Clarification of creep-fatigue damage mechanism of high-temperature structural materials. Pt. 5. Distribution properties and mechanical parameter of cavity nucleation

Creep-fatigue damage at high-temperature on SUS304 austenitic stainless steel is characterized by nucleation and growth of creep cavities on grain boundaries. These microscopic damages progress during start-up, steady state and shut-down operation in the high-temperature power plants. In this study, in order to clarify a mechanical factor on cavity nucleation under the creep-fatigue condition, creep-fatigue tests with CP waveform were conducted on SUS 304 stainless steel and the failure specimens were examined by SEM. As a result, it was found that the cavitated grain boundaries existed almost within 10deg from a plane normal to a stress axis regardless of tensile strain-rate and the volumetric density of damaged grain boundary per one cycle was proportional to time-dependent strain. A rate equation, in which rate of the volumetric density of damaged grain boundary is expressed as function of time-dependent strain rate, was proposed. The volumetric density of damaged grain boundary under strain-hold waveform condition could be predicted by the equation with good accuracy. (author)