Multi-level Modelling of Plastic Anisotropy of Aluminium Alloys Using Crystal Plasticity Models and Advanced Yield Functions

This thesis aims to accurately describe the plastic anisotropy of aluminium alloys through a hierarchical multi-level method. Robust and efficient integration schemes have been proposed for the explicit numerical integration of rate-dependent crystal plasticity models. On the mesoscale, the plastic anisotropy is modelled by crystal plasticity models considering a representative volume element (RVE). The RVE consists of a number of single grains and inherits the microstructural information of the polycrystalline material, e.g. crystallographic texture, grain size and shape and grain boundary misorientation. Five crystal plasticity models have been used in this work, namely the full-constraint (FC) Taylor model, the Alamel model, the Alamel model with so-called type III relaxation (Alamel Type III), the visco-plastic self-consistent (VPSC) model and the crystal plasticity finite element method (CPFEM). The accuracy and applicability of these crystal plasticity models when predicting the plasticity anisotropy have been investigated for three different aluminium alloys. On the continuum scale, the yield surface of the material is represented by advanced yield functions. Two yield functions have been employed and investigated for this purpose, namely the Yld2004-18p yield function and the Facet yield function. The yield function is a key component of an anisotropic model in a finite element method (FEM) code, in addition to the flow rule and work hardening law, for simulating plastic deformations. Advanced yield functions, like Yld2004-18p, are conventionally identified by experiments, e.g. uniaxial tensile tests, biaxial tension/compression tests and shear tests. However, the number of available experimental tests is limited for sheet metals and most of the stress space is not covered by the experiments. The multi-level modelling was made through identifying the parameters of the advanced yield functions partially or fully by stress points at yielding provided by crystal plasticity calculations. The accuracy and applicability of this multi-level modelling scheme were evaluated for describing the plastic anisotropy of three aluminium alloy sheets in this thesis.