A 6-COMPONENT YIELD FUNCTION FOR ANISOTROPIC MATERIALS

In classical flow theory of plasticity, it is assumed that the yield surface of a material is a plastic potential. That is, the strain rate direction is normal to the yield surface at the corresponding loading state. Consequently, when the yield surface is known, it is possible to predict its flow behavior and, associated with some failure criteria, to predict limit strains above which failure occurs. In this work a new six-component yield surface description for orthotropic materials is developed. This new yield function has the advantage of being relatively simple mathematically and yet is consistent with yield surfaces computed with polycrystal plasticity models. The proposed yield function is independent of hydrostatic pressure. So, except for such cases, strain rates can be calculated for any loading condition. Applications of this new criterion for aluminum alloy sheets are presented. The uniaxial plastic properties determined for 2008-T4 and 2024-T3 sheet samples are compared to those predicted with the proposed constitutive model. In addition, for 2008-T4, the predictions of the six-component yield function are compared to those made with the plane stress tricomponent yield criterion proposed by Barlat and Lian. Though rather good agreement between experiments and predicted results is obtained, some discrepancies are observed. Better agreement could result if the isotropic work-hardening assumption associated with the yield criterion were relaxed. Nevertheless, the proposed yield function leads to plastic properties similar to those computed with polycrystalline plasticity models and can be very useful for describing the behavior of anisotropic materials in numerical simulation of forming processes.