Numerical and experimental investigations of forming limit diagrams in metal sheets

A strain forming limit criterion is widely used throughout the sheet metal forming industry to gauge the stability of the deformed material with respect to the development of a localized neck prior to fracture. This criterion is strictly valid only when the strain path is linear throughout the deformation process. There is significant data that shows a strong and complex dependence of the limit criterion on the strain path. But unfortunately, the strain path is never linear in secondary forming processes. Furthermore, the path is often to be nonlinear in localized critical areas in the first draw die. Therefore, the conventional practice of using a path independent strain forming diagram criterion often leads to erroneous assessments of forming severity process. In this study, the numerical results are based on MARC K7.1-3D finite element analysis (FEA) software using rigid-plastic flow method. This general purpose of FEA code is using both the implicit and explicit procedure. The model is employed to predict the stress–strain forming limit diagrams and thickness distribution of a deep drawing quality steel (DDQS) and aluminum alloy (AL) sheet. The predicted results agree well with those determined experimentally.