Experimental Approach to a Theory of Plasticity at Elevated Temperatures

Abstract

The problem of providing an experimental basis for an isothermal theory of plasticity of metals at elevated temperatures has been examined. The key objectives are considered to be the establishment of a steady‐state function and the discovery of a nonsteady deformation law. Constant stress and constant strain rate are judged the best loading conditions for both of these tasks since they provide paths, under the simplest thermomechanical histories, to the steady‐state deformation. The use of plots of strain rate and its time derivative for constant stress tests and plots of stress and its time derivative for constant strain rate tests is advocated as a means of best displaying the transient nonsteady behavior. It is even suggested that the functions represented by such plots may be independent of the loading history prior to the period of constant stress or constant strain rate applications. Constant stress and constant strain rate tests are limited in the early (nonsteady) period by lack of control of structure on initial loading, and generally in the later period by the nonuniformity of deformation. It is suggested that the former difficulty may be overcome by employing loading rates sufficiently rapid to permit sensible constancy of structure. Nonuniformity of deformation imposes a severe restriction on the examination of steady‐state deformation.