Slow crystallographic fatigue crack growth in a nickel-base alloy

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm^-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.