PSI - Issue 68

Aleks Vainionpää et al. / Procedia Structural Integrity 68 (2025) 279–284 A. Vainionpää et al. / Structural Integrity Procedia 00 (2025) 000–000

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systems are 0.3 vs 0.07 µm/cycle at a crack depth of 1 mm. Moreover, PUL/POL waveform significantly accelerates the initiation of a fatigue crack in comparison to the CA specimens (5000 vs 180000 cycles). 4. Discussion The environment has an impact on fatigue crack initiation and propagation, as evidenced by changes in striation spacing (FCGR) and cycle-to-initiation. Exposure to a simulated PWR primary water environment systematically results in a reduction of fatigue life and cracking resistance in austenitic stainless steel 316L. The deformation layer formed beneath the as-machined specimen surface facilitates accelerated oxidation in a PWR water environment. Dissolution along the slip bands can promote micro-crack initiation and short crack growth of EAF cracks through a crevice corrosion effect (Que 2021). Stress and strain concentration can accelerate passive oxide film rupture via plastic deformation slips, leading to increased dissolution and higher FCGR, and increase hydrogen uptake at crack tips, potentially causing hydrogen accumulation and local mechanical property degradation (Que 2022). PUL accelerates both the FCGR and the initiation of a fatigue crack compared to CA loading. Fatigue life is directly affected by the waveform and sequencing of the loading cycles, particularly when the cycling incorporates complex patterns of loading that can influence mean stress or the cyclic hardening behaviour of the material. VA fatigue is associated with situations where a component is subjected to different combinations of alternating loading at multiple different stress or strain amplitudes during operation. This variability more accurately represents the service conditions experienced by in-service plant (e.g. during load follow) as opposed to the single constant amplitude loading conditions applied to test specimens, which are used for the development of standard fatigue endurance curves. This paper indicates that VA loading as a waveform relevant for NPP operation can alter fatigue endurance lifetimes and deserves more attention in future studies. 5. Conclusions The effects of environment and hydrogen and cyclic waveform on the LCF behavior in a nuclear grade 316L austenitic stainless steel were evaluated. The main conclusions can be summarized as follows: • The implementation of "VA-maps" provides a consistent method for evaluating the striation spacing and cycles to-depth of VA specimens. This approach facilitates fast and robust estimations of crack propagation rates throughout the crack progression history, and the cycle number where the crack has initiated. • The exposure to a PWR environment results in a decreased low-cycle fatigue lifetime, an enhanced fatigue crack initiation and an accelerated fatigue crack growth rate of 316L austenitic stainless steel. • The effect of waveform (PUL, POL and CA) was evaluated. PUL reduces the low cycle fatigue lifetime, accelerates the fatigue crack growth rate and advances the cycle where initiation of a fatigue crack occurs compared to CA loading. Acknowledgements This project has received funding from the Euratom research and training program 2019-2020 under grant agreement No. 945300. The authors would also like to acknowledge and sincerely thank the INCEFA-SCALE Project partners for their contributions to this work. The authors would like to sincerely thank J.C. le Roux (Electricité de France), I. Schnablova (UJV), V. Zbyněk (UJV), L. Doremus (Framatome), B. Connolly (University of Manchester), J. Beswick (Jacobs), A. McLennan (Jacobs), T. Lehtikuusi (VTT), J. Lukin (VTT) and all participants in the INCEFA- SCALE Project for their valuable contributions to this work. References Arrieta, S., Cicero, S., Mottershead, K., Cicero, R., Mclennan, A., Courtin, S., Que, Z., 2022. Environmental Fatigue Analysis of nuclear components within the framework of INCEFA-SCALE project, Procedia Structural Integrity, vol. 42, no. 2, pp. 27-34. Ehrnstén, U., Andresen, P., Que, Z., 2024. A review of stress corrosion cracking of austenitic stainless steels in PWR primary water, Journal of Nuclear Materials, vol. 588, no. 154815.