Laboratory data have indicated that light water reactor environments can significantly reduce the fatigue life and crack growth performance of austenitic stainless steels. These environmental effects have been codified into design procedures and documents such as NUREG/CR-6909 Rev 1 (fatigue life) and ASME code case N-809 (crack growth). However, there is considered to be significant conservatism in these methods when applied to plant relevant loadings.

The Weighted K-Rate, WKR, method was initially developed by J. Emslie et.al (PVP2016-63497) to address the influence of waveform shape as one of the potential sources of the over-conservatism in code case N-809. This method was found to significantly reduce the over-conservatism associated with ASME code case N-809. However, this method was based solely on isothermal data, and was shown to also retain significant over-conservatism, especially for out-of-phase non-isothermal waveforms typical of many thermally induced loading transients. The WKR method was further evolved into the Weighted Temperature and K-rate (WTKR) method, by Currie et.al (PVP2019-93855), further updated by Mann et.al (PVP2020-21585), which partitions the damage across the loading cycle, under non-isothermal conditions, and has been shown to significantly reduce the perceived over-conservatism associated with ASME code case N-809 when applied to many plant-relevant loading waveforms. This paper describes work that was done to investigate the impact of non-isothermal temperature / loading waveforms, and forms the bulk of non-isothermal data from which the WTKR method was derived.

The data presented in this paper indicate that for out-of-phase transient loading (typical of most thermally induced plant loadings), and simple isothermal loading at low temperatures and longer rise times, the WTKR method provides a more accurate prediction of fatigue crack growth rates than the application of ASME code case N-809.

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