Stress

When an external force or system of forces is applied on a body, it undergoes a change in the shape or size according to nature of the forces. In the process of deformation, internal restoring force is developed due to molecular displacements from their positions of equilibrium. The internal restoring force opposes the deforming force.

The internal restoring force acting per unit area of cross-section of a deformed body is called stress.

In equilibrium, the restoring force is equal in magnitude and opposite in direction to the external deforming force. Hence, stress is measured by the external force per unit area of cross-section when equilibrium is attained.

If the magnitude of deforming force is F and it acts on area A,

Stress = restoring force / area

Stress = deforming force (F) / area (A)

Stress = F/A

The unit of stress is Nm^–2. The stress may be longitudinal, normal or shearing.

Longitudinal Stress

If the deforming forces are along the length of the body, the stress produced is called as longitudinal stress.

Normal Stress

If the deforming forces are applied uniformly and normally all over the surface of the body so that the change in its volume occurs without change in shape, the stress produced is called as normal stress.

You may produce normal stress by applying force uniformly over the entire surface of the body. Deforming force per unit area normal to the surface is called pressure while restoring force developed inside the body per unit area normal to the surface is known as stress.

Shearing Stress

If the deforming forces act tangentially or parallel to the surface so that shape of the body changes without change in volume, the stress is called shearing stress.