The Role of Texture and Elastic-Plastic Anisotropy in Metal Forming Simulations

Abstract

Metals mostly occur in polycrystalline form where each grain has a different crystallographic orientation, shape, and volume fraction. The distribution of the grain orientations is referred to as crystallographic texture. The discrete nature of crystallographic slip along certain lattice directions on preferred crystallographic planes entails an anisotropic plastic response of such samples under mechanical loads. While the elastic-plastic deformation of single crystals and bicrystals can nowadays be well predicted, plasticity of polycrystalline matter is less well understood. This is essentially due to the intricate interaction of the grains during co-deformation. This interaction leads to strong in-grain and grainto- grain heterogeneity in terms of strain, stress, and crystal orientation. Modern metal forming and crash simulations are usually based on the finite element method. Aims of such simulations are typically the prediction of the material shape, failure, and mechanical properties during deformation. Further goals lie in the computer assisted lay-out of manufacturing tools used for intricate processing steps. Any such simulation requires that the material under investigation is specified in terms of its respective constitutive behavior. Modern finite element simulations typically use three sets of material input data, covering hardening, forming limits, and anisotropy. The current research report issued by the Max-Planck-Institut für Eisenforschung is about the latter aspect placing particular attention on the physical nature of anisotropy. The report reviews different empirical and physically based concepts for the integration of the elastic-plastic anisotropy into metal forming finite Raabe, Texture and Anisotropy in Metal Forming Simulations Raabe, edoc Server, Max-Planck-Society - 3 - MPI Düsseldorf element simulations. Particular pronunciation is placed on the discussion of the crystallographic anisotropy of polycrystalline material rather than on aspects associated with topological or morphological microstructure anisotropy. The reviewed anisotropy concepts are empirical yield surface approximations, yield surface formulations based on crystallographic homogenization theory, combinations of finite element and homogenization approaches, the crystal plasticity finite element method, and the recently introduced texture component crystal plasticity finite element method. The report presents the basic physical approaches behind the different methods and discusses engineering aspects such as scalability, flexibility, and texture update in the course of a forming simulation. Published overviews on this topic can be found in Raabe, Klose, Engl, Imlau, F. Friedel, Roters: Advanced Engineering Materials 4 (2002) 169-180, Raabe, Zhao, Mao: Acta Materialia 50 (2002) 4379–4394, and Raabe, Roters: International Journal of Plasticity 20 (2004) p. 339-36