高温高サイクル疲労におけるfish-eye破壊

抄録

Fish-eye fracture is observed on the smooth specimens of carbon and low alloy steels fatigued under rotating bending at temperatures from 200 to 500°C. The mechanism of the fish-eye fracture and the effect of oxidation on high-cycle fatigue behavior were discussed here.
In the relationship between σ_a/σ_B and N_f, where σ_a is the stress amplitude, σ_B the tensile strength at each test temperature and N_f the number of cycles to failure, stepwise curves were obtained at 200 to 300°C and curves with an inflection point at above 400°C. At lower stress levels than the stress level of the stepwise or the inflection point, the fish-eye was observed at the fracture surface. At above 400°C, a region covered with thick oxide was observed independently of stress levels, shape of which looks like the cross section of a convex lens.
A surface crack is not easy to initiate at elevated temperatures under low stress levels, as oxide at the specimen surface prevents dislocations from slipping off from the surface. During this period, the fish-eye fracture occurs from an inclusion in the interior of specimen. The stepwise curve comes from the difference in crack initiation site. At 400 and 500°C, the crack propagation process at the early stage is prolonged by oxide formed at the crack surface.