Towards the effective behaviour of polycrystalline microstructures at finite strains

It is well known that metals behave anisotropically on their microstructure due to their crystalline nature. FE-simulations in the metal forming field however sometimes lack the right macroscopic anisotropies as their type can be unspecific. In order to find a suitable effective elastoplastic material model, a finite crystal plasticity model is used to model the behaviour of polycrystalline materials in representative volume elements (RVEs) representing the microstructure, taking into account the plastic anisotropy due to dislocations occurring within considered slip systems. A multiplicative decomposition of the deformation gradient into elastic and plastic parts is performed, as well as the split of the elastic free energy into volumetric and deviatoric parts resulting in a compact expression of the resolved Schmid stress depending on the slip system vectors. In order to preserve the plastic incompressibility condition, the elastic deformation gradient is updated via an exponential map scheme. To further circumvent singularities stemming from the linear dependency of the slip system vectors, a viscoplastic power-law is introduced providing the evolution of the plastic slips and slip resistances. The model is validated with experimental microstructural data under deformation. Through homogenisation and optimisation techniques, effective stress-strain curves are determined and can be compared to results from real manufacturing and fabrication processes leading to an effective elastoplastic material model which is suitable for metal forming processes at finite strains.