Effect of strain path change on limits to ductility of anisotropic metal sheets

Localized necking in thin metal sheets is analyzed by using the M–K-model approach, and the effect of a number of different non-proportional strain paths prior to the occurrence flow localization are considered. The analyses account for plastic anisotropy, using four different anisotropic plasticity models to fit a set of experimental data for cold-rolled steel sheet. The predicted forming limit diagrams show strong dependence on whether or not the load on the sheet is removed between two load steps on a non-proportional strain path. This dependence is investigated in detail for one of the anisotropic plasticity models, and it is shown that elastic straining plays a large role, as the stresses quickly move from one point of the yield surface to another. When the load is removed between steps, the stress point moves in a different manner, which results in quite different flow localization response.