Relationship between Microcrack Initiation and Propagation and Microstrain Distribution under Creep and Creep-Fatigue Condition.

Abstract

Structural components operating at high temperatures in thermal and nuclear power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to comprehend detailed microscopic damage evolution under creep-fatigue condition. We had observed continuously the growth and coalescence of cavity and microcrack on specimen surface during the test. In this study, creep and creep-fatigue tests with tensile hold-time were conducted on SUS304 stainless steel, with large grain size producted by a particular heat treatment, by using a high-temperature fatigue machine combined with a scanning electron microscope. Microstrain distribution in the grains on the specimen surface during the test was measured by analyzing the distortion of microgrids. The grid pattern with a pitch of about 10 μm was induced by a diamond stylus on the specimen. As result, it was found that the grain deformed uniformly in a direction of stress axis under creep condition. The deformation progressed with inhomogeneous microstrain distribution under creep-fatigue condition. Initiation of microcrack under creep-fatigue condition was mainly related to strain concentration in the grain near grain boundaries. The microcrack under creep condition propagated after reaching a critical crack opening displacement. The crack opening displacement under creep-fatigue condition was smaller than that under the creep condition.