Simulation of shear fracture in sheet metal forming of thick plates under triaxial stress states

In industrial sheet forming of high strength steel sheets, shear fracture has been reported to occur in narrow die shoulders where the sheet is subject to excessive bending and stretching. For high strength steel plates, which generally show low ductility, the die curvature becomes relatively large for thin sheets, increasing the risk of fracture. Under these circumstances, failure is caused by the shear stresses induced by the high through-thickness stress gradient. Since shear fracture cannot be predicted by the strain-based forming limit diagram, efforts have been made to overcome this limitation and eventually led to the triaxiality failure diagram. In this study, we investigate shear fracture occurring in the forming of thick high strength steel sheets by means of finite element analysis. To do so, we implement the TFD in the commercial finite element analysis software AFDEX through a subroutine. We compare and analyze the predicted damage values obtained for diverse damage models with those obtained via the subroutine so to gain better insight in the theory and background of shear fracture. Finally, the influence of the mesh quality on the predicted stress triaxiality-dependent damage parameter is investigated.