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Fatigue d’un acier inoxydable austénitique 304L : étude des effets de l’environnement (air/eau primaire rep) à 300°c, de la contrainte moyenne et de l’état de surface

Abstract : The fatigue life estimation of nuclear components is an important subject for the operation license extension of nuclear power plants. In the current codifications, the influence of several factors (surface finish, mean stress, etc.) is taken into account to estimate the fatigue life. However, the effect of pressurized water reactor (PWR) primary water, as well as its interaction with other as-mentioned factors, have not been unambiguously demonstrated.Thus, this work aims to develop further knowledge of the impact of these different parameters on the kinetics and fatigue damage mechanisms of austenitic stainless steel 304L which are widely used for the cooling pipes of the primary circuit in PWR. Three parameters are investigated: the environment (air at 300 ° C/PWR primary water), the mean stress (0 MPa/20MPa/50 MPa), and the surface finish (as-received state/degraded state).Strain amplitude-controlled fatigue tests were carried out. To investigate the kinetics of crack initiation and propagation, two approaches are considered: the establishment of phenomenological macroscopic cracking laws, based on crack depth measurements in until failure fatigue tests and in stopped fatigue tests; the establishment of microscopic laws, based on measurement of the fatigue striation spaces of specimens tested to failure. To understand themodification of the cracking kinetics observed under different conditions, micro cracking mechanisms were discussed, based on the SEM, EBSD and TEM characterization.The fatigue tests reveal a decrease of fatigue life in PWR primary water, independent of the surface finish and the applied mean stress. This is attributed to the acceleration of the initiation phase of crack propagation. Compared to the cracking in air, the cracking in PWR water presents an aspect more "fragile", in terms of plasticity localization and reduction around the crack, as well as a more crystallographic cracking path. Applying a mean stress under a given strain amplitude reduces fatigue life in air, due to accelerated crack initiation and crack propagation. This detrimental effect is related to ductile and rapid cracking in the presence of mean stress. However, in PWR water, the application of the mean stress shows a minor influence on the fatigue crack propagation rate. The reduction of fatigue life due to the presence of mean stress in PWR water is mainly caused by the accelerated crack initiation.The harmful influence of the surface finish on fatigue life is observed, independent of the environment and mean stress. The crack propagation rate in PWR water again shows little sensitivity to the surface finish.
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Submitted on : Friday, May 21, 2021 - 2:59:08 PM
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Ziling Peng. Fatigue d’un acier inoxydable austénitique 304L : étude des effets de l’environnement (air/eau primaire rep) à 300°c, de la contrainte moyenne et de l’état de surface. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2021. Français. ⟨NNT : 2021ESMA0005⟩. ⟨tel-03232355⟩

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