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