Abstract
The effects of small (0·007–0·020 in.) holes on the fatigue strength of thin sheets of a wrough nickel-base superalloy loaded in pulsating tension have been investigated. Fatigue crack initiation, propagetion, and critical crack length were separately analyzed using surface replicas taken periodically throughout each test. The effect of hole size on the stress intensity range ( ΔK) for various crack lengths was theoretically calculated. Analyzed in this way, the experimentally-measured crack growth rates ( da/dN) showed the same dependence on ΔK [da/dN = A (ΔK) B] for all hole sizes. Using the experimental materials constants A and B, this crack propagation law was integrated from the hole perimeter to the critical crack length to predict the number of cycles spent in propagation for various hole size and stress range combinations. The analysis predicts, as observed, that the time spent in crack propagation decreases rapidly with increasing hole size as well as nominal stress range. Larger holes also produce earlier crack initiation even though the stress concentration factor is no higher. The microscopically planar slip character of the alloy and the various strain gradients adjacent to the holes are believed responsible.
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