CNRS Laboratory for Thermostructural Composites

Bordeaux, France

CNRS Laboratory for Thermostructural Composites

Bordeaux, France

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Vignoles G.L.,CNRS Laboratory for Thermostructural Composites | Ortona A.,CIMSI
International Journal of Thermal Sciences | Year: 2016

Effective thermal conductivities (ETC) under vacuum were computed numerically on 3D blocks of opencell foams either obtained by X-ray tomography or generated by Computer-Aided Design (CAD) with ideal geometries. For the first time a Monte-Carlo/Random Walk code accounting for the coupling of conduction in the solid phase and of radiation in the pore space has been used. The whole range of conduction/radiation ratio, parameterized by a Nusselt number, has been scanned; a law relating the ETC to this parameter has been obtained. In all cases the conductive and radiative contributions are additive. The slope of the radiative contribution to the ETC is found to display a distinct behavior, depending on whether radiation or conduction dominates. The low-temperature regime has an emissivity-dependent ETC slope, while the high-temperature regime does not. The critical ratio between both regimes is related to the ratio between cell and strut diameters. In all cases, it is found that the ETC anisotropy decreases with temperature. Closing some windows enhances conduction parallel to the closing walls and reduces radiation perpendicular to them. This effect is shown to influence the ETC eigendirections in actual media. © 2016 Elsevier Masson SAS.


Martin E.,CNRS Laboratory for Thermostructural Composites | Leguillon D.,CNRS Jean Le Rond d'Alembert Institute | Carrere N.,ENSTA Bretagne
International Journal of Solids and Structures | Year: 2012

The strength of an open holed composite plate subjected to tensile loading is analysed with the help of a coupled strength and energy criterion. Analytical and numerical models are used to determine the stress distribution and the energy released by crack nucleation in the vicinity of the hole. It is shown that the hole size effect can be described and that the crack length at nucleation both depends on the local geometry and the fracture characteristics of the specimen. These characteristic values can be identified with the help of traction tests performed on unnotched and holed samples. © 2012 Elsevier Ltd. All rights reserved.


Leyssale J.-M.,CNRS Laboratory for Thermostructural Composites | Vignoles G.L.,CNRS Laboratory for Thermostructural Composites
Journal of Physical Chemistry C | Year: 2014

We report on the dynamical behavior of single divacancy defects in large graphene sheets as studied by extensive classical molecular dynamics (MD) simulations at high temperatures and static calculations. In the first part of the paper, the ability of the used interatomic potential to properly render the stability and dynamics (energy barriers) of such defects is validated against electronic structure calculations from the literature. Then, results from MD simulations are presented. In agreement with recent TEM studies, some mobility is observed through a series of Stone-Wales-like bond rotations involving the 5-8-5, 555-777, and 5555-6-7777 reconstructions. Although these three structures are by far the most probable structures of the DV defect, not less than 18 other full reconstructions, including the experimentally observed 55-66-77 defect, were occasionally observed in the ≈1.5 μs of MD trajectories analyzed in this work. Most of these additional reconstructions have moderate formation energies and can be formed by a bond rotation mechanism from one of the aforementioned structures, with a lower activation energy than the one required to form a Stone-Wales defect in graphene. Therefore their future experimental observation is highly probable. The results presented here also suggest that the barrier to a conventional Stone-Wales transformation (the formation of two pentagon/heptagon pairs from four hexagons) can be significantly reduced in the vicinity of an existing defect, strengthening a recently proposed melting mechanism for graphene based on the aggregation of Stone-Wales defects. From a structural point of view, in addition to pentagons, heptagons, and octagons, these new DV reconstructions can also contain four- and nine-member rings and show a particularly large spatial extent of up to 13 rings (42 atoms) against three (14 atoms) for the original 5-8-5 defect. © 2014 American Chemical Society.


Lamon J.,CNRS Laboratory for Thermostructural Composites
Composites Science and Technology | Year: 2010

The present paper compares the fragmentation strength distributions predicted using various models including:. -The well-known Monte Carlo simulation method based on chain-of-segments model and fiber strength distribution. This model has been widely used to simulate fragmentation of fibers in polymer or metal matrix.-Fragment dichotomy model based on failures and flaw-strength distributions in successive fragments. This model has been validated in previous papers by comparison to experimental results, and-Bayesian chain-of-elements model based on flaw-strength distribution in the fiber. This model is proposed in this paper.These models are discussed. They were compared to experimental series of fragmentation stresses obtained during tensile tests on SiC matrix composites reinforced with SiC or C fibers. Matrix failures were detected using either acoustic emission counts or SEM inspection during the tests. The influence of preponderant factors was anticipated. © 2010.


Herb V.,CNRS Laboratory for Thermostructural Composites | Martin E.,CNRS Laboratory for Thermostructural Composites | Couegnat G.,CNRS Laboratory for Thermostructural Composites
Composites Part A: Applied Science and Manufacturing | Year: 2012

Thin 3D-woven SiC f/SiC samples were subjected to low velocity impact tests at room temperature. For this purpose, hemispherical impactors and circular supports of various diameters were used. The extent of damage was evaluated with the help of optical microscopy. Formation of micro-cracks initiating from the indented site is observed. The predominant internal damages (fiber bundle and matrix cracking) remain localized beneath the impactor. This is confirmed by thermography analysis and post-impact tensile tests. The diameter of the damaged zone can be related to the energy absorbed by the specimen during the impact event. © 2011 Elsevier Ltd. All rights reserved.


Reinisch G.,CNRS Laboratory for Thermostructural Composites | Leyssale J.-M.,CNRS Laboratory for Thermostructural Composites | Vignoles G.L.,University of Bordeaux 1
Journal of Chemical Physics | Year: 2010

We present an extension of some popular hindered rotor (HR) models, namely, the one-dimensional HR (1DHR) and the degenerated two-dimensional HR (d2DHR) models, allowing for a simple and accurate treatment of internal rotations. This extension, based on the use of a variable kinetic function in the Hamiltonian instead of a constant reduced moment of inertia, is extremely suitable in the case of rocking/wagging motions involved in dissociation or atom transfer reactions. The variable kinetic function is first introduced in the framework of a classical 1DHR model. Then, an effective temperature and potential dependent constant is proposed in the cases of quantum 1DHR and classical d2DHR models. These methods are finally applied to the atom transfer reaction SiCl3 + BCl3 → SiCl4 + BCl2. We show, for this particular case, that a proper accounting of internal rotations greatly improves the accuracy of thermodynamic and kinetic predictions. Moreover, our results confirm (i) that using a suitably defined kinetic function appears to be very adapted to such problems; (ii) that the separability assumption of independent rotations seems justified; and (iii) that a quantum mechanical treatment is not a substantial improvement with respect to a classical one. © 2010 American Institute of Physics.


Chollon G.,CNRS Laboratory for Thermostructural Composites | Delettrez S.,CNRS Laboratory for Thermostructural Composites | Langlais F.,CNRS Laboratory for Thermostructural