physical and mechanical properties and its application in orthodontics

GOOD MORNIN

G

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. It is common to report stress in units of Megapascals (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 = Δ L

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 a corresponding deformation in a body.

Tensile stress produces tensile strain.

Compressive stress produces compressive strain.

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 the relative STIFFNESS or RIGIDITY of a material which is measured by the elastic region of stress – strain diagram.

It is denoted by letter E

o Determined from stress stain curve by calculating ratio of stress to strain or slope of linear 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 associated with “springiness”.

Definition: It is defined as the amount of energy absorbed by a structure when it is stressed to its proportional limit.

Area bounded by the elastic region is measure of 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 ratio is constant upto the proportional limit, the constant in this linear stress-strain relationship is Modulus of Elasticity.

Below PL no permanent deformation occurs in a structure.

Region of stress stain Curve. Below PL – ELASTIC REGION Above PL – PLASTIC REGION

Elastic limit (EL)

Definition: It is defined as maximum stress that a material can withstand before it undergoes permanent deformation.

For all practical purposes PL and EL represent same stress. But they differ in fundamental 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 be identical although their experimental values may differ slightly.

Yield Strength(yield stress or proof stress)

It is defined as the stress at which a material exhibits a specified limiting deviation from proportionality 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 specimenand 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.


Thus, Light from object

Incident on eyes

Focused in retina →rods and cones

Converted into nerve impulses

Transmitted to brain


Three dimension of color are: 1. Hue 2. Value 3. Chroma


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.


Value

Relative lightness or darkness of color.

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


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.


Color Solid:

Central rod = value

Spikes = hue

Volume = chroma


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-b

axis


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.


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


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


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


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

Documents