Rubber does not obey Hooke's Law. The rubber band returns to its original length when the force is removed but the loading and unloading curves are different.The loop formed by the loading and unloading curve is called a hysteresis loop and represents the difference between the work done when loading and the energy released when unloading. This energy is released as heat energy.The change in length during unloading is greater than during loading for a given tension.There is a low limit of proportionality.