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Stress, defined as force per unit area, is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces.

stress = Restoring force/ area

There are three main types of stress. 

1) If we stretch or compress an object, we are subjecting it to a TENSILE STRESS. 

2) If an object is subjected to a force along an entire surface, changing its volume, then it is said to be experiencing a BULK STRESS.

3) if the force is acting tangentially to the surface, causing it to twist, then we are subjecting it to a SHEAR STRESS.

Strain is the geometrical expression of deformation caused by the action of stress on a physical body.

Strain = Change in dimension/ original dimension

There are three types of strain:

(Classification based on types of change)

1) LONGITUDINAL (Change in length)

2) VOLUME OR BULK STRAIN (Change in volume)

3) SHEARING STRAIN (Change in shape)