Stress is defined as force per unit area within materials that arise from externally applied forces, uneven heating, or permanent deformation and that permits an accurate description and prediction of elastic, plastic, and fluid behavior.

The force applied per unit area in mechanics is known as stress. The following formula represents it



σ is stress applied

F is force applied

A is the area of force applied

Stress is measured by unit N/m2

The ratio of internal force F, produced when a substance is deformed, to the area A where force is applied is known as stress. At equilibrium, the internal force is equal to the magnitude of the externally applied force.

The newton per square meter (Nm2) is the SI unit for stress. Dyne-cm2 is the CGS unit in which stress is measured. ML-1T-2 is the dimensional formula for stress

Types of Stress

  • Normal stress: It is the restoring force per unit area perpendicular to the surface of the body.  It is of two types: tensile and compressive stress.
  • (Tangential stress: When the elastic restoring force or deforming force acts parallel to the surface area, the stress is called tangential stress.


It is the ratio of the change in size or shape to the original size or shape. It has no dimensions, it is just a number. Strain is the amount of deformation experienced by the body in the direction of force applied, divided by the initial dimensions of the body.

The following equation gives the relation for deformation in terms of the length of a solid:


where, ϵ

is the strain due to the stress applied, δl is the change in length and L is the original length of the material? The strain is a dimensionless quantity as it just defines the relative change in shape.

Types of Strain

  • Longitudinal strain: If the deforming force produces a change in length alone, the strain produced in the body is called longitudinal strain or tensile strain.
  • Volumetric strain: If the deforming force produces a change in volume alone, the strain produced in the body is called volumetric strain
  • Shear strain: The angle tilt caused in the body due to tangential stress expressed is called shear strain
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