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”.

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 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

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

 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.

 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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