Abstract

Current Equivalent Single Crack (ESC) tables in ASME Code, Section XI, Nonmandatory Appendix L were developed using a probabilistic fracture mechanics (PFM) approach which included outdated K solutions and fatigue crack growth rates. Recent study demonstrated that the current ESC tables can be non-conservative when using the latest K solutions and fatigue crack growth rates. However, significant effort is needed to update and maintain the ESC tables. Hence, the ASME Code Committee Task Group on Appendix L has been discussing potential approaches to eliminate ESC from Appendix L. Present study was performed to develop an alternative bounding evaluation approach without using the ESC tables. Fatigue crack growth calculations for the ranges of pipe dimensions considered in Appendix L were performed. The objective was to identify options for potentially simplifying the Appendix L methodology. Two possible options were proposed. Option 1 was to use the allowable flaw depth from a 1:6 flaw and apply it to evaluate the fatigue crack growth of an initial 360° flaw. Option 2 was to use the allowable flaw depth of a 360° flaw and apply it to evaluate the fatigue crack growth of an initial 1:6 flaw. The time for a postulated initial flaw to reach the allowable flaw size for both options was calculated. The current study demonstrated that the flaw tolerance analysis using ESC is not bounded by either Option 1 or Option 2 when using the membrane-to-gradient cyclic stress ratio Δσm /Δσg = 0.0. However, when a more reasonable lower bound membrane-to-gradient cyclic stress ratio (i.e., Δσm /Δσg = 0.1) was used to determine the ESC, Option 1 results bounded most of the cases considered in the present study.

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