Comments on normalizing the reduced strain rate for stress reduction analysis

Abstract

The stress reduction behavior of a class M material can be analyzed on a semilogarithmic plot of reduced strain rate (kr) normalized by the strain rate prior to the stress reduction (ko) versus reduced stress (o) normalized by the stress prior to the stress reduction (Oo) [1]. The stress reduction behavior of class M materials on such a plot consists of two branches. A low stress sensitivity branch is observed at large stress reductions (small O/Oo) and a high stress sensitivity branch is observed at small stress reductions (large O/Oo) [2]. The slope of the high stress sensitivity branch can be used to determine the true activation area of the deformation mechanism [2,3]. Usually, stress reduction experiments are performed in the steady state strain rate region, i.e, region II on a semilogarithmic plot of strain rate versus strain, and therefore the reduced strain rate is normalized by a constant. However, for some materials a steady state strain rate region does not occur. The question that needs to he answered is whether the strain rate prior to the stress reduction should he used to normalize the reduced strain rate when a steady state strain rate does not occur? The objective of this paper is to determine what strain rate should he used to normalize the reduced strain rate for materials without a steady state strain rate region, and to determine what effect the choice of normalization factor has on the true activation area.