High-temperature nuclear design codes, such as Section III, Division 5 of the American ASME Boiler and Pressure Vessel Code and the French RCC-MRx, require evaluating fatigue damage for qualifying high-temperature components. Both codes provide clear guidance for counting cycles under uniaxial loading conditions, but neither provides a cycle counting procedure for multiaxial loading conditions. The ASTM E1049 also does not address multiaxial cycle counting. However, several widely utilized multiaxial cycle counting procedures are available in the open literature, but there is no agreement on the most appropriate method for high-temperature applications. Applying the different cycle counting methods to the same loading history generally produces different results. Comparisons between cycle counting procedures are available for low-temperature high-cycle fatigue but not for high-temperature low-cycle dwell-fatigue applications. This work presents an extensive comparison between different multiaxial cycle counting procedures potentially suitable for high-temperature low-cycle dwell-fatigue applications. Furthermore, how to conservatively assemble design transients to construct a loading history is also an open question. This work also investigates the uncertainty related to the loading sequence. The results guide the selection of the most appropriate cycle counting procedure, strain range metric, and cycle distribution for ASME Section III, Division 5 applications.

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