Effects of frequency on fatigue crack growth at elevated temperature

The effects of frequency on fatigue crack growth behaviour have been studied in a prealloyed powder material, Udimet 720Li, at 650 °C. Fracture mode and fatigue crack growth behaviour were studied at frequencies ranging from 0.001 to 5 Hz using a balanced triangular waveform. Tests were carried out under constant Δ K control, with load ratio and temperature being held constant. A mechanism map was constructed where predominantly time, mixed and cycle-dependent crack growth behaviour were identified. The results were verified by SEM analyses. Cycle-dependent crack growth data were obtained at room temperature, while fully time-dependent crack growth data were generated under sustained loads at 650 °C.
It was found that mixed time/cycle-dependent behaviour is of most significance for this material at the temperature and frequencies studied. Data for other nickel-based superalloys from various sources in the literature were compiled and compared with those of U720Li alloy at a given stress intensity and temperature in the mixed regime. An analysis was developed to rationalize the observed effect of frequency on fatigue crack growth rate.