Fig. 11: Model predicted cavity nucleation field in stress amplitude and frequency space for Type 316 stainless steel at 550 and 600 ˚C.
It is also interesting to note that the optimum frequency and the whole frequency range are positively correlated with temperature, Fig. 11. This can be explained by the time constant τ, Eq.. τ is inversely proportional to temperature, given that k e is temperature independent but both the η b and are inversely proportional to temperature. As commented in18, a lower τ means that a higher stress variation rate would be required to achieve the same level ofσ n leading to cavitation. Therefore, the nucleation frequency increases with the temperature (i.e. decreasing τ).
Taplin, Tang 46 and Tang, Taplin 47developed a mechanism map for cavity nucleation in the stress amplitude and frequency space. For GB sliding induced cavitation, their predicted nucleation frequency for Type 304 stainless steel was in the range from 3×10-2 to 1 Hz under stress amplitude of 180 MPa, and 3×10-2 to 5×10-1 Hz under stress amplitude of 140 MPa, respectively. This agrees with our model prediction in a sense that a higher stress amplitude leads to a wider range of nucleation frequencies. Also, the frequency ranges obtained at 650 ˚C in their predictions are generally higher than our model predictions at 600 ˚C (3×10-3 to 5×10-2 Hz), Fig. 11. Thus, our model prediction successfully captures the positive correlation between the nucleation cycle frequency and temperature.

4.3 Unequal ramp rate

In the past two decades, high-temperature fatigue testing with unequal ramp rate has emerged as alternative method to study cavitation damage during creep-fatigue interaction, as demonstrated in5. Yamaguchi and Kanazawa 48 studied effects of unequal ramp rate on the strain-controlled high-temperature fatigue in Type 316 stainless steel at 600 ˚C. It was found that the test with a slow-fast cycling waveform exhibited an intergranular fracture mode, and the fatigue life was reduced significantly due to creep cavitation. By comparison, the test with a fast-slow waveform fractured in a transgranular manner, and limited fatigue life reduction was found.