In the case of ASME Class 1 pressure vessels and piping, as in other similar other codes, the design adequacy for fatigue is based on the cumulative usage factor (*CUF*), with recent augmentation to account for possible environmental effects. This deterministic quantification utilizes several engineering parameters (inputs) and (multiplicative) empirical factors. Although the fixed values of some of these design factors and S–N curves are based on underlying experimental data, the associated uncertainties are not explicit in the resulting fatigue assessment that is effectively based on the singular, calculated quantities of *CUF* and *F~en~*, projected for a specified service. As such, the resulting fatigue margin and associated conservatism remain implicit or inconsistent and unquantifiable.

The increasing demand for either extending the life of existing systems or for new systems with economically viable or better optimized fatigue designs has promoted the development of more realistic evaluation of fatigue performance as offered by the use of probabilistic techniques that explicitly account for the various uncertainties. One such approach to supplement the deterministic analysis was recently proposed by the author keeping the existing and familiar framework of *CUF* based fatigue assessment, while satisfying the target or acceptable component reliability to meet the fatigue design adequacy. The *CUF* formulation includes an explicit consideration of the k-factors (for material, loading history, surface and size effects) as adjustments to the S–N data curve. The objective of this paper is to provide an assessment of the impact of k-factors and their uncertainty on the realistic safety factor over a range of target reliability values. Also, for a typical application of the *CUF* analysis of a safety injection nozzle safe-end, similar assessment is made to quantify the impact of strain-rate variable and its uncertainty on the estimated safety factor. The approach taken consists of parametric analysis of the fatigue margin, for a given target reliability, by individually removing the factors and/or their uncertainty and comparing the results with the base case where all factors and associated uncertainties are maintained at their original values. Results of this analysis and their implications are discussed in relation to the *CUF* based evaluation of fatigue and associated margins.


An Assessment of Uncertainty in Design Factors and Strain-Rate Inputs for Environmentally-Assisted Fatigue and Related Margins

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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