Modeling of damage in unidirectional ceramic matrix composites and multi-scale experimental validation on third generation SiC/SiC minicomposites

International audienceThe purpose of this paper is to experimentally validate a 1D probabilistic model of damage evolution in unidirectional SiC/SiC composites. The key point of this approach lies in the identification and validation at both local and macroscopic scales. Thus, in addition to macroscopic tensile tests, the evolution of microscopic damage mechanisms - in the form of matrix cracks and fiber breaks - is experimentally analyzed and quantified through in-situ scanning electron microscope and computed tomography tensile tests. A complete model, including both matrix cracking and fiber breaking, is proposed on the basis of existing modeling tools separately addressing these mechanisms. It is based on matrix and fiber failure probability laws and a stress redistribution assumption in the vicinity of matrix cracks or fiber breaks. The identification of interfacial parameters is conducted to fit the experimental characterization, and shows that conventional assumptions of 1D probabilistic models can adequately describe matrix cracking at both macro- and microscopic scales. However, it is necessary to enrich them to get a proper prediction of ultimate failure and fiber break density for Hi-Nicalon type S fiber-reinforced SiC/SiC minicomposites

Estimation de la diffusivité thermique de composites C/SiC sous chargement mécanique

L'accès aux propriétés thermiques des matériaux composites thermostructuraux destinés aux applications nucléaires est un enjeu important pour prévoir leur comportement en condition d'utilisation. Dans un objectif prévisionnel mais également de compréhension des transferts de chaleur, ce travail s'intéresse à l'évaluation des diffusivités thermiques transverses de composites Cf/SiC soumis à un chargement mécanique de type traction. La démarche mise en place s'appuie sur l'utilisation de la méthode flash associée à une analyse du signal par thermographie IR permettant de générer des cartographies de propriétés

Caractérisation expérimentale de l'endommagement dans les minicomposites SiC/SiC

National audienceLes composites SiC/SiC sont étudiés pour leur usage potentiel comme matériau de gainage dans les réacteurs nucléaires de génération future. Afin de valider un modèle multiéchelle d'endommagement à l'échelle microscopique, une caractérisation expérimentale de l'endommagement à l'échelle du toron est mise en œuvre. Des essais de traction in-situ sur minicomposite permettent d'obtenir des données statistiques sur la cinétique d'apparition des fissures matricielles et l'évolution de leur ouverture en fonction de la contrainte. Ces observations de surface sont complétées par des observations microtomographiques réalisées à l'ESRF sur un minicomposite en traction. L'analyse des images 3D permet alors d'étudier la propagation des fissures matricielles au sein du minicomposite. Les ruptures de fibres sont également observables grâce à cette technique d'observation

Relation microstructure/propriétés à haute température dans les fibres et matrices de carbone

One of the main difficulties encountered with C/C composites, is the lack of data on their constituent's properties at high temperatures (up to 3000°C). This information is of interest to using models of mechanical behavior for prediction or optimization of composition and structure of these composites. The main objective of this work was to measure physical and mechanical properties of carbon fibers and matrix with different structures and texturesfrom room temperature to at least 2000°C. This needed the development of a specific device and specific techniques because of the shape and dimension of samples as well as the experimental difficulties involved in high temperature testing. At last, results obtained from different fibers and matrix were discussed in relation to their structure and texture.Un des problèmes posés par les composites C/C, en particulier, est le manque de données expérimentales sur les propriétés de leurs constituants aux hautes températures d'utilisation (jusqu'à 3000°C). Ces données sont nécessaires pour les modèles de comportement. L’objectif de ce travail était donc, de déterminer les caractéristiques physiques et mécaniques, depuis la température ambiante jusqu'à plus de 2000°C, de fibres et matricesde carbone, en fonction de leur structure et de leur texture. Pour cela il a fallu développer une machine et des techniques spécifiques en raison de la géométrie, de la taille des échantillons, et des difficultés expérimentales posées par la haute température. Enfin, les résultats obtenus sur différentes fibres et matrices ont été corrélés aux paramètres décrivant la structure et la texture

Relation microstructure/propriétés à haute température dans les fibres et matrices de carbone

One of the main difficulties encountered with C/C composites, is the lack of data on their constituent's properties at high temperatures (up to 3000°C). This information is of interest to using models of mechanical behavior for prediction or optimization of composition and structure of these composites. The main objective of this work was to measure physical and mechanical properties of carbon fibers and matrix with different structures and texturesfrom room temperature to at least 2000°C. This needed the development of a specific device and specific techniques because of the shape and dimension of samples as well as the experimental difficulties involved in high temperature testing. At last, results obtained from different fibers and matrix were discussed in relation to their structure and texture.Un des problèmes posés par les composites C/C, en particulier, est le manque de données expérimentales sur les propriétés de leurs constituants aux hautes températures d'utilisation (jusqu'à 3000°C). Ces données sont nécessaires pour les modèles de comportement. L’objectif de ce travail était donc, de déterminer les caractéristiques physiques et mécaniques, depuis la température ambiante jusqu'à plus de 2000°C, de fibres et matricesde carbone, en fonction de leur structure et de leur texture. Pour cela il a fallu développer une machine et des techniques spécifiques en raison de la géométrie, de la taille des échantillons, et des difficultés expérimentales posées par la haute température. Enfin, les résultats obtenus sur différentes fibres et matrices ont été corrélés aux paramètres décrivant la structure et la texture

Mechanical behavior of SiC/SiC composites reinforced with new Tyranno SA4 fibers: Effect of interphase thickness and comparison with Tyranno SA3 and Hi-Nicalon S reinforced composites.

