Assessment of forming limit stress curves as failure criterion for non-proportional forming processes

The forming limit stress curve (FLSC) is often recommended as failure criterion for the virtual tryout of forming processes which include non-proportional loading. However, parameters influencing position and shape of the forming limit stress curve are not fully known. Up to today it has not been proved if the forming limit curve is strain path-independent, or at least approximately path-independent. In this study the influence of the parameters yield criterion, flow curve extrapolation and level of pre-stretching on the applicability of the FLSC as failure criterion are assessed. The work is performed using the aluminum alloy AA6014 based on forming limit curves for proportional and non-proportional loading published in Werber et al. Key Eng Mater 502–506:71–76, (2012). FLSCs are generated for yield criteria according to von Mises and Hill’48, for flow curve extrapolations according to Ghosh and Hockett-Sherby as well as for an experimentally measured flow curve. In order to be able to assess the influence of the described parameters on the failure criterion based on the FLSC the application of a mean forming limit stress curve (MFLSC) is used. This method is based on the assumption that all FLSCs gained for proportional as well as non-proportional loading map to one single curve. The influence of yield criterion and flow curve approximation on the FLSCs is addressed; the strain path dependency of FLSCs is proved and the forming limit curves for non-linear loading calculated with an assumed mean forming limit stress curve are compared to the experimentally gained forming limit curves.