Evaluation of Micro Crack Initiation and Propagation Behavior in Ultra High Strength Steels with Artificial Defects

Abstract

Ultra-high tensile steels are increasingly utilized to produce thinner automotive bodies to reduce vehicle weight. However, its application in suspension systems has been delayed due to concerns regarding reduced fatigue strength at stress concentration sites in complex shapes. Additionally, recent reports have highlighted cases of ultra-high tensile steel cracking under compressive loads during press forming. It is critical to address these cracking issues to ensure the reliable use of ultra-high tensile steel in suspension components. Prior studies have attributed such cracking to surface depressions due to local buckling, though the influence of depression geometry on crack formation remains unclear. This study conducted compression tests on ultra-high tensile steel specimens with artificially induced defects of varying shapes and sizes, followed by microstructural evaluations of the areas surrounding the cracks. The relationship between surface defect geometry and crack initiation mechanisms was investigated by analyzing the microstructure before and after testing. The results demonstrated that the crack length at the bottom of the artificial defect in the ultra-high tensile steel was shorter than in the high tensile steel. Furthermore, the Electron Backscatter Diffraction (EBSD) analysis revealed that the grain refinement at the bottom of the artificial defect was refined in an ultra-high-strength steel sheet. These results suggest that grain refinement increases the crack propagation resistance.