This document discusses the mechanical properties of orthodontic biomaterials. It defines mechanical properties as measures of a material's resistance to deformation or fracture under applied forces. The key mechanical properties discussed are: 1. Elastic properties like elastic modulus, resilience, and Poisson's ratio which measure a material's elastic deformation in response to stress. 2. Plastic properties like percent elongation and hardness which measure a material's permanent or irreversible deformation after stress is removed. 3. Strength properties like yield strength and fracture toughness which indicate the stress level required to cause plastic deformation or fracture. Standard tests for measuring properties like hardness, strength and toughness are also outlined. In summary, the document provides an overview

1. Good afternoon
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2. MECHANICAL
PROPERTIES OF
ORTHODONTIC
BIOMATERIALS
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3. content
 Introduction
 Expression of mechanical properties.
[A] Elastic or reversible deformation –
Proportional limit
Resilience
Modulus of elasticity
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4.  [B]Plastic or irreversible deformation –
 Percent elongation
 Hardness
 [C] Combination of elastic and plastic deformation –
 Toughness
 Yield strength
 conclusion
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5.  Mechanical properties are defined by law of mechanics
that is the physical science that deals with energy and
forces and their effect on bodies1.
 Thus all the mechanical properties are measures of the
resistance of a material to deformation or fracture under
the applied force.
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6.  Mechanical properties are the measured responses
both elastic (reversible on force removal) and plastic
(irreversible or non elastic) of materials under an
applied forces or pressure.
 Mechanical properties are expressed most often in
units of stress and/ or strain.
Keneth j.anusavis,Chiayi shen, H.Ralphs:Phillips’ science of dental materials.Elsevier Inc2013.
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7. Expression of mechanical properties:
 Expressed in units of stress and strain.
 Represents measurement of:
 [A] Elastic or reversible deformation –
Proportional limit
Resilience
Modulus of elasticity
Keneth j.anusavis,Chiayi shen, H.Ralphs:Phillips’ science of dental materials.Elsevier Inc2013
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8.  [B]Plastic or irreversible deformation –
Percent elongation
Hardness
 [C] Combination of elastic and plastic deformation –
Toughness
Yield strength
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9. To discuss these properties, first we have
to understand the concept of stress and
strain
Based on Newton’s third law of motion (i.e.
for every action there is an equal and opposite
reaction), when an external force act on a solid,
a reaction occurs to oppose this force which is
equal in magnitude but opposite in direction to
the external force. The stress produced within
the solid material is equal to the applied force
divided by the area over which it acts
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10. Stress : Force per unit area within a structure
subjected to an external force /pressure
For dental applications there are several types of
stress that develop according to the nature of
applied force and object shape.
Nature of force Stress
Tensile force tensile stress
Compressive force compressive stress
Shear /bending force shear stress
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11. 11

12. Strain : change in length per unit initial
length
Relative deformation of an object subjected to a stress
Strain may be
[a] Elastic [b] Plastic [c] Elastic and plastic
Reversible deformation Permanent deformation when object deformed past the
elastic limit certain
amount of elastic recovery
Occurs.
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13. Force is applied
at the distance
d/2 from
interface [a-b]
Diagram showing elastic shear strain
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14. Shear
stress
Force is applied
along interface
Diagram: showing plastic shear strain
plastic
strain
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15. Tensile stress:
Ratio of tensile force to the original cross sectional
area perpendicular to the direction of applied
force.
 A tensile stress is always accompanied by a tensile
strain, but it is very difficult to generate pure
tensile stress in a body _that is, a stress caused by a
load that tends to stretch or elongate a body.
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16.  There are few pure tensile stress situations
in dentistry. The deformation of a bridge and
the dimeteral compression of a cylinder can
represent examples of this complex stress
situation.
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17. Diagram : showing stresses induced in a three unit bridge
by a flexural force
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18. Diagram showing stresses induced in a two unit
cantilever bridge
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19. Compressive stress:
if a body is placed under a load that tends to compress or
shorten it, the internal resistance to such a load is called
compressive stress.
Compressive stress is associated with compressive strain.
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20. Shear stress : a shear stress tends to resist the
sliding or twisting of one portion of a body over
another .
Example : if a force is applied along the surface of
the tooth enamel and an orthodontic bracket ,the
bracket may debond by shear stress failure of the
resin luting agent .
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21. S
f
Plastic
Shear
strain
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22. Mechanical properties based on elastic deformation :important
mechanical properties and parameters that are measures of
elastic strain or plastic strain, behaviors of dental materials .
These are :
[a] Elastic modulus / Young’s modulus/ Modulus
of elasticity
[b] Dynamic young’s modulus
determined by measurement of ultrasonic wave velocity
[c] Shear modulus
[d] Flexibility
[e] Resilience
[f] Poisson’s ratio
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23. Elastic modulus :
Describe the relative stiffness or rigidity of a material
which is measured by slope of the elastic region of
stress -strain graph.
It is independent of the ductility of a material since it
is measured in the linear region of the stress –strain
plot , and it is not a measure of its plasticity or
strength .
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24. Conventional stress –strain curve
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25. Because the elastic modulus represents the ratio of
elastic stress to the elastic strain, it follows that
the lower the strain for a given stress, the greater
the value of the modulus. For example if one wire
is much more difficult to bend than another of the
same shape and size, considerably higher stress
must be induced before a desired strain or
deformation can be produced in the stiffer wire .
More horizontal the slope-----more springiness.
More vertical the slope------ more stiffness.
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26. 26
RANGE
Distance that the wire will bend elastically before
permanent deformation occurs.
If the wire is deflected beyond this limit, it will
not return to its original shape, but clinically
useful SPRINGBACK will occur unless the failure
point is reached.
Orthodontic wires are often deformed beyond
their elastic limit, so spring back properties are
important in determining clinical performance.
STRENGTH=STIFFNESS × RANGE.

