Stress - Delta University!

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Mechanical properties of
materials
By
Dr. Reham Mohammed Abdallah
Items to be covered
• Introduction on mechanical
properties
• Force
• Stress & types
• Strain
• Stress-strain curve
• Stress terms
Proportional limit
Elastic limit
Yield stress
Ultimate strength
Fracture strength
Strain terms
Flexibility
Ductility
• Energy terms
Resilience
Toughness
• Mechanical properties are important
in
understanding
and
predicting
a material's behavior under load.
• Quantities of force, stress, strain,
strength,
toughness,
hardness,
friction and wear can help to
Identify the properties of a material
(polymer, ceramic and metal).
Understand reasons of failure.
Select
and
design
of
dental
restorations and appliances.
• Standardization of laboratory tests is
essential
to
control
permit
comparison
between investigators.
quality
of
and
results
Force
• One
body
interacting
with
another
generates force. Forces may be applied
through actual contact of the bodies or
at a distance (e.g., gravity).
• The International System Unit of force
(SI unit) is the Newton (N).
Stress
 When
a
force
acts
on
a
body
tending
to
produce
deformation, a resistance is developed to this external force
application.
 Definition: It is the Internal resistance to the externally
applied force.
 It is denoted by (σ)
 Stress (σ)= Force/Area (fig.1).
 Pascal = 1 N / m².
 Commonly stress is reported in terms of
megaPascals (MPA). MPA=106 Pascal.
Fig.1:Stress measurements
Types of stresses
1. Axial stresses
Compressive stress
Compression results when the body is subjected
to two sets of forces directed towards each
other in the same straight line. (fig.2)
Tensile stress
Tension results in a body when it is subjected
to two sets of forces directed away from each
other in the same straight line. (fig.2)
Fig.2.Types of axial stresses
• Axial
Compressive
Tensile
2. Non axial stresses
Shear stress
Shear is the result of two sets of forces
directed towards each other but not in the
same straight line.(fig.3)
Torsion
It results from the twisting of the body. (fig.3)
Bending
It results by applying bending movement.(fig.3)
Fig.3.Types of non axial stresses
• Non Axial
Shear
Torsion
Bending
Strain
• Definition: It is the change in length per unit
length.
• It represents the relative deformation of
an object that is subjected to stress.
• It may be elastic, plastic or both elastic and
plastic.
• It is denoted by “ε”
• Designated as ∆L / L. So, it is unitless.
• Strain(ε)= Deformation/Original length
• When the force is applied, the rod's
length
changes
from
its
original
length L0, to the extended length
L1. The resulted strain, ɛ, is given by
ɛ = (L1- L0)/ L0 (fig.4)
Fig.4:strain
In an object
subjected to
stress
Stress-Strain Relationship
• If a bar of material is subjected to an applied
force, F, the magnitude of the stress and the
resulting deformation (ɛ) can be measured.
• This is done with tensile, compressive or shear
loading
of
samples
using
universal
testing
machine (Fig.5).
• Graph representing stress (or load) and strain
(elongation) can be obtained (Fig.6).
Fig.5: Universal testing
machine
Fig.6: Stress-strain curve for a material
subjected to tensile stress.
Stress-Strain Behavior (types of strain)
1. Elastic deformation
• Reversible: When the stress is removed, the
material returns to the dimension it had
before the loading.
2. Plastic deformation
• Irreversible: When the stress is removed, the
material does not return to its previous
dimension.
I. Stress terms
1.Proportional Limit
• It is the maximum stress up to which,
the stress is linearly proportional to
strain. (fig.6)Point A
• A
material
with
high
value
proportional
limit
can
withstand
greater
stress
deformation.
without
of
permanent
Significance
• Dental
restorations
should
be
constructed from materials with a
high proportional limit. Any dental
restoration
deformed
that
through
is
permanently
the
forces
of
mastication is usually a functional
failure to some degree.
• For example, a fixed partial denture that
is
permanently
deformed
by
excessive
occlusal forces would exhibit altered occlusal
contacts.
2. Elastic Limit
• Maximum stress a material can withstand
without
undergoing
permanent
deformation.
• It describes the elastic behavior of the
material. (fig.6)Point B
• The elastic and proportional limits
have nearly the same values, as they
represent the same phenomena.
3. Yield Stress or Proof stress
• It is the stress at which materials
start
to
show
permanent
deformation.(fig.6) Point C
Significance
• Permanent deformation and stresses
in excess of the elastic limit are
desirable
an
when
orthodontic
arch
shaping
wire
or
adjusting a clasp on a removable
partial denture.
