Strain- and stress-based forming limit curves of DP440 steel sheet with application to the cross-die forming test

Abstract

In this study, experimental and numerical forming limit curves (FLC) and forming limit stress curves (FLSC) for DP440 high-strength steel (HSS) sheets were developed and analysed in depth. Initially, an FLC was experimentally determined by means of the Nakajima stretch-forming test. Subsequently, two more FLCs were numerically derived through the Marciniak–Kuczinsky (M–K) model. During the numerical processes, Hill’s 48 was considered the anisotropic yield criterion for one and Yld2000-2d for the other. Both, however, analogously took the Swift model as their hardening law. In addition, all of those experimental and M-K based FLCs were transformed into the corresponding FLSCs, yet again, with the help of both Hill’s 48 and Yld2000-2d. It was found that the resulting FLSCs were significantly affected by the selected choice of the anisotropic yield model. The M-K-Yld2000-2d-based FLC and FLSC agreed clearly well with the experimental FLC and experimental-Yld2000-2d FLSC, respectively, than the M-K-Hill’48- based FLC and FLSC. Finally, a cross-die forming test was conducted in order to prove those FLCs’ and FLSCs’ practical applicability. It could apparently be concluded that the FLSCs could more precisely define the formability behaviour of DP440 than the FLCs. Furthermore, Yld2000-2d represented the anisotropic plastic deformation better than Hill’48, regarding the strain distributed on the formed part from the cross-die forming test.