What is Stress and Strain?

Stress and strain are two important parameters associated with a body that is subjected to an external force.

Strength of Material

When a body is subjected to some external force, it undergoes some form of deformation. The resistance of the body to prevent such deformation forms the strength of the material.

What is Stress?


The force of resistance per unit area of the body is called stress. It is measured as the ratio of force and the area of the cross-section. The external force acting on the body is termed as load (P). If the area of cross-section is (A), then stress is given by,

σ = P/A

Unit of Stress

The unit of stress is N/m2 or kgf/m2 or kgf/cm2.

The S.I Unit is N/m2. Where, 1N/m2 = 1 Pascal 

What is a Strain?

The ratio of change in dimension of the body to its original dimension is called strain. The strain is a unitless quantity. Which is given by the relation,

ε = ∆l/l
∆l = Change in Dimension
l   = Original Dimension


These are of 4 Types:
  1. Tensile Strain
  2. Compressive Strain
  3. Volumetric Strain
  4. Shear Strain

1. Tensile Strain

This is the ratio of change in length of the body to its original length due to the deformation caused by a pulling force or tensile force.
e = dl/l

2. Compressive Strain

This is the ratio of change in dimension of the body to the original dimension due to the deformation caused by a pushing force or compressive force.
e = -dl/l

3. Volumetric Strain

This is the ratio of change in the volume of the body to its original volume.
e = dV/V

4. Shear Strain

This is the strain produced due to shear stress.

Shear Strain = Φ= dx/h
Shear Strain
Body Under Shear Stress Fp



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