Modeling of Directional Hardening Based on Non-Associated Flow for Sheet Forming

Abstract

This work describes a material model for sheet metal forming that takes into account anisotropic hardening under conditions of proportional loading. Conventional isotropic and kinematic hardening models constrain the shape of the yield function to remain fixed throughout plastic deformation, which is not consistent with most test data from aluminum alloys obtained under proportional loading. Conventional hardening models are shown to introduce systemic errors in stresses in different loading conditions at low and high levels of strain that tend to amplify the effect of stress miscalculation on the prediction of springback. A new model is described in which four stress‐strain functions are explicitly integrated into the yield criterion in closed form solution. The model is based on non‐associated flow so that this integration does not affect the accuracy of the plastic strain components. The model is expected to lead to a significant improvement in stress prediction under conditions dominated by proportional loading, and this is expected to directly improve the accuracy of springback prediction for these processes.