Effect of pre-strain on work-hardening behavior of magnesium alloy sheets upon cyclic loading

This paper reports the effect of pre-strain on the work-hardening behavior of rolled AZ31 magnesium alloy sheets during in-plane cyclic loading. The work-hardening behavior of the alloy remained almost unchanged when tensile strain was applied before cyclic loading. However, the work-hardening behavior was significantly affected when a compressive strain was applied. First, the resulting stress–strain curve was not sigmoidal upon tension in some cases, depending on the magnitudes of the applied compressive pre-strain owing to the inversion of the loading direction from tension to compression before the second increase in the work-hardening rate. In other words, a sigmoidal stress–strain curve certainly arose upon tension when compressive pre-strain was applied and the loading direction was not inverted from tension to compression. It was found that the strain at the beginning of the second increase had a high correlation with the volume fraction of twins. Second, the change in the rate of work-hardening at the beginning of tension became sharp as compressive pre-strain increased, probably owing to the effect of activation of detwinning on the stress–strain curve, which became increasingly significant as the compressive pre-strain increased the volume fraction of twins.