Hysteresis
in the context of deforming a material is the loss of energy in the
form of heat when a sample is loaded then unloaded. This occurs due to internal friction, and so the effect is larger for a material such as rubber, which requires the movement of large molecular chains each time it is stretched or compressed. If a piece of rubber is deformed several times in the hands, this loss of energy can be felt directly as the sample heats up.
As can be seen on the graph, the deformation of a viscoelastic material follows a different path on the load and unload cycles. The unload cycle shows that the material is slower to recover compared to the load cycle. This difference in recovery is known as hysteresis, which is due to energy dissipation by the generation of heat.