International audienceThe development and availability of a new 3rd generation SiC fiber, the Tyranno SA4 (SA4), are promising for the processing of higher neutron and/or corrosion resistant SiC/SiC composites. Despite its promising properties, especially the higher crystallinity and thermal conductivity than the Hi-Nicalon S fiber, the previous Tyranno SA3 (SA3) reinforcement leads to low damage tolerant SiC/SiC composites, restraining its use as a reinforcement. The latter is the consequence of very high interfacial shear stress, whatever the pyrocarbon interphase thickness. In this work, tubular samples where produced with both reinforcements and with two different pyrocarbon interphase thicknesses for tensile mechanical characterizations to access the potential benefit of the new SA4 fibers. The tensile mechanical properties of SA4 composites are highly enhanced compared to SA3 composites. The low damage tolerance drawback of SA3 composites is solved with high ductility tensile mechanical behavior for SA4-based composites. Tensile mechanical tests also highlight an unusual influence of pyrocarbon interphase thickness on the composites tensile modulus and proportional limit stress. The thinner interphase (≈ 60 nm) is the most interesting for repeatable mechanical properties and induces high proportional limit stress. Unloading █ reloading cycles during tensile mechanical tests also highlight the benefit of this new fiber compare to Hi-Nicalon S. This work demonstrates that the substitution of SA3 by the new SA4 SiC fiber reinforcement in the processing of SiC/SiC composites is a great opportunity for the ceramic matrix composites development and especially for nuclear applications

Tensil creep behavior of SiC-based fibers with low oxygen content

International audienceThe creep behavior of Hi-Nicalon, Hi-Nicalon S and Tyranno SA3 fibers is investigated at temperatures up to 1700DC. Tensile tests were carried out on a high capability fiber testing apparatus in which the fiber is heated uniformly under vacuum. Analysis of initial microstructure and composition of fibers was performed using various techniques. All the fibers experienced a steady-state creep. Primary creep was found to be more or less significant depending on fiber microstructure. Steady-state creep was shown to result from grain boundary sliding. Activation energy and stress exponents were determined. Creep mechanisms are discussed on the basis of activation energy and stress exponent data. Finally, tertiary creep was observed at very high temperatures. Tertiary creep was related to volatilization of SiC. Results are discussed with respect to fiber microstructure

Influence of Texture and Thickness of Pyrocarbon Coatings as Interphase on the Mechanical Behavior of Specific 2.5D SiC/SiC Composites Reinforced with Hi-Nicalon S Fibers

(This article belongs to the Special Issue Advanced Composites and Coatings for Nuclear Applications)International audienceIn the framework of nuclear applications of SiC/SiC composites, the influence of pyrocarbon interphase texture and thickness on the mechanical behavior both on as processed materials and on irradiated materials is a major concern. Thus, the PyC interphase influence has to be clearly addressed to define its optimal chemical vapor infiltration processing parameters. For this pur-pose, specific 2.5D SiC/SiC composites reinforced with Hi-Nicalon S fibers and with two kind of PyC texture and thickness were produced. Transmission electronic microscopy allowed PyC thickness and microstructure/texture characterizations, whereas push-out and tensile tests were employed as experimental mechanical procedures. Original result is that PyC nature directly in-fluences the Interfacial shear stress and failure mode of the weakest interface, regardless of PyC thickness. Adhesive failure or cohesive failure are highlighted depending on the PyC CVI deposition mechanisms. Similar post irradiation characterizations will be required to assess the role of irradiation on the PyC microstructure/texture evolution and mechanical behaviors of these materials

Investigation of the neutron irradiation effects on the SIC/SIC composites at high temperature

International audienceSiC/SiC composites have aroused interest for fusion and fission nuclear applications due to their thermomechanical properties at high temperature and behavior under irradiation (low swelling, neutron transparency, …), essentially as core materials. In this aim, the CEA has been working on the manufacture and the development of tubular SiC/SiC composites cladding elements for more than a decade. During this period, an optimization of the mechanical properties and the mastering of the manufacturing process have been conducted. Third generation SiC fibers, Hi-Nicalon S and Tyranno SA3, and high-purity CVI pyrocarbon interphase and SiC matrix have been employed for their expected behavior under irradiation. However, the latter consequences have to be properly evaluated. Different types of materials have then been produced for several irradiations. Firstly, the manufacturing process and the materials characterization will be described as the irradiations conditions. The CROCUS irradiation was aimed to determine the mechanical behavior of SiC/SiC minicomposites and of various brazing fillers while the CEDRIC irradiation had as objective to follow their creep at high temperature (600 – 1000°C). The TIRAMISU purpose was to evaluate the effect of high-doses (> 100 dpa) at the Sodium Fast Reactor temperature (550°C). The aftermath of these irradiations on the materials dimensions (swelling) as well as on its structure (fibers, interphase) and mechanical properties will then be evaluated