Instabilities of Dynamic Strain Diagrams Predicted by the Relaxation Model of Plasticity

To predict of appearance and disappearance of yield drop effects with regard to different dynamics, temperatures or other factors, the wide spectrum of the initial plastic stage of stress–strain diagrams for homogeneous materials is considered. Compared with unchanged quasi-static stress–strain diagrams, the dynamic changes in stress–strain diagrams depending on loading history are classified. In addition to a group of monotonic diagrams, varying only in yield strength, a group of three nonmonotonic diagrams, with the appearance or disappearance of the yield drop effect at different strain rates, is predicted on the basis of the relaxation model of plasticity. It is shown that unlike classical dynamic plasticity models, which are able to construct only the first set of diagrams, the relaxation model of plasticity allows the prediction of any set of deformation curves on the basis of a minimal number of parameters, which are invariant to the strain rate and generally to the loading history. Based on experimental data from the literature, dynamic deformation dependencies with an emerging yield drop at a fixed strain rate for different metals are predicted. Similar dynamic effects on stress–strain diagrams for materials with different strain rate sensitivities and structural-temporal parameters are revealed.