For a polymer subjected to a cyclic loading$/$unloading$,$ the work density that the external stress done to the material in one cycle can be calculated as $0+0,$ where t$0$ is an arbitrary starting time, and T is a period. Please show that for an elastic solid, the work density is zero. Please provide a simple physical explanation for the result. For a simple fluid, the work is always positive. Where does the work go to$?$ For a viscoelastic material, the work is determined by the loss factor.