Fatigue design of nuclear power plant pressure boundary components necessitates the use of design curves, where the allowable number of cycles is a function of the applied stress intensity. Design curves are derived from best-fit curves to small-specimen data, which comprises a range of materials, heats, temperatures and test techniques.

This paper continues the series of papers most recently published in PVP2020-21136. At VTT, heat specific strain-life data has been obtained. In this paper, using our data and literature data we demonstrate the significance these heat and temperature specific best-fit curves can have on the predicted number of cycles, when also considering the detrimental effect of environment through Fen factors. Example calculations show that in most cases, a simple change of the reference curve from the Code best-fit curve to a more realistic one adds considerably to the number of predicted cycles, or alternatively, reduces the cumulative usage at equivalent number of cycles. Mainly for high cycle fatigue at high temperature, best-fit heat-specific curves may lie below the reference, taken as the mean curve in NUREG/CR-6909. However, refinement of calculation criteria and/or the Fen methodology is considered to provide additional relief to these cases so that the cumulative usage factor calculation can still be kept below unity.

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