Logarithmic and Andrade creep

The exhaustion theory of logarithmic creep is re-examined for crystalline materials in which the obstacles to dislocations are localized, small and have maximum activation energies of not more than about 1 eV. When the theory is generalized to include the regeneration, as well as the exhaustion, of weakly obstructed dislocation segments, and also to take account of thermal fluctuations which, although realizable, are larger than the minimum activation energies of weakly obstructed dislocations, it no longer generates logarithmic creep by the process originally envisaged. If there is no strain hardening it gives steady-state creep. Andrade creep is obtained when the strain hardening is weak; logarithmic creep when it is strong. The original form of the exhaustion theory may be applicable in materials hardened by larger obstacles, as in precipitation hardening, where the maximum activation energies are too large to be met by thermal fluctuations.