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

Abstract

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.

Overview and comparison of various test methods to determine formability of a sheet metal cut‐edge and approaches to the test results application in forming analysis

“…An optical online measurement system was used with a gauge length of 2 mm for the determination of the fracture strain as well as a punch force measurement [59]. The curve shape of the punch force displacement curve can be used to differentiate between the edge-crack sensitivities.…”

“…A symmetrical specimen with a width of 50 mm and semicircle notch with a radius of 10 mm was proposed by [59]. Here edges made by water jet, laser and punching were compared.…”

“…A hole with a diameter of 10 mm in a rectangular specimen with a length of 150 mm and a width of 20 mm (cutting clearance 7, 13 and 30 %) was pierced. Specimen with a width of 50 mm and a hole of 10 mm diameter was produced [59]. The holes were made by water jet cutting, Laser cutting and punching.…”

“…Another method to adjust a tensile test specimen to achieve an edge-crack is to punch a hole in the middle of the specimen [59,61].…”

Advanced Characterization of HAZ Softening of AHSS for Crash Modeling

“…It has been shown that edge quality can affect fracture strain, and materials with punched edge exhibit failure initiation from edge, showing lower fracture strain than samples with higher quality edge preparation. 14) Figures 9(b) and 9(d) present the DIC images of the weld samples containing a HAZ just before failure initiation. Failure initiated in the HAZ and center of sample regardless of edge quality, and propagated through HAZ and base metal until full material separation was achieved.…”