Free-surface ductility in bulk forming processes

Free-surface ductility for bulk forming processes is investigated by finite element (FE) simulations of micromechanical representative volume elements (RVEs) subjected to typical deformation histories. Evidence is provided that failure in bulk forming might be explained by stability considerations rather than by void growth criteria. A readily applicable condition for stability is derived; the strain path curvature is shown to delay significantly the onset of instability. The criterion is used to obtain bulk forming limit diagrams both in principal strain and in principal stress space.