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Abstract
While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors (ΔKth) for defect assessment is relatively well established for many circumstances, this is not the case for materials and conditions which are sensitive to time dependent mechanisms. In particular, current testing standards do not address determination of the property for very high R (Kmin/Kmax) ratios, and those conditions when the environment can be influential and ΔKth is sensitive to the adopted da/dN(ΔK) criterion. The incidence of creep crack growth in association with very slowly growing high-R high cycle fatigue cracks is not always discernable by the direct examination of near-to-threshold da/dN(ΔK) test records, but can be predicted by means of a time dependent failure assessment diagram analysis. In addition to a general state of knowledge review, particular attention is paid to circumstances relating to high-R ΔKth in power plant steels at high temperatures for which oxide induced closure and creep cracking can be influential. Evidence for a low alloy 1%Cr steel, a martensitic 9%Cr steel and an austenitic 18%Cr steel is considered, and the practical implications are examined.