Effect of necking types of sheet metal on the left-hand side of forming limit diagram

As boron-steel sheets (22MnB5) deform at elevated temperatures on the left-hand side (LHS) of forming limit diagram (FLD), localized necking band of the sheets is always perpendicular to the major strain direction. This necking type is not in accordance with the hypothesis of Hill's zero extension. It is called 0-angle type in the present work. According to Storen–Rice's vertex theory, forming limits prediction models are deduced under the hypotheses of Hill's zero extension and 0-angle. Both models can be applied to rate-dependent as well as rate-independent materials. FLD tests of boron-steel sheets were performed at elevated temperatures. Test results indicate that the forming limit prediction under 0-angle hypothesis is consistent with the experimental data. The effects of parameters in the models on the LHS of FLD were evaluated, and the evolution of the difference between two models was discussed.