Analysis of the Pattern of Fracture Surface and the Life Evaluation in Creep-Fatigue Interaction under Ultrahigh-vacuum Environment.

Following the creep-fatigue tests with Modified 9Cr-1Mo steel in a high-vacuum environment, creep-fatigue fracture surfaces were investigated by SEM. In the cases where the creep-fatigue life is time-or rate-independent, the fracture mode is the transgranular type, but some differences were observed in the number of dimples, which depends on the strain wave form. When time-or rate-dependent life reduction takes place, the primary fracture mode is found to be of the intergranular type which is observed on the inner side of the specimen, but becomes transgranular on the outer side. The area fractured in the intergranular manner is correlated to the time-dependent damage variable calculated by the creep-fatigue life evaluation model based on the overstress.