2. Grain Boundary Cavity and Damage Evaluation in Creep-Fatigue

This chapter presents analytical and experimental results on representative creep damage parameters for grain boundary cavities that initiate under creep-fatigue conditions. First, equations relating creep damage parameters to cavity and grain boundary distributions were derived using a probabilistic model. Then, the size and shape distribution of the grain boundary cavities that initiated under creep-fatigue conditions in SUS304, which is a typical polycrystalline heat-resistant steel, and their change from the initial to the middle of the life of the steel were shown. Based on the observations of cavities, four kinds of creep damage parameters (area fraction of cavities on cross section, fraction of cavities on grain boundary lines, areal cavity density, and A-parameter) were evaluated. For all damage parameters, the damage value increases with increasing life fraction. However, under creep-dominated conditions, where only the grain boundary is preferentially damaged, only the fraction of cavities on grain boundary lines has a clear physical meaning as creep damage.