Strain Energy

When a material is stretched, the work done on it is stored as elastic strain energy. The strain energy is equal to the area under the force-extension graph.For a material that obeys Hooke's law, the graph is a straight line through the origin, so the strain energy is E = ½FΔL, where F is the load and ΔL the extension. Since ΔL is proportional to F, the strain energy is proportional to F².If the material is loaded and then unloaded, the unloading curve may not follow the loading curve. The area under the loading curve is the work done on the material, and the area under the unloading curve is the energy released.The area between the two curves is the energy that is not recovered. It is dissipated as heat inside the material, or goes into permanently deforming it if it has been stretched beyond its elastic limit.