27. Calculation of elastic modulus
E is the elastic modulus
P is the applied force or load
A is the cross-sectional area of the material under
stress
Δl is the increase in length
Lo is the original length
By definition: stress=P/A=σ
Strain= Δl/ Lo= ϵ
Thus,
E=stress/strain= σ/ ϵ =[ P/A]/ Δl/ Lo
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28. Dynamic Young’s modulus
Elastic modulus can be measured by dynamic method
as well.
Since the velocity at which sound travels through a
solid can be readily measured by ultrasonic
longitudinal and transverse wave transducers and
appropriate receivers, the velocity of sound wave
and the density of the material can be used to
calculate the elastic modulus and Poisson’s ratio
values .
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29. Flexibility
The maximum flexibility is defined as
the flexural strain that occurs when the
material is stressed to its proportional
limit.
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30. There are instances in which a larger strain or
deformation may be needed with moderate or slight
stress. For example, in an orthodontic appliance, a
spring is often bent considerable distance under the
influence of small stress.
In such a case, the structure is said to be flexible and
it possesses the property of flexibility.
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31. Resilience:
Resilience can be defined as the amount of
energy absorbed within a unit volume of
structure when it is stressed to its
proportional limit.
As the interatomic spacing increases, the internal
energy increases.
As long as the stress is not greater than the
proportional limit, this energy is known as resilience.
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32. The area bounded by elastic region is a measure of
resilience, and the total area under the stress-strain is
toughness.
Amount of permanent deformation that a wire can
withstand before failing is FORMABILITY. 32
FORMABILITY

33. The resilience of two materials can be
compared by observing the areas under the
elastic region of their stress- strain plots,
assuming that they are plotted on the same
scale. The material with the larger elastic
area has the higher resilience.
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34. Poisson’s Ratio
when a tensile force is applied to a cylinder or rod, the
object becomes longer and thinner. Conversely, a
compressive force act to make the cylinder or rod
shorter and thicker.
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35. If an axial stress σ in the z [long axis ]direction of a
mutually perpendicular xyz coordinate system
produces an elastic tensile strain, and
accompanying elastic contraction in x and y
direction [σ and σ respectively], the ratio of σ /σ
or σ/σ is an engineering property of the material
called Poisson’s Ratio.
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36. Strength properties:
Strength is equal to the degree of stress necessary to
cause either fracture(ultimate strength) or a specified
amount of plastic deformation(yield strength).
The strength of a material can be described by one or
more of the following properties
A. Proportional limit
B. Elastic limit
C .Yield strength or proof stress
D. Ultimate tensile stress
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37. A. Proportional limit
Proportional limit is the greatest elastic stress
possible in accordance with hooks law, it represents
the maximum stress above which stress is no longer
proportional to strain.
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38. B. Elastic limit
The elastic limit of a material is defined as the
greatest stress which the material can be
subjected such that it returns to it original
dimensions when the force is release.
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39. C. Yield strength
Yield strength is used in cases where the proportional
limit cannot be determined
with sufficient accuracy.
Yield strength often is a property that represents
the stress value at which a small amount(0.1% or
0.2%) of plastic strain has occurred.
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40. A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred
to as percent offset. 40

41. D. Ultimate tensile strength
It is the maximum stress that a material can withstand
while being stressed or pulled before failing or
breaking.
UTS determines the maximum force the wire can deliver if used
as SPRING.
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42. Permanent (plastic) deformation
If a material is deformed by stress at a point above the
Proportional limit before fracture , removal of the
applied force will reduce the stress to zero but the
plastic deformation remains .Thus the object does not
return to its original dimension when the force is
removed.
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43. 1. Cold working:
When metal alloys have been stressed beyond their
proportional limits, their hardness and strength
increase at the area of deformation, but their
ductility decreases . as dislocation move and pile up
along grain boundaries , further plastic deformation
in these areas become more difficult. As a result ,
repeated plastic deformation of the metal.
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44. 2. Diametral tensile strength
Tensile strength can gnarly be determined by
subjecting a rod, wire, or dumbbell-shaped specimen
to tensile loading. Since the test is quite difficult to
perform for brittle materials because of alignment and
griping problem, another test can be used to
determine this property for brittle dental materials .It
is referred to as DIAMETRAL COMPRESSION TEST.
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45. 45

