Effect of specimen geometries on the C* versus da/dt master curve for type 316L stainless steel,

International audienceThis work deals with engineering components made of stainless steels working at high temperature and subjected to creep-fatigue loading history. The defect assessment procedures generally use the crack growth properties curve da/dt versus C* parameter for estimating the creep-crack growth. The ASTM E 1457-98 [ASTM E 1457-98. Standard test method for measurement of creep crack growth rates in metals, 1998] procedure proposes the rule to establish such a master curve. In particular, it is stipulated that this rule only applies for CT specimens. Previously [Laiarinandrasana L, Kabiri R, Drubay B. In: Gupta A, editor. Proceedings of the 16th international conference on structural mechanics in reactor technology, Washington, USA, 2001], some practical methodology to produce this crack growth curve on CT specimens has been described by introducing the way to determine the upper and lower limits of relevant experimental points and by adopting the ASTM E 1457-98 method to estimate the creep component of the load line displacement rate (dδ/dtbehavior). This latter is the interesting part of the total displacement rate recorded during the test. This paper focuses on the application of the procedure proposed in [Laiarinandrasana L, Kabiri R, Drubay B. In: Gupta A, editor. Proceedings of the 16th international conference on structural mechanics in reactor technology, Washington, USA, 2001] on specimen geometries other than CT, such as circumferentially cracked round bar (CCRB) and double edged notched in tension (DENT) specimens. The da/dt versus C* curves issued from all of these specimens are compared. Discussion about the effect of geometry on these curves is carried out. Additionally, some finite element analyses have been performed in order to simulate the creep crack growth using the node release technique. These simulations allow to verify the validity of the proposed expressions of C* and consequently the master curve of the 316L(N) stainless steel

Fissuration en relaxation des jonctions soudées en aciers inoxydables austénitiques

Cet article représente une synthèse des études menées au CEA/SRMA, en collaboration avec l'ENSMP, sur la fissuration en relaxation (F.E.R.) des aciers inoxydables austénitiques. Initialement, l'étude a porté sur l'endommagement des Zones Affectées (Z.A.) des soudures en aciers stabilisés au titane de type AISI 321. Une démarche de simulation expérimentale de Z.A. par des traitements thermo-mécaniques a été mise en place. Un essai de relaxation sur éprouvette CT a également été mis au point afin de reproduire en laboratoire ce type de fissuration. Outre la validation de cette démarche, il a été montré que l'écrouissage joue un rôle prépondérant sur la fragilisation intergranulaire et que le vieillissement atténue cette fragilisation contrairement à ce qui est couramment admis dans la littérature. Cette remise en cause du rôle premier de la précipitation des carbonitrures de titane permet d'élargir le champ d'investigation aux aciers non stabilisés pour lesquels il existe un certain nombre de cas de fissuration similaires à ceux attribués à la F.E.R. En appliquant la démarche précédente à des aciers de type AISI 316 et AISI 304, on montre aussi que l'écrouissage est un facteur de fragilisation intergranulaire. Parallèlement, on établit, par approche locale de la rupture, un modèle d'endommagement intergranulaire utilisable pour l'évaluation du risque de fissuration sur des composants réels

A leakage model to design seals for solid oxide fuel and electrolyser cell stacks

International audienceAlthough planar solid oxide fuel cell and electrolyser technology is a key perspective for the next energy systems, it still suffers from a lack of efficient tightness solutions due to the need for the use of a mix of brittle ceramics and stiff metallic materials at high temperatures. In order to design new well adapted metallic seals, an original computational model is proposed. It links the evolution of the local mechanical fields to the leakage rate. It remains purely macroscopic and does not require a fine description of roughness. As the model is designed to deal with high temperature systems, it takes into account the strain rate dependence of the seal materials. High temperature leakage tests are realized under load control conditions using a seal consisting of a 0.3 mm thick Fecralloy sheet lying between two elastic bearings made of Udimet 720 nickel alloy. One of the bearings presents a boss for which several geometries are used. Finite element calculations are performed to describe the mechanical state of the seal as a function of time. These results are post-processed using the proposed model to derive an estimation of the leakage rate. The model is tuned against the experimental results. Finally the validity of the model is checked by comparing its predictions to additional experimental results in which seal geometry, loading history, gas pressure or gas composition are varied

A study of sulfide-based solid electrolytes (SSEs) sensitivity towards humidity: gas evolution quantification and degradation mechanisms investigation

International audienc

Seal between two elements having different coefficients of thermal expansion

The disclosure relates to a seal inserted between two elements having different coefficients of thermal expansion, the seal includes a first and a second metallic contact portion spaced along an axial stacking direction orthogonal with respect to a radial direction. Tight connection means between the first and second contact portions allow a relative movement between the first and second contact portions along the radial direction. A first and a second sliding part are respectively coupled in translation along the radial direction with the first and second contact portions and are stacked so as to be able to slide in relation to each other along the radial direction

Systematic investigation of sensitivity of sulfide-based solid electrolytes for lithium-ion batteries towards humidity

International audienceAll-solid-state lithium-ion batteries (ASSLIBs) attract huge interest of researchers and battery manufacturers since they are the most promising candidates for future safe energy storage devices with high energy density for electric transport. However, an important obstacle for wide commercialization of sulphide-based solid electrolytes (SSEs) for LIBs is their extremely high reactivity with traces of humidity leading to toxic H2S evolution. Therefore, a deep understanding of reactivity of SSEs with water is a keyfactor to optimise intrinsic characteristics of SSEs as well as a production process of ASSLIBs ensuring its safety and reliability. Very limited amount of literature data is available on the topic .In this work, reactions between SSEs and water were investigated with high precision using a home-made test assembly. Commercially available SSEs, such as Li$_3$PS$_4$, Li$_7$P$_3$S$_{11}$ Li$_{10}$GeP$_2$S$_{12}$, Li$_6$PS$_5$Cl and Li$_6$PS$_5$Br were investigated at different scales (from few milligrams to 1g). The developed test assembly is based on gas flow-through cell for the analysis of SSE powders and pellets and continuous H2S measurements thanks to ultra-sensitive laser detection equipment. Specific attention was paid to reproducibility ofthe results. The levels of humidity were chosen to mimic a dry room atmosphere. A strong relationship was found between the composition of SSEs, their surface area and reactivity towards humidity. Passivation phenomena were observed in some cases upon exposure to H$_2$O. Additional advanced characterisation methods helped to understand the differences in reactivity of SSEs with water

A study of sulfide-based solid electrolytes (SSEs) sensitivity towards humidity: gas evolution quantification and degradation mechanisms investigation

International audienc

Investigation of sulfide-based solid electrolytes (SSE) sensitivity towards humidity

International audienceThe purpose of the present work is to study in details reactivity of SSEs towards humidity by followingtwo main directions:- The development of a flow-through setup that allows a better identification and highly precisequantification of gases (H2S or others) generated during reaction between water (humidity)and commercial SSEs: this flow-through setup intends to solves problems usually encounteredwhen using closed-type setups, which are the most used setups in the literature[3, 4]. Theseproblems include over-concentration of detected gases and low reproducibility ofmeasurements;- Physico-chemical and electrochemical characterizations of pristine and humidity exposed-SSEsin order to reveal the main factors affecting sensitivity towards humidity and to understand thedegradation mechanisms