In order to investigate the very high cycle fatigue properties of high cleanliness valve spring steel, rotating bending fatigue tests were performed by means of dual-spindle rotating bending fatigue testing machine. Fracture surfaces of failed specimens were observed by a scanning electron microscope (SEM) and fracture mechanisms of this material were discussed from view points of fractography and fracture mechanics. As results, it was found that the interior-initiated fracture would occur even for specimens without non-metallic inclusions inside the material. This fact suggests that the interior clack takes place at a certain kind of microstructural defect.
