It has been generally understood that the grain boundary cavitation is one of the detremental processes for the degradation of the materials to reduce the creep-fatigue life at high temperature. In 1985, Hong and Nam proposed a model for life prediction under creep-fatigue condition in terms of the cavity nucleation and growth. In that model, the cavity nucleation factor, P, was introduced to correlate between the number of cavities and the plastic strain range from which athermal vacancies are generated, and was considered as a material's constant which was independent on the experimental conditions.
However, in this study, it is found that the cavity nucleation factor is a function of the plastic strain range but does not depend on the testing temperature at the temperature range near $0.5T_m$. Considering this dependency, a new cavity nucleation factor, P', is introduced as a real material's constant. Using this new cavity nucleation factor(P'), a modified equation for the life prediction is proposed, and is shown to be in good agreement between the predicted lives and the experimental ones for various creep-fatigue test results of austenitic stainless steels.
And the interesting approach has been made to find the physical meaning of the new cavity nucleation factor(P'). Using the creep-fatigue test results of three heats which have the different linear density of grain boundary precipitates, it is attempted to find the relationship between the P' and the distribution of grain boundary precipitates. According to this study, it is suggested that the new cavity nucleation factor, which is regarded as a material constant, is found to be strongly related to the density of the grain boundary precipitates to give the linear relationship between them.
Additionally, from the Hong-Nam's model for the creep-fatigue life prediction, a correlation between creep-fatigue life and tensile hold time for austenitic stainless steels is derived to be expressed by the power-law dependence where the value of exponent is found to be about -0.4 in this investigation.
A damage function based on the model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed for the first time. The creep-fatigue data from the present and other investigations are used to check the validity of the proposed damage function and it is shown that they satisfy the normalized Coffin-Manson plots at all levels of tensile hold time and temperature under strain controlled creep-fatigue tests. This damage function is composed of the terms related to the cavitational damage in the life prediction equation and is generally applicable to the materials in which failure is controlled by the grain boundary cavitational damage.
