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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.

Tubular hydroforming provides a number of advantages over conventional stamping processes, including reduction in part counts and weight reduction, improved strength and stiffness of structural components, lower tooling costs, and higher dimensional accuracy. In order to provide hydroforming guidelines for product designers and process engineers, and to obtain fundamental understandings of such forming process, we begin a series of study on failure analysis of tubular hydroforming under internal pressure and end feeding. The focus of this paper is placed on the condition of the onset of bursting. Bursting is an instability phenomenon where the tube can't sustain any more internal pressure. Splitting usually follows due to extreme deformations in the bursting area. Onset of bursting depends on process parameters such as pressure and end feeding, as well as on material properties. A mathematical analysis is conducted in this paper for the conditions of onset of bursting.