46. In this method, a compressive load is placed by a flat
plate against the side of a short cylindrical specimen
(disc). The vertical compressive force along the side
of the disc produces a tensile stress that is
perpendicular to the vertical plane passing through
the center of the disc. Fracture occurs along the
vertical plane (the dashed vertical line on the disc).
In such a situation, the tensile stress is directly
proportional to the compressive load applied.
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47. It is calculated by the following formula :
Tensile stress= 2F /πDt
Where F= applied force
D=diameter
t=thickness
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48. 3 .Flexural strength
Also called transverse strength and modulus of rupture,
is essentially a strength test of a bar supported at each
end or a thin disc supported along a lower support
circle under a static load.
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49. 4. Impact strength:
This property may be defined as the energy
required to fracture a material under an impact
force. The term impact is used to describe the
reaction of a stationary, object to a collision with a
moving object.
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50. OTHER IMPORTANT PROPERTIES
A: Toughness
Amount of elastic and plastic deformation energy required
to fracture a material. Fracture toughness is a measure of
energy required to propagate critical flaws in the
structure. Measured as the total area under the stress
strain graph.
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51. B: Fracture toughness
Fracture toughness, or the critical stress intensity, is
a mechanical property that describes the resistance
of brittle materials to the catastrophic propagation
of flaws under an applied stress.
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52. C:Brittleness
Is the relative inability of a material to sustain
plastic deformation before fracture of a material
occurs. In other words, a brittle material fractures at
or near its proportional limit.
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53. D: Ductility and malleability
Ductility represents the ability of a material to
sustain a large permanent deformation under a tensile
load up to the point of fracture.
For example, a metal can be drawn readily into a long
thin wire is considered to be ductile.
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54. Malleability is the ability of a material to sustain
considerable permanent deformation without
rupture under compression, as in hammering or
rolling into sheet.
Gold is the most ductile and malleable pure metal,
and silver is the second. Platinum ranks third in
ductility, and copper ranks third in malleability.
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55. E: Hardness
Resistance to indentation.
In mineralogy the relative hardness of a substance
is based on its ability to resist scratching.
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56. HARDNESS TESTS
Hardness tests are included in numerous specifications
for dental materials developed by the American Dental
Association (ADA) and standards promoted by the
International Organization for Standardization(ISO).
There are several types of surface hardness tests. Most
are based on the ability of the surface of a material to
resist penetration by a diamond point or steel ball
under a specified load.
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57. The most frequently used tests are:
A : BRINELL TEST
B : ROCKWELL TEST
C : VICKERS TEST
D : KNOOP TEST
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58. BRINELL TEST
 Used extensively for determining the hardness of metals and
metallic materials used in dentistry.
 A hardened steel ball is passed under a specific load into the
polished surface of material.
 The load is divided by the area of the projected surface of the
indentation, and quotient is referred to as BRINELL HARDNESS
NUMBER [BHN]
 For a given load , smaller the indentation , the larger is the number
and the harder the material.
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59. ROCKWELL TEST
 Similar to Brinell test in that a steel ball or a conical
diamond point is used.
 The depth of penetration is measured directly by a dial
gauge on the instrument.
 The Rockwell hardness number [RHN] is designated
according to the particular indenter and load employed.
 The convenience of this test , with direct reading of the
depth of the indentation.
 Also called as BRALE test.
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60. VICKERS TEST
 136 Diamond pyramid test
 Employs the same principle of hardness testing as Brinell
test.
 Square based pyramid is used.
 The load is divided by the projected area of indentation.
 The length of diagonals of the indentation are measured
and averaged.
 Employed in the testing of dental casting gold alloy.
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61. KNOOP TEST
 Employs a diamond-tipped tool that is cut in the
geometric configuration .
 The impression is rhombic in outline and the length of the
largest diagonal is measured .
 The load is divided by the projected area to give the
KNOOP HARDNESS NUMBER [KHN] or [HK].
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62. Brinell and Rockwell hardness tests are classified as
macro hardness test and knoop and Vickers test are
classified as micro hardness test.
Both Knoop and Vickers test employ loads less
than 9.8 N.Resulting indentations are small and
limited to depth of 19 micron meter .
Rockwell and Brinell tests give average hardness
values over much larger areas .
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63. The principle of hardness tests is based on
resistance to indentation .
The equipment generally consists of a spring-
loaded metal indenter point and a gauge from
which the hardness is read directly.
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64. THANK YOU
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