Anelasticity is a characteristic of viscoelastic materials that depends on some physical properties of the material itself. Anelasticity is the opposite of elasticity.
An anelastic material subjected to stress undergoes deformations that are not proportional to the stresses and that do not disappear when the force is zeroed. In other words, an anelastic material will fully recover to its original state on the removal of load.
Inelastic behavior occurs both in materials commonly considered plastic and in those so-called elastic, but above a certain maximum load value. In elastic materials this anomaly of behavior manifests itself in different ways through yielding, subsequent elasticity, work hardening, elastic hysteresis. If we examine the resistant behavior of an elastic material, we can clearly see, after an initial elastic phase, the beginning of yielding in correspondence to the onset of strong deformations which increase faster than the load.
Subsequently, with the permanence of the load, the material undergoes further deformation in time (subsequent elasticity), deformation which, when the causes of the agents cease, partly reabsorbs, partly remains. For materials that are not perfectly elastic, but have already undergone permanent deformation, an increase in the elastic limit occurs when a succession of loading and unloading is applied. If loading and unloading cycles are repeated successively, a closed cycle is achieved, in which no more permanent deformation occurs.
The response of a solid to stress such that the stress deformation dependence contains, in addition to an instantaneous elastic part, a component whose equilibrium value is reached after a certain time (relaxation time) depending on the nature of the solid and on the temperature; this delayed response is due to the achievement of equilibrium by defects (impurities, dislocations, etc.) or excitations (electrons, phonons, etc.), whose energy depends on the effort.