A study has been made of the near-threshold fatigue crack propagation behavior of a wroght Ni-base superalloy, Rene 95, with reference to the effect of crack size on the threshold stress intensity ..delta..K/sub 0/ for no detectable crack growth. Measured threshold ..delta..K/sub 0/ values at low load ratios (R = 0.1) for physically short cracks (0.01 to 0.20 mm) were found to be 60% smaller than the corresponding ..delta..K/sub 0/ values for long cracks (approx. 25 mm). 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 Ni-base alloys. The large difference observed in 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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