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Stress and Strain
In a simplistic sense, stress may be
thought of as Force/Area.
Similarly, strain is the deformation of the
component/original length.
A stress may be direct, shear or torsional -
leading to corresponding deformations.
Stress cannot be measured directly, but
deformation can be.
Shear Stress, Shear Strain,
shear stress, = Shear Load / Area
shear strain, = angle of deformation (radians)
shear modulus, G = / (elastic region)
Load, P
P
Area
Ao
Lo
L/2
L/2
Tensile Stress
Load, P
P
Area
Ao
Lo
L/2
L/2
Compressive Stress
S P
Ao
e L
Lo
Engineering Stress
Engineering Strain
Load
, P
(kN
)
Uniform Deformation
Total Elongation
Elastic
Deformation
Maximum
Load, Pmax
Load,
Pf
Elongation, DL (mm)
Hooke’s Law
Elastic Deformation
Elasticity : Elastic deformation is not permanent; it means
that when the load is removed, the part returns
to its original shape and dimensions.
For most metals, the elastic region is linear. For
some materials, including metals such as cast
iron, polymers, and concrete, the elastic region is
non-linear.
If the behavior is linear elastic, or nearly linear-
elastic, Hooke’s Law may be applied:
Where E is the modulus of elasticity (MPa).
S Ee
• Property of body by which it retains to its
shape after removing stresses.
• Plasticity = %age elongation + %age
reduction in area.
“Ability of a material to resist
deformation.”
Tensile strength: Measure of level of
tensile stress required to make
material fail.
Compressive strength: Maximum
compressive stress that a material
can resist without being crushed.
Tensile Strength
Compressive Strength
Ductility & Brittleness
Ability of a material by
which it can be drawn
into wires.
Opposite to ductility.
Tendency of a body to
break without being
distorted.
Malleability Ability of a body to be plastically extended in all
directions without breaking under compressive
forces only.
Property by which metals drawn into sheets.
Resistance to the plastic deformation.
o Measure of amount of energy that a material
can absorb before fracturing.
o Work done to propagate a crack.
Stiffness
“Ability of a material to resist
bending.”
• Progressive deformation of a material
under constant load with time.
• Important for some type of engineering
design particularly those operating on
high temperature.
• Tertiary creep > Primary creep >
Secondary creep.
l0lF
Constant
Load
Fixed
When stress is applied on a material then after
certain time it becomes unfit as compared to new one.
It occurs due to repeated loading and
unloading.
Modulus of
Elasticity
Ratio of stress to strain.Young’s Modulus.Stress/strain.N/m2.
Typical Mechanical
Properties
Material Yield Stress
(MPa)
Ultimate
Stress (MPa)
Ductility
EL%
Elastic Modulus
(MPa)
Poisson’s
Ratio
1040 Steel 350 520 30 207000 0.30
1080 Steel 380 615 25 207000 0.30
2024 Al Alloy 100 200 18 72000 0.33
316 Stainless Steel 210 550 60 195000 0.30
70/30 Brass 75 300 70 110000 0.35
6-4 Ti Alloy 942 1000 14 107000 0.36
AZ80 Mg Alloy 285 340 11 45000 0.29
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