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
 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”.

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)

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.

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 

 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

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

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

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

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

 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

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

(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

(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.

(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.

Physical and mechanical properties and its application in orthodontics

Physical and mechanical properties and its application in orthodontics

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