A perspective on the Wilshire creep equations

The Wilshire creep equations were introduced around fifteen years ago. Their aim was to address the non-physical extrapolation of power-law models, especially to high stresses, and the unrealistic values for activation energy and stress exponent that often arise from simple fits to data. In application they have met with some success, also with some difficulties which have largely been addressed empirically. No detailed mathematical analysis of the model seems to have been performed. This paper considers the fundamental characteristics of the Wilshire equations, as originally given, commencing with their internal consistency. It is found that the strain-time equation is incompatible with those for minimum creep-rate and rupture life. A consistent rate equation is derived, enabling the model to address the creep process rather than merely its results. Predictions made using the original and developed models are compared with actual materials behaviour; this reveals aspects of the approach which require reconsideration. The upper limit imposed by the ultimate tensile strength, and departures from a simple power law emerge as the key characteristics to be preserved and considered further.