Takayuki Sakakibara

Interior-Induced Fracture Mechanism of High Cleanliness Spring Steel (JIS SWOSC-V) in Very High Cycle Regime

In order to investigate the interior-induced fatigue crack propagation behavior of high cleanliness valve spring steel (JIS SWOSC-V), rotating bending fatigue tests were performed for various kinds of specimens with different hardness or surface finishings. The harder specimen with higher compressive residual stress showed longer fatigue life. The electrochemical polished specimen pre-treated with shot peening showed almost same fatigue life as the shot-peened specimen in spite of the difference in surface roughness. After fatigue tests, fracture surfaces were observed using a scanning electron microscope (SEM) to evaluate the fatigue fracture mechanism. Most specimens failed in surface-induced fracture mode due to high cleanliness; however, some specimens failed in interior-induced fracture mode in the very high cycle regime. Although non-metallic inclusions were not observed at interior fatigue crack initiation sites, 2 types of significant microstructures (with smooth surface or granular surface) were observed. EBSD analysis, profile analysis and computational simulation using a fracture surface topographic analysis (FRASTA) method were performed to investigate the mechanism of the interior-induced fatigue fracture caused by the microstructure at defect without any inclusion.