Characterization of Edge Fracture in Various Types of Advanced High Strength Steel

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 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.