Damage dependent material properties in a Finite Element Simulation of a hybrid forward extrusion process

The occurrence of damage and fracture limits modern forming processes. An example for such a process is the full forward extrusion of steel, where accumulating damage results in characteristic chevron cracks. To prevent chevron cracks, in conventional extrusion, the billet is pre-heated before forming. As an alternative development in a hybrid approach the workpiece is heated resistively, by applying an electric current during extrusion. This results in an inhomogeneous temperature distribution in the workpiece, influenced by the local resistivity of the material, which in turn is dependent both on temperature and damage. While the press force is measured and a cut through the final part gives information on the occurrence of failure, it is not possible to measure local temperatures within the workpiece during forming. Since experimental results show that press force does not significantly change with resi stive heating, numerical methods have to be used to investigate the influence of property changes of the material on the local temperature in the workpiece. In this work a 3D Finite Element Method model using Abaqus/Standard is presented that is capable of simulating the highly nonlinear hybrid forward extrusion process (thermomechanical and electric field) incorporating a remeshing routine. Subsequently, this model is extended to couple a strain based damage with electrical resistivity. Validation is performed by comparisons with experimental results for the conventional cold, pre-heated and hybrid approach. Notably, the results show a significant influence on the temperature distribution in the workpiece for simulations with strain coupling compared to those with unaffected material properties. This in conclusion shows how the presented method of coupling material properties to strain values enables new possibilities to design and optimize complex forming processes.