Verification of the damage model of effective stresses in cold and warm forging operations by experimental testing and FE simulations

In order to predict ductile material failure during bulk metal forming processes, several different damage models were modified and integrated into the FE Software MSC.SuperForm 2002. The so-called macromechanical strain-dependent damage criteria failed to predict ductile damage as they are not able to give a critical reference value for an occurring crack that could be transferred from experiments to complex simulations. The model of effective stresses (MES) by Kachanov and Lemaitre showed good performance for this task in the FE evaluation of experiments with tensile and collar compression specimens. The damage location and the fracture propagation could be simulated with high coincidence. The appearance of a first crack in the experiments was measured by passive ultrasonic testing with subsequent filtering. From the visible necking and the drop of the stress–strain curve in the tensile test the damage related signals were extracted and taken as a reference for the collar compression test. At elevated temperatures a high-speed CCD-camera allowed for the identification of cracks.