Strain energy:

The external work done on an elastic member in making it to distort from its unstressed state is transformed into strain energy which is a form of potential energy.  The strain energy in the form of elastic deformation is recoverable mostly in the form of mechanical work. The potential energy stored in a body by the virtue of an elastic deformation, equal to the work that should be done to produce this deformation. Strain energy is the form of potential energy. The work done to distort the elastic member is stored as the strain energy. Some energy can be lost in plastic deformation of the member and some can be converted into heat instead of stored as strain energy, but rest is recoverable. Strain energy is measure of the energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress-strain curve, and is the measure of toughness of material.

Potential Energy stored in a strained solid equal to the work performed in deforming solid from its unstrained state less any energy dissipated by inelastic deformation. It can also be defined as the potential energy stored in the body because of elastic deformation. It is the mechanical energy stored up in the stressed materials. The change in configuration of the body depends on the type of stress. The ratio of the change in configuration to original configuration is known as strain. In a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that the angle strain, torsional strain, or steric strain, Allylic strain, and pentane interference are all reduced.