Influence of limit stress states and yield criteria on the prediction of forming limit strains in sheet metals

Several researchers have proposed analytical methods for predicting the forming limit curve (FLC), which has been successfully used as a diagnostic tool in sheetmetal forming. However, these approaches lack ease of adaptability to various situations and also involve considerable complexity. Sing and Rao proposed a new FLC modeling approach based on limit stress states derived from yield criterion and material properties from a simple tensile test. The first aspect of this study addresses the influence of the shape of the forming limit stress curve (FLSC) upon the FLC. The FLC modeled from a singly linear FLSC exhibits good agreement with the experimental curve, unlike those modeled from an elliptical or a piecewise linear FLSC. It is, thus, established that a linearized limit stress locus describes adequately the actual localized neck condition for the materials chosen in this study. Second, the study focuses on the suitability of the different cases of Hill's yield criterion for satisfactory prediction of FLCs. The FLCs, predicted using different cases of Hill's criterion are compared with experimental FLCs in the case of steel and copper. Different cases of Hill's criterion provide a wider choice for FLC modeling for different classes of materials. The sensitivity of Hill's stress exponent is also thoroughly explored for achieving a close correspondence between the predicted and experimental FLCs.