Clarification on Definition of Membrane-to-Gradient Stress Ratio Used in the Development of ASME Code Section XI Appendix L Flaw Tolerance Procedures

Abstract ASME Code, Section XI, Nonmandatory Appendix L prescribes flaw tolerance procedures for determining an allowable operating period for operating plant components and piping subjected to cyclic (fatigue) loadings. The procedures require qualified nondestructive examination (NDE) confirmation that the location of interest has an absence of flaws deeper than the Section XI, IWB-3500 Flaw Acceptance Standards. The flaw tolerance analysis postulates a pre-existing flaw at the location of interest. The flaw depth is based on the Acceptance Standards, and the flaw aspect ratio is determined based on the material type and the ratio of the cyclic membrane-to-gradient (M-G) stress. Currently, Appendix L does not provide a procedure or guidance on how to specifically calculate M-G stress ratios for individual cyclic stress profiles, which can lead to uncertainties and inconsistencies in Appendix L flaw tolerance evaluations. A detailed review of the technical basis documents for the revision to Appendix L published in the 2008 Addenda was performed to obtain clarification on the definition of the M-G cyclic stress ratio for determining the equivalent single crack (ESC) tables that currently reside in Appendix L. From this review, it was confirmed that the original Appendix L technical basis calculations used a fixed stress profile of through-wall linear and nonlinear gradient stresses, and the M-G ratio was based on the membrane and gradient stress values for a fixed total stress at the component internal surface. Independent calculations were performed to confirm these findings. This paper reports the review findings and suggests an improved approach for determining and calculating the various M-G ratios from stress gradients in individual cyclic stress profiles for use in Appendix L flaw tolerance evaluations.