Criticality in Andrade creep

The transition from instantaneous plastic yielding to rapid creep, under constant applied stress, is a form of critical state behaviour, analogous to the avalanching of sandhills of critical slope. A model is proposed for this in which localized obstacles give way to dislocations driven against them. Strain hardening moves the system from the critical state into the creep regime. When this hardening is weak the strain increases as the cube root of time, as observed originally by Andrade. The circumstances under which creep sometimes develops differently from this are briefly discussed.