General orthotropic yield functions based on linear stress deviator transformations

Two non-quadratic yield functions for orthotropic sheet metals based on linear transformations of the stress deviator components are presented. Both yield functions are convex and are formulated in terms of the general (3D) stress state. The first yield function contains 18 adjustable anisotropy parameters which may be calculated from selected mechanical input data such as directional in-plane stresses, r-values, equibiaxial data and out-of-plane yield stresses (obtained e.g. from polycrystal plasticity calculations). For situations where fewer input parameters are available a second yield function containing 13 adjustable anisotropy parameters is proposed. The predictive capabilities of both yield functions are demonstrated by modelling the plastic anisotropy of aluminum alloy sheets.