A plasticity model with yield surface distortion for non proportional loading

In order to enhance the modeling of metallic materials behavior in non prop ortional loadings, a modification of the classical elastic-plastic models i ncluding distortion of the yield surface is proposed. The new yield criteri on uses the same norm as in the classical von Mises based criteria, and a " distorted stress" S-d replacing the usual stress deviator S. The obtained y ield surface is then "egg-shaped" similar to those experimentally observed and depends on only one new material parameter. The theory is built in such a way as to recover the classical one for proportional loading. An identif ication procedure is proposed to obtain the material parameters. Simulation s and experiments are compared for a 2024 T4 aluminum alloy for both propor tional and non-proportional tension-torsion loading paths. (C) 2001 Elsevie r Science Ltd. All rights reserved.