Thermal activation and Andrade creep

Becker and Mott have provided different formulae for the thermal activation energy of a process that occurs under stress. As the applied stress tends towards the critical stress, Becker's formula requires the smaller activation energy. We argue that, while the Mott formula represents the energy required for the system to cross the barrier, the Becker formula represents the energy required for the barrier to be temporarily suppressed. Work of at least the Mott value has to be done to move the system from its position before the barrier to one beyond the barrier, so the Becker formula does not control the process. Andrade creep is a process in which the deformation increases with time with an exponent 1/3. Cottrell provided a model, based on the Becker formula, which explained this result. The Mott formula, applied to a modified physical model, yields an exponent 4/13, experimentally indistinguishable from Andrade's Law.