Plasticity evolution of an aluminum-magnesium alloy under abrupt strain path changes

During the forming and manufacturing of engineering materials, plasticity behavior could be evolving significantly due to complex deformation history. Therefore, this study aims to characterize the plasticity evolution of an aluminum-magnesium alloy under simple monotonic and non-monotonic loading with abrupt strain path changes. Instead of focusing only on one single stress state in the first-step loading for most of the studies in the literature, the current non-monotonic strain path testing program investigates three stress states - uniaxial, plane-strain, and biaxial tension - in the first-step loading and combines them with a second-step uniaxial loading along and orthogonal to the initial loading direction. This combination generates non-monotonic stress-strain data in a quite large and distributed spectrum in terms of the Schmitt parameter. It is found that the aluminum-magnesium alloy shows a unique phenomenon with a lower yield strength at reloading compared to monotonic cases coupled with a steady increase of stress overshooting the monotonic one at large strains. This increase of stress as well as the strain hardening rate lasts till the uniform strain and is therefore referred to as permanent hardening. The comprehensive non-monotonic behavior delivered by the new experimental program in this study could further assist the development of material models and an in-depth understanding of the underlying mechanisms.