Rate sensitivity of plastic flow and implications for yield-surface vertices

When crystalline slip is considered as the micromechanism of plastic deformation in polycrystals, and the slip process is assumed to be rate-independent, the formation of a vertex is predicted on the current yield surface in stress-space. Experiments which were conducted to confirm the existence of such a vertex are known to have produced ambiguous results; e.g. a superimposed shear during compressive loading was found to produce an initially elastic response. If we consider that the crystalline slip process is slightly rate-dependent, however, then we can furnish a reasonable explanation for this elastic behavior in shear. We illustrate the point by considering first a single crystal model undergoing double slip, and then a polycrystal model based on the slip concepts of Batdorf and Budiansky. The results presented, based on the assumption that the slip process is not rate-independent, but rather at least slightly rate-dependent, give qualitative agreement, and reasonable quantitative agreement, to experimental results for superposed shear during compressive loading. They also suggest that the actual rate sensitivity of plastic flow may be central to understanding the ambiguous conclusions from experimental attempts to find yield surface vertices.