Microscopic Damage Process in Notched Specimen under Creep-Fatigue Condition Using High-Temperature Fatigue Testing Machine Combined with SEM

摘要

In order to evaluate the crack initiation life precisely in the stress concentration portion of high-temperature components in power plants, it is important to comprehend the microscopic damage process under localized stress distribution. In this study, fatigue and creep-fatigue tests with tensile hold time were conducted on SUS304 stainless steel specimens, having V shaped double-edge notch, by using a high-temperature fatigue testing machine combined with a scanning electron microscope (SEM). As the results, it was found that the microscopic damage under the fatigue condition progressed by the initiation and propagation of a single main crack due to the accumulation of slip bands caused by strain concentration in the limited region around the notch root. On the other hand, under the creep-fatigue condition, many microcracks initiated in grain boundaries around the root corresponding to the microstrain distribution with a smooth gradient from the notch root. The main crack at the root was formed by coalescence of those microcracks and propagated by coalesing with the microcracks distributing ahead of the microcrack.