The time constant of logarithmic creep and relaxation

Under certain conditions, the plastic extension of a sample subjected to a constant stress is to a good approximation proportional to the logarithm of the time. Similarly, if a sample is plastically strained and unloaded, there are changes in its length and hardness which vary logarithmically with time. For dimensional reasons, a logarithmic variation must involve a time constant τ characteristic of the process, so that the deformation is proportional to ℓn(t/τ). stinct mechanisms of logarithmic creep have been proposed, the work-hardening of a set of barriers to dislocation motion, all having the same activation energy, or the progressive exhaustion of the weaker barriers in a set which has a distribution of activation energies, these energies remain constant during the process of creep. It has been suggested that the experimentally observed value of τ can be used to decide which of the two mechanisms is operative. It is shown here that the work-hardening mechanism expresses τ in terms of parameters which are not easy to estimate, while, if the exhaustion mechanism operates, the observed value of τ is determined by the experimental conditions rather than by the parameters of the dislocation mechanism.