5.
Conclusions
A mechanistic based cavitation model that considers nucleation,
early-stage growth and sintering under multi-cycle creep-fatigue
interaction has been developed to provide predictions of both the number
density of cavities and the area fraction of cavitated grain boundaries
in Type 316 stainless steel. The model replies on the local normal
stress to connect the three concurrent or sequential cavitation events,
providing important insights about how to design the creep-fatigue load
waveform so that the creep cavitation can be enhanced. The following
conclusions can be reached:
- When tensile hold time is longer than that of compression, the
creep-fatigue test with initial compression is advantageous in terms
of creating more cavities.
- Unbalanced stress hold time in favour of compression most likely
closes all of the cavities nucleated during the previous tension
phase.
- Effect of compressive hold time or stress level on the number density
of cavities in creep-fatigue tests is not monotonic. The underlying
mechanism is their competing effect on nucleation and sintering.
- There is an optimum value for the stress variation rate to obtain the
highest number density of cavities, and their presence can be
attributed to sintering effect.
- Our model can satisfactorily explain several interesting experimental
observations, including nucleation incubation time, effective
nucleation field in a stress-frequency space for high-temperature
fatigue, and the effect of unequal ramp rate on cavitation.