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.