Fatigue Crack Growth for Typical CV Siderail Hole-making Processes Applied to Heat-treated Steel (MET1123) and Ultra High Strength Low Alloy Steel (120XF)

摘要

In order to provide the best siderail chassis value to the global commercial vehicle (CV) market two key aspects were investigated in this study: 1. A mechanical characterization for the industry standard heat-treated steel (MET1123) and a comparable ultra-high strength steel (120XF). 2. The impact common hole making processes have on commercial vehicle siderails - specifically crack growth. For acceptance in the global CV industry it was key to collaborate with recognized research and testing facilities for the respective areas of evaluation. This study was divided into 2 phases: Material evaluation and fatigue crack growth. The material evaluation phase determined monotonic properties such as strength, impact and % elongation. As part of the material evaluation the low cycle fatigue (LCF) properties were also evaluated. The heat-treated steel (MET1123) has an industry strength requirement for yield and ultimate of 120 ksi (838 mpa) and 130 ksi (910 mpa) respectively and this was met. The ultra high strength steel (120XF) is a new grade, currently in the development phase and these respective properties were slightly less than the MET1123, although very close to the industry standard. The LCF results of the MET1123 and 120XF were comparable. The next phase of the study was the fatigue crack growth. The holes were cut with: a hydraulic Punch press, CO_2 laser, numerically-controlled (NC) drill and O_2 Plasma torch. The coupons used were extracted from actual siderails and the holes were processed in the same fashion as the holes cut into a typical production siderail. The specimens were configured into SAE keyhole coupons. The threshold stress intensity, K_(th), and crack growth rates were most favorable for the punch press. The HT (MET1123) steel outperformed the HSLA (120XF) for threshold, crack growth rates and fracture toughness for all processes.