WRC 587 PDF

Fatigue assessment procedures for pressurized components are provided in API 579-1/ASME FFS-1, Part 14. These assessment procedures provide both smooth bar and welded joint based fatigue methods. The existing welded joint fatigue curve is based on data up to 107 cycles, a straight-line extrapolation is assumed above 107 cycles. This Bulletin describes an extension of the Master S-N curve to account for run-out (censored) data. Few data points in the set have lives extending beyond 107 cycles, and no data points have lives extending beyond 108 cycles. As a result, using the Master S-N curve in the Very High Cycle Fatigue (VHCF) regime can lead to overly conservative risk evaluations and costly mitigation measures. An iterative least-squares approach is discussed in this Bulletin that statistically accounts for the effects of censored data. By including runouts in the analysis, and assuming an extra degree of freedom in the curve used to fit the data, a nonlinear expression for fatigue life is presented that closely matches the Master S-N curve below 107 cycles. The continuous nature of the nonlinear curve avoids any ambiguities related to determining the transition point in a bilinear curve and eliminates discontinuities in the standard deviation of the fit, which is pertinent when applying probabilistic approaches.