal energy.

Metal restoration – increase conductivity compared to other materials.

Thermal Diffusivity

The value of thermal diffusivity of a material controls the time rate of temperature change as heat passes through a material.

It is a measure of the rate at which a body with a nonuniform temperature reaches a state of thermal equilibrium.

For a given volume of material, the heat required to raise the temperature , to a given amount depends on its heat capacity or specific heat and the density.

Thermal Diffusivity (cont)..

The formula that related thermal diffusivity to

thermal conductivity is

h = k / cpρ

h = Thermal diffusivity

k = Thermal conductivity

cp = Heat capacity

ρ = temperature dependent density

Thermal Diffusivity (cont)..

Square root of thermal diffusivity is indirectly

proportional to thermal insulation ability.

SI unit is square meter per second commonly used.

Coefficient of thermal

expansion

Coefficient of thermal expansion, is defined as the change in length / unit of the original length of a material when its temperature is raised 1degree K.

SI unit μm /m0 K or ppm / k0

A tooth restoration may contract or expand more than the tooth during the change in temp which may cause micro leakage or debond of restoration of teeth.

To reduce this, selection of material whose expansion or contraction coefficient should be matched approximately within 4%.

PFM

Color and color perception

(cont)..

Sensation induced from color of various wavelength reaching the eye.

Eye is sensitive to wavelength of

400nm(violet) to 700nm(dark red).

For an object to be visible, it must reflect and transmit incident light at certain wavelength.

Color is measured using munsell system.


(cont)..


(cont).. Thus,

Light from object

Incident on eyes

Focused in retina →rods and cones

Converted into nerve impulses

Transmitted to brain


(cont)..

Three dimension of color are:

1. Hue

2. Value

3. Chroma


(cont)..

Hue:

Dominant color of an object

E.g. red, blue, green (dominant wavelength).

The normal human teeth have hue range of

6.3

yellow red to 9.3 yellow red.


(cont)..

Value

Relative lightness or darkness of color.

The human teeth have a value in the range of 0-7.


(cont)..

CHROMA

Degree of saturation of particular hue.

Higher the chroma, more intense and mature

the color.

Chroma cannot exist itself and it is always

associated with hue and value.

Normal human teeth has chroma of 4 to 7.


(cont)..

Color Solid:

Central rod = value

Spikes = hue

Volume = chroma


(cont).. CIE SYSTEM:

Commission International Eclairage.

Based on Adam system

Colour in L*a*b

L = value

a = measure along r-g

axis

b= measure along y-


(cont)..

Shade Guide :

In the dental laboratory, color matching is

usually performed by the shade guide.

The most commonly used guide is VITA

shade guide.

The range is from A1 to D4 .From left to right

the darkness increase.


(cont)..

Metamerism:

Object that appear to be color matched under

one type of light may appear different under

another light source.

Day light, incandescent lamps, fluorescent

lamps are most common source of light in

dental operatory.

Two or more sources of light should be used

to prevent metamerism causing wrong

selection of

Metamerism


(cont)..

Near ultraviolet radiation:

Natural tooth structure absorbs light at wave

lengths too short to be visible at human eye.

These wave lengths between between 300nm-

400nm are referred as near ultraviolet

radiation.

Sources are natural sunlight, photoflash

lamps, UV light


(cont)..

Fluorescence:

Energy that the tooth absorbs is converted into

light with longer wavelength in which case the

tooth actually becomes a light source.

The phenomenon is called Fluorescence.

Ceramics, composites – fluorescent agents

are added.

Fluorescence


(cont)..

BEZOLD BRUCKE EFFECT:

At low light levels, rods of human eye are

dominant and color perception is lost. As the

brightness becomes more intense , color

appears to change.

BEZOLD BRUCKE EFFECT

physical and mechanical properties and its application in orthodontics

Health & Medicine