The mechanical equation of states: Dislocation creep due to stresses varying in magnitude and direction

Abstract

The model of creep, as a process in which dislocation movements cause work-hardening and recovery, which was shown in earlier work to explain the effects of raising, lowering or temporarily removing the stress, has been used to predict the effects of stress-reversal. A temporary change in the direction of stressing is theoretically a method of remobilizing dislocations whose glide had been halted by barriers. So, for example, intermittent compression should restore tension mobility to some dislocations. Analytical solutions of the pertinent differential equations are derived which permit the graph of creep strain versus time to be predicted for any sequence of temperatures and for a stress that varies both in magnitude and sign. For the special, isothermal case where the sign of the stress (but not its magnitude) is varied to produce an infinite sequence of identical stress-cycles, the theory predicts that the associated strain-time cycles will approach an equilibrium form. This is often observed. Although the strain rate falls during the period following each stress reversal, the total strain due to a sequence of identical, equilibrium stress reversals is theoretically equal to that which secondary creep would, by itself, have produced. Experiments support this deduction. The state of the specimen is explicitly defined by the model in terms of its previous states and the current stress and temperature, i.e. the model leads to a Mechanical Equation of States.