Stress based failure criterion for formability characterisation of metastable steels

To better predict material forming behaviour, especially sheet metal forming, an approach has been developed to characterise the material, which is independent of the forming procedure and deformation history. This approach is based on a stress based failure criterion: the forming limit stress diagram (FLSD). It also takes into account strain hardening and the anisotropy of the material and thus, this criterion is more resistant to changes in the strain path during the forming process. To determine failure critical stress curves, the Nakazima-strip-test is simulated using FEM. At the point in time, when the strains from the crack-critical elements in the simulation reach the forming limit curve (FLD criterion), the maximum stresses on these elements are evaluated. This procedure is validated with a two-step forming test and a hole expansion test (HET). Both experiments offer forming processes with changing deformation paths in negative and positive range of the strain diagram. The numerical simulations of the HET and two-step forming test are carried out in order to evaluate the applicability of the FLD and FLSD. The results show that the stress-based criterion (FLSD) better characterises the formability than the strain-based FLD-failure criterion.