Longitudinal strain: If there is a change in the length of a body due to the applied force, then the strain is called longitudinal strain. It is the ratio of the change in length of a body to the original length of the body. It is measured by the change in length per unit length. This kind of strain is also called torsion strain.
Let the initial length of a body = L
The change in length due to applied force = l (Figure)
Longitudinal strain = l/L
If the length increases due to tensile stress, the corresponding strain is called tensile strain. If the length decreases due to compressive stress, the strain is called compressive strain. The longitudinal strain, e, is measured in terms of the changes in the lengths of lines.