When a wire is stretched work is done against the interatomic forces.
This work is stored in the wire in the form of elastic potential energy. W=21× stress × strain × volume of wire
Also when strain is small, ratio of longitudinal stress to corresponding longitudinal strain is called Youngs Modulus of material of body. Y= longitudinal strain longitudinal stress ∴W=21× stress ×Y stress × volume =2Y( stress )2× volume