Development of an Empirical Model to Characterize Fracture Behavior During Forming of Advanced High Strength Steels Under Bending Dominated Conditions

Higher strength advanced high-strength steels (AHSS) such as DP780 and DP980 are more susceptible to fractures at bend radii during press stampings in comparison with more ductile low carbon sheet steels used by the automotive industry. Most research work to develop predictive guidelines for preventing failures at bend radii have centered on determining critical R/t ratios to avoid failures caused by bending. In this paper, results from bending tests with and without applied tension conducted on a number of AHSS steel lots to generate different conditions for fracture are presented. For bending tests with applied tension, measures of overall formability as a function of R/t ratio of the punch are presented. Consistent with other studies reported in literature, the overall formability was found to increase with increasing R/t ratio reaching saturation for higher R/t ratios. In addition, local formability was determined for all the bending tests by measuring the thickness strains at failure using an optical microscope. It was observed that the thickness strain at failure was dependent on the R/t ratio and the loading mode. Examination of fracture surfaces from the different tests using an SEM reveals that fracture initiation occurs primarily at the ferrite/martensite interphase boundary. To analyze the local loading conditions leading to fracture, 2D finite element analyses (FEA) of the different bending tests using ABAQUS standard were conducted. Results of the FEA were analyzed, and a parameter describing bending dominance in a stamping process was isolated. An empirical fracture criterion relating the thickness strain at fracture as a function of this parameter was developed. Implications of the generated results and their applications for part design and evaluation of stamping feasibility are also discussed.