Laboratory data has 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). As a result of this observation, environmentally assisted fatigue (both fatigue life and fatigue crack growth) of austenitic stainless steels in high temperature water environments has become a significant concern.  However, current operational experience indicates that there is a considerable level of conservatism in the fatigue life design procedure in NUREG/CR-6909 Rev 1 and the crack growth relationships in ASME code case N-809.  

The Weighted K Rate, WKR, method was 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 for fatigue crack growth.  However, this method was based solely on isothermal data, and was shown to still retain significant over-conservatism, especially for out-of-phase non-isothermal waveforms typical of many thermally induced loading transients.  The Weighted Temperature and K-rate (WTKR) method has been developed, 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.  Nonetheless, the results indicate that WTKR still retains a degree of conservatism especially for out-of-phase temperature and loading waveforms.


Thermo-Mechanical Fatigue Crack Growth Testing

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


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

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