Abstract
Near-threshold fatigue crack growth behavior for distributed cracks in Hastelloy-X has been examined with reference to the effects of crack size and environment on the threshold stress intensity ΔK.The threshold stress intensity value for a large crack in air was lower than in vacuum. This result was same for a small crack in air and vacuum experiments. But the value for a small crack was about 50% lower than for a large crack in vacuum. In air, the small crack threshold was lower than the large one. These results were discussed in terms of fatigue crack closure. Observation of crystallographic fracture surface in vacuum suggested the roughness induced crack closure mechanisms.The distributed-small-cracks specimen was obtained by low cycle fatigue loading tests. The threshold stress intensity for the distibuted-fatigue-crack growth could be estimated by the maximum crack size of the distributed cracks. The coalescence of small cracks did not affect the near-threshold fatigue crack growth behavior.
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