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In vehicle crash events there is the potential for fracture to occur at the processed edges of structural components. The ability to avoid these types of fractures is desired in order to minimize intrusion and optimize energy absorption. However, the prediction of edge cracking is complicated by the fact that conventional tensile testing can provide insufficient data in regards to the local fracture behavior of advanced high strength steels. Fracture prediction is also made difficult because there can be inadequate data on how the cutting processes used for hole piercing and blanking affect the edge condition. In order to address these challenges, research was undertaken to analyze edge fracture in simple test pieces configured with side notches and center holes. Test specimens were made from a number of advanced high strength steels including 590R (C-Mn), 780T (TRIP), 980Y (dual phase) and hot stamp 1500 (martensitic). Edges were prepared by three different cutting processes: shearing, laser, and water jet ablation. The specimens were pulled to failure and local fracture strains were measured by digital image correlation. Component level tests were also done on simple hat sections that featured a notch cut into the flange and side wall by either water jet or punching. These hat sections were made from select steel grades and were deformed in a three-point bend crush mode to initiate failure at the notch. The results indicate that edge fracture in high strength steels is highly influenced by both edge condition and specimen geometry. In addition, it was concluded that certain material grades can be more notch or punch sensitive than others depending on their metallurgical structure.A prime example of this type of complication is the possibility for AHSS components to fracture at cut edges during crash deformation. This susceptibility can evolve from the limited ductility of the steel, the pre-existing damage from the edge cutting process, and the geometrical stress effect of the feature itself. The potential for edge fracture can be a consideration when developing body structures. If the cracking is not accounted for in the overall design, the load paths through the vehicle frame can be misdirected and the resultant intrusion levels can exceed target levels. It is beneficial if fracture prone areas can be identified in early design layouts through FEM modeling, as opposed to expensive and time-consuming crash tests. However, establishing the proper material data and analytic methods needed for edge fracture prediction has been one of the challenges in the advent of advanced high strength steel. The basic aim of this research was to broaden the understanding of edge cracking by analyzing how different material compositions, geometrical stress states, and edge process conditions affect fracture limits. The investigation promotes the use of new optical measurement techniques, such as digital image correlation (DIC), as an innovative method to acquire strain data in a localized area.

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Development of an Empirical Model to Characterize Fracture Behavior During Forming of Advanced High Strength Steels Under Bending Dominated ConditionsHigher 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 RIt 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 RIt ratio of the punch are presented. Consistent with other studies reported in literature, the overall formability was found to increase with increasing RIt ratio reaching saturation for higher RIt ratios, ¡n 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 RIt ratio and the loading mode. Examination of fracture surfaces fi-om the different tests using an SEM reveals that fracture initiation occurs primarily at the ferritelmartensite interphase boundary. To analyze the local loading conditions leading to fracture, 2D flnite element analyses (FEA) of the different bending tests using ABAQUS standard were conducted. Results of the EEA 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.