Forming limit model evaluation for anisotropic sheet metals under through-thickness normal stress

The M–K method has been used with Barlat׳s 1989 anisotropic yield surface to predicate forming limit to study the effect of the normal stress and material anisotropy. The 3D stress state is converted to plane-stress state according to the hypothesis that hydrostatic stress has no effect on plastic deformation. The predicted forming limits correlate well with experimental data for AA5XXX under plane-stress condition and AA6011 aluminum alloy under three-dimension stress condition. Analyses show that normal stress increases the forming limit described both in strain space and in stress space. Moreover, the sensitivity of forming limit curve to normal stress is associated with the material׳s hardening effect. •M–K theory and Barlat׳s 1989 yield function based 3D FLC model is developed.•FLCs under plane stress and 3D stress conditions are validated with experiment.•It is found that normal stress enhances the formability in both strain and stress spaces.•Results show sensitivity of FLC to normal stress is associated with hardening effect.