A study was made of the near-threshold fatigue crack propagation behaviour of a wrought nickel-base superalloy, René 95, with reference to the effect of crack size on the threshold stress intensity ΔK0 no detectable crack growth. Measured threshold ΔK0 values at low load ratios (R = 0.1) for physically short (0.01 – 0.20 mm) cracks were found to be 60% smaller than the corresponding ΔK0 values for long (about 25 mm) cracks. However, short crack threshold values at R = 0.1 were found to be similar to long crack thresholds at R = 0.8. Such behavior is rationalized in terms of fatigue crack closure, specifically involving the role of fracture surface roughness from crystallographic crack growth in nickel-base alloys. The large difference observed between the threshold values for long and physically short cracks serves to illustrate the potential problems in applying conventional (long crack) fatigue data to defect-tolerant lifetime predictions for structural components containing small flaws.
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