Abstract
To clarify the effect of hardness on fatigue characteristics of structural steel in an Extremely High Cycle Fatigue(EHCF) regime, rotational bending fatigue tests were carried out for two types of specimen with different hardness of microstructure. Fracture surfaces were observed using a Scanning Electron Microscope to examine the fracture mechanisms. Two types of fracture modes, surface fracture and internal fracture, were observed for high-strength steel. In middle strength steel, however, surface fracture mode alone was observed instead of fatigue crack started at nonmetallic inclusion. Computer simulation was also performed to discuss the fatigue properties in EHCF regime. The proposed model was possible to predict fatigue properties of both high-and middle-strength steel and showed that the possibility of internal failure (fish-eye failure) in EHCF of middle-strength steel.
