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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environmentally assisted fatigue (EAF), a progressive and cumulative degradation mechanism that can be a life limiting factor for the structural integrity of NPP primary pressure boundary components manufactured from austenitic stainless steels (Vainionpää 2023; Vainionpää 2024; Ehrnstén 2024). Although, low cycle fatigue (LCF) is considered in the NPP design codes, the practical implications of this degradation mode in the operation of NPPs is evidently underestimated. The fatigue damage in NPPs is the result of non-monotonic cyclic loading conditions and can be influenced by a number of environmental (exposure environment, water chemistry, temperature, etc.), mechanical (waveform, stress state, strain rate, strain amplitude, etc.), and material factors (chemical composition, microstructure, stability of austenitic structure, etc.). Nevertheless, critical knowledge gaps persist, particularly in understanding the mechanisms of high-temperature PWR water containing hydrogen and cyclic loading parameters on LCF initiation and propagation of austenitic stainless steels. INCEFA-SCALE (INcreasing safety in NPPs by Covering gaps in Environmental Fatigue Assessment - focusing on gaps between laboratory data and component SCALE) is a five-year European Horizon 2020 collaborative project that aims to develop an improved mechanistic understanding of how environment, loading waveform, strain rate, and temperature impact the LCF behavior of austenitic stainless steels (Arrieta 2022; Mclennan 2023; Howe 2022). In this work, the effects of a PWR environment and cyclic loading parameters on the LCF behavior of 316L stainless steel were explored through detailed characterization of the specimens with various waveforms (periodic underload PUL, periodic overload POL and constant amplitude CA) tested in simulated PWR water, high-temperature air (HTA),

or room-temperature air (RTA) environments. 2. Material and experimental procedure

A non-stabilized solution annealed slab of austenitic stainless steel 316L was investigated. The measured room temperature yield strength, tensile strength, and modulus of elasticity were 235 MPa, 560 MPa, and 197 GPa, respectively. The chemical composition of the studied 316L material is presented in Table 1. The average grain size and hardness of the investigated 316L material are 95 µm and 139 HV1, respectively.

Table 1. Chemical composition (wt.%) of the stainless steel 316L investigated in this study. Fe C Mn Si S P Ni Cr Mo Cu N bal. 0.03 1.84 0.38 0.004 0.03 11.47 17.34 2.57 0.02 0.05

Solid bar specimens were manufactured in the longitudinal direction from the rolled plate by turning. The specimen gauge lengths and transition radii were polished. The polishing resulted in fine axial polishing marks at a 45° angle relative to the tensile axis. The surface roughness Ra, Rt and Rz were measured as 0.17, 1.9 and 1.5 µm, respectively. LCF testing was performed in a simulated PWR primary water environment and HTA at 300 °C or RTA at 20 °C, as specified in Table 2. LCF standard ASTM E606 and alignment standard ASTM E1012 were followed in the tests. The cyclic loading was performed under direct total strain control until specimen failure. The fatigue life (N 25 ) criterion for failure was defined as the cycle at which the tensile peak stress of a cycle drops by 25% from its steady state tensile peak stress. In the INCEFA SCALE project, 25 specimens, considered representative of the testing program phase I results (a total of 88 valid LCF tests) and validated by the expert panel from the consortium, were characterized in detail. The loading used for the testing consists of triangular or sawtooth CA waveforms and variable amplitude (VA) waveforms, i.e. PUL and POL. The PUL waveforms consist of a single underload at 0.6% strain amplitude followed by a set of 200 baseline cycles (either using a strain amplitude of 0.18% or 0.12%). Similarly, POL waveforms consist of a single overload at 0.6% strain amplitude followed by a set of 200 baseline cycles (either using a strain amplitude of 0.18% or 0.12%).