Development of Shear Fracture Criterion for Dual-Phase Steel Stamping

Forming Limit Diagrams (FLD) have been widely and successfully used in sheet metal stamping as a failure criterion to detect localized necking, which is the most common failure mechanism for conventional steels during forming. However, recent experience from stamping Dual-Phase steels found that, under certain circumstances such as stretching-bend over a small die radius, the sheet metal fails earlier than that predicted by the FLD based on the initiation of a localized neck. It appears that a different failure mechanism and mode are in effect, commonly referred to as “shear fracture” in the sheet metal stamping community. In this paper, experimental and numerical analysis is used to investigate the shear fracture mechanism. Numerical models are established for a stretch-bend test on DP780 steel with a wide range of bend radii for various failure modes. The occurrences of shear fracture are identified by correlating numerical simulation results with test data. The study suggest that the shear fracture is due to the failure of martensitic grains at micro-level when certain criteria are reached, and the macroscopic failure mode during forming operations is viewed as the competition between localized necking and the shear fracture, whichever criterion is satisfied first. Same studies are also repeated for a HSLA grade steel, which consists entirely of ferritic phases, to emphasize the critical role played by martensitic grains in the shear fracture development. The relevance of the shear fracture criterion to industrial stamping is discussed.