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 ΔK 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.

Stress intensity factors (SIFs) presented in the literature for corner cracks are limited to ideal quarter‐circular and quarter‐elliptical crack shapes. This paper presents SIF solutions for corner cracks that exhibit tunnelling, extending the range of corner crack shapes illustrated in the literature. Solutions were developed in a parametric form, obtained by empirically fitting polynomials to numerical values of SIF obtained from the FEM. A parameter was defined to quantify the extent of tunnelling. It was observed that crack shape has a significant effect on the SIF, so the consideration of equivalent quarter‐circular cracks can produce inaccurate results when significant tunnelling occurs. SIF solutions for quarter‐circular cracks are also presented and compared with those quoted in the literature.