Predicting breakage on a die radius with a straight bend axis during sheet forming

As the use of high strength sheet steels for automotive applications increases, there have been increasing occurrences of breakage on die radii at strains that are less than predicted by a conventional forming limit curve. To develop a better understanding of the problem, data from an angular stretch bend test were analyzed to determine the applied stress on a bend that caused breakage for a range of die radius, sheet thickness, and steel grade. The forming limit stress was calculated using a modified version of Stoughtonʹs stress-based forming limit curve. The ratio of the applied failure stress to the forming limit stress is defined as the failure stress ratio. Regression analysis was used to determine the effect of the sheet thickness to die radius ratio (T/R) on the failure stress ratio for the different steel grades ranging from DDQ+ to DP980. The results were grouped due to commonality among various steel grades. Three predictive equations for (1) DP800, (2) DP980, and (3) all the other steel grades describe the behavior for all the steels in the study. It is found that breakage on a die radius for high strength steels such as DP600, DP800, and DP980 is typically a result of low values of the forming limit stress.