4. Ultimate(Tensile or compressive)
Strength or stress
• Maximum stress that the material
can
withstand
(fracture)
under
before
tension
failure
or
compression respectively.
• The material could not withstand
any more stresses, as it will fracture.
(fig.6) Point D
• The yield strength is often of greater
importance than ultimate strength
in
design
and
material
selection
because it is an estimation of when a
material
will
start
to
deform
permanently.
5. Fracture strength or stress
• It is the strength at which
material fractures. (fig.6) Point F
the
II. Strain terms
1. Flexibility
• The maximum flexibility is defined as the
strain occurring when the material is
stressed to its proportional unit.
Significance
• A larger strain or deformation with
slight
stresses
is
an
important
consideration in orthodontic appliances.
• Impression
large
materials
flexibility
should
or
have
elastic
deformation to withdraw through
severe undercuts without permanent
deformation.
2. Ductility
• The
amount
produced
fracture.
in
of
the
plastic
specimen
strain
before
• Or the ability of a material to be
drawn and shaped into wire by
means of tension.
• When tensile forces are applied, the
wire
is
formed
deformation.
by
permanent
• However, malleability of a substance
represents
its
ability
to
be
hammered or rolled into thin sheets
without fracturing.
Significance
• High ductility and malleability are
useful
in
restorations
burnishing.
adapting
to
the
metallic
margins
by
• Very thin pure direct filling gold foil
is available for restorations.
• Orthodontic wires are drawn from
cast ingot.
Brittleness :
 If a material showed no or very little
plastic deformation on application of load
it is described as being brittle.
 A brittle material
fractures at or near
its proportional limit.
Ductile material
1) Is the ability of a
Material to withstand
Plastic deformation
Under tensile stress
Without fracture.
Fracture occur far
Away from P.L
Brittle material
1) brittle material
fractures at or near
its proportional limit.
Fracture occur at or
near P.L
Elastic Modulus (E)
It is the constant of proportionality
between stress and strain.
It represents the slope of the elastic
portion of the stress – strain curve.
It is a measure of rigidity or stiffness
Materials with higher Young’s modulus
value are said to be stiffer or more rigid
than those of low Young’s modulus values
because they require much more stresses
to produce the same amount of strain.
It is measured by
stress/strain
 It is measured by GPA=109 Pascal.
Elastic Modulus of material (A) is higher
than that of material (B)
Significance
• Oral
appliances
materials
should
proportional
modulus
and
to
mastication
deformation.
limit
restorative
have
and
high
elastic
resist
forces
of
and
permanent
• In orthodontic, when rapid large
forces need to be applied to cause a
tooth to move, a high stiffness wire
is used. However, flexible wires are
used when a low force is needed for
slow movement of the tooth.
III. Energy terms
1. Resilience
• Resilience is the resistance of a material to
permanent deformation.
• It
indicates
the
amount
of
energy
necessary to deform the material to the
proportional limit.
• Resilience is therefore measured by
the area under the elastic portion of
the stress strain curve.
Significance
• Resilience has particular importance
in the evaluation of resilient-denture
lining
materials,
tissue
conditioners
and maxillofacial materials.
2. Toughness
• Toughness,
which
is
the
resistance
of
a
material to fracture, is an indication of the
amount
of
energy
fracture.
• It is represented by
the area under the elastic
and plastic portions of
a stress-strain curve.
necessary
to
cause
Significance
• Addition of zirconia, alumina and
leucite to dental porcelain to resist
crack propagation and increase the
fracture toughness.
ANALYSIS FOR A STRESS
STRAIN CURVE
STIFFNESS & FLEXIBILITY
1) If longitudinal portion of the curve
is closer to the long axis, the
material is stiff & not flexible.
2) If it is away from the long axis the
material is flexible.
TOUGHNESS & BRITTLENESS
1) If material fractures after a long
concave portion of the curve, it
donates that the material is tough
& ductile.
2) If elastic portion of the curve is
minimal, it shows the brittleness of
the material.
STRENGTH & WEAKNESS
• If longitudinal portion of curve is long, it
means that the material is strong.
• If longitudinal portion is short, it means
that the material is weak.
 HENCE FROM THE ANALYSIS OF THE
STRESS STRAIN CURVE, IT IS POSSIBLE
TO
HAVE
AN
IDEA
ABOUT
PROPERTIES OF A MATERIAL.
THE
Comparison of materials
properties
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