The Bauschinger effect and cyclic hardening in copper

Abstract

The Bauschinger Effect and cyclic hardening were studied during the first few cycles in copper single crystals of an “easy glide” orientation. Dislocation etch pitting was used to augment the mechanical measurements. It was found that significant rearrangements of the dislocation distribution occurred during stress reversal. Sharply defined kink bands which formed during forward straining broadened, and the overall dislocation distribution became more uniform during subsequent cyclic deformation. For cycling at constant strain amplitude, it was found that hardening above the prestress level occurred almost as rapidly as during unidirectional deformation. However, the absence of hardening during increments of Bauschinger strain yielded anomalously low hardening in terms of the cumulative strain. A cyclic hardening model is proposed in which the Bauschinger strains below the prestress level result from relaxation of internal stresses whereas “hardening strains” occur only above the prestress level. The non-symmetry, which causes higher Compressive stresses than tensile stresses during cycling at constant strain amplitude, is shown to result from larger Bauschinger strains below the prestress levelduring tensile half-cycles. Cell sizes and bundle spacings reported in the literature for cyclic tests have been correlated with the corresponding flow stresses and are consistent with predictions of the Long Range Stress Theory.