On the representation of rheological results with special reference to creep and relaxation

Abstract

The remarkable qualitative similarity in the response of solids of widely different structures to applied stresses, particularly in creep and relaxation, is ascribed to relaxation centres all of which, in a given solid, have approximately the same heat of activation, but, on account of variations in their geometry such as shape and size, have different entropies of activation. A normal distribution of entropies of activation is shown to lead to a log-normal spectrum of relaxation times of the type originally proposed by Wiechert on heuristic grounds. Boltzmann's superposition principle and the relation between creep and relaxation derived by Zener are used to show that this distribution yields a form of stress relaxation curve common to many solids, as well as Andrade's t1/3-creep law. Experimental data, given in the literature, on the creep and relaxation in pure polycrystalline aluminium, rubber, polymethylmethacrylate and ceramics are shown to be in good agreement with the theory. The significance of Nutting's law of deformation is discussed.