Strain

A) Tensile Strain :- it is a ratio of :- Increase in Length/Original Length

A question may arise – Why this ratio ? An answer is that it is the material property which determines how a metal (or rubber band etc ) recovers after it is expanded. Upto the elastic limit, the elongation does not depend on the original length we start with , hence the effect of original length is neutralized by dividing all elongation measurements by the original length.

See animation in introduction to observe how a wire first expands and then recovers after load is removed.

B) Volumetric strain : a similar approach, here the increase in volume is divided by original volume. This is also depending on material property.