A unified model for inelastic deformation of polycrystalline materials — application to transient behavior in cyclic loading and relaxation

Hart proposed a unified state variable model which is capable of predicting the nonelastic deformation for low and high temperatures over a very large strain rate range. He developed these relations from experimental evidence and a heuristic consideration of micro-mechanism of deformation in polycrystalline materials. Some of the important parameters can be uniquely determined using load relaxation tests in the fully loaded regime. The model was based on the realization that the dislocation motion is limited by the dislocation glide friction and by the resistance to strong barriers to dislocation motion represented by the hardness parameter, σ∗. A transient model based on the non-steady state condition for the frictional glide process is introduced here using a microhardness parameter as a new state variable. The microhardness parameter represents the strength or the average lifetime of the mobile dislocations relative to the frictional glide viscous drag process. The evolution of the microhardness parameter is very similar to that proposed for the hardness parameter except that the recovery plays a major role. The power law relationship for the frictional glide process is shown to be a steady state condition for the transient behavior in this model. The results show that the model can predict the transient behavior for both cyclic loading and the reloading phenomena during inelastic deformation and load relaxation.