Reference fatigue crack growth rate curves have been contained in Appendix A of Section XI since its inception. The curves have been designed to be applicable to carbon and low alloy pressure vessel steels exposed to either air or light water reactor coolant environments. Data obtained over the past several years have shown a different behavior of these steels in the LWR environment than that predicted by the present reference curve. A revised set of reference curves have been formulated, incorporating a new curve shape as well as a dependency of growth rate on R ratio (minimum load/maximum load). This article provides the background and justification for such a revision, details the methodology used to develop the revised curves, and includes an evaluation of the adequacy and impact of the revised curves as compared with the single curve which they replace.

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