2016 年 2016 巻 61 号 p. 7-12
Fatigue cracks, leading to catastrophic fracture of metal structure, generally initiate by the cyclic stress even under yield stress. At the first stage of fatigue process, movement and increase of lattice defects (dislocations), namely microscopic sliding takes place, and crack initiation follows thereafter. The positron annihilation spectroscopy has been widely used for probing open volume lattice defects in various materials. In this study, rotating bending fatigue test of Type 316 stainless steel was conducted, and fatigue damage accumulation has been investigated using S parameters defined in positron annihilation spectroscopy. S-parameter increased with increasing number of loading cycles due to the fatigue damage accumulation, namely increase of dislocation density. When fatigue crack initiated, S-parameter measured at the crack initiation site became higher than that at the location without fatigue crack. The plastic deformation zones at the fatigue crack tips had extremely higher dislocation densities. Higher S parameter at the crack initiation site was attributed to those plastic zones at the fatigue crack tips.