High-R Fatigue Crack Growth Threshold Stress Intensity Factors at High Temperatures

Abstract

While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors 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. There are two well-established international standard procedures for the determination of this fracture mechanics parameter, although their respective crack growth rate criteria differ by an order of magnitude. Unfortunately neither specifically addresses determination of the property for very high-R (K min/K max) ratios, under conditions when the environment can be influential, and Δ K th can be even more sensitive to the da/dN(ΔK) criterion adopted for its determination. In addition to a general state of knowledge review, particular attention is paid to circumstances concerning high-R Δ K th in power plant steels at high temperatures for which oxide-induced crack closure and creep cracking can be influential. Evidence for low-alloy 1%Cr, martensitic 9%Cr and austenitic 17%Cr steels is examined.