Modelling of cavity nucleation under creep-fatigue interaction

A cavity nucleation model under creep-fatigue interaction, abbreviated as CH model, has been established. The key aspect of the CH model is accounting for the creep deformation through a self-consistent scheme. This allows the combined effect of grain boundary sliding (GBS) and crystal deformation on cavity nucleation to be assessed in Type 316 stainless steel over the temperature range of 500–600 °C. The CH model predicts that the local normal stress asymptotically approaches a non-zero saturation value over a long period of stress hold. This aligns with the continuous nucleation process under creep. The CH model has been used to calculate the cavity nucleation rates under various load-waveform scenarios, which include the stress and time applied during the transient and steady loadings. As a result, the load waveform characteristics that can maximise the cavity nucleation are identified. First, the pre-compressive hold time needs to be long, whereas the load reversal time needs to be short. Second, the tensile hold time can be shortened to some extent if time efficiency is the primary concern. Third, all the above-mentioned time-related parameters have their optimum values depending on the temperature and creep rate. Fourth, the unbalanced stress hold in favour of the tension enhances the nucleation rate, and the stress range is the controlling factor. In summary, the CH model provides an important guide to the design of creep-fatigue testing programme with the desire to promote creep cavitation damage. This mechanistic-based model provides underlying interpretations to the complexity of creep-fatigue interaction.