PSI - Issue 57

Stéphan Courtin et al. / Procedia Structural Integrity 57 (2024) 4–13

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1. Introduction Austenitic stainless steel large nuclear components, with high thicknesses, may experience cyclic Large Scale Yielding (LSY) due to mechanical and thermal loadings during their service life. International codes and standards only provide inadequate and very conservative methods for dealing with the thermal fatigue crack growth associated with this situation. Non-codified alternatives, such as  J approaches recently discussed by Li et al. (2019) or Hojo and Kumagai (2022), then appear to be good candidates for assessing plasticity phenomena occurring during the thermal fatigue crack growth beyond Small Scale Yielding (SSY). To make progress on the validation of this kind of approaches for nuclear applications, this paper aims to apply a  J approach on the so-called PACIFIC experiment presented by Léopold et al. (2017) and permitting thermal fatigue crack propagation in LSY conditions. In this paper, numerical protocol and modelling assumptions will be detailed, after a short reminder of the experimental data. Then, a particular focus will be given, through sensitivity analyses, on the choice of the cyclic elastic-plastic stress-strain curve used in the Finite Element Analyses (FEA). Finally, comparisons between numerical results and experimental fatigue crack growth data will be provided and discussed.  J Difference between the extreme J-integral values during the virtual monotonically increasing thermal loading Sy Yield strength T Kmin Thermal state generating the K min value T Kmax Thermal state generating the K max value ΔT Difference between the two thermal states T Kmin and T Kmax (ΔT = T Kmin - T Kmax ) FEA Finite Element Analyses LSY Large Scale Yielding PWR Pressurized Water Reactor SSY Small Scale Yielding 2. The PACIFIC experiment The PACIFIC experimental device (see Fig.1(a)), presented by Léopold et al. (2017), was developed by EDF R&D to study the thermal fatigue crack propagation in LSY conditions. The principle of a PACIFIC experiment is to propagate 4 initial cracks of a given length by applying thermal cycles. The mock-up is a 316L austenitic stainless steel disk composed of 2 main parts (see Fig. 1(b)): • A central disk, also named “mock - up”, containing 4 initial cracks, initially propagated in the whole thickness of this disk by a fully mechanical fatigue process (pre-cracking step). • An external ring, also named “complementary ring”, which is welded to the central disk on its whole thickness. Loads, which are both thermal and mechanical, are applied using 4 circuits and a hydraulic jack (see Fig.1(b)). There are 2 water circuits and 2 oil circuits. All of them are totally independent from each other. Water circuits are used to heat and cool the upper and lower surfaces of the mock-up (i.e. the central disk illustrated in Fig. 1(b)). Oil circuits help to heat and cool the complementary ring (i.e. the external ring illustrated in Fig. 1(b)) by a conduction phenomenon. Two kinds of thermal gradients can be generated into the quasi-structure (see Fig. 2): Nomenclature E Young’s modulus K min K max ΔK eff Minimum stress intensity factor Maximum stress intensity factor Effective stress intensity factor derived from the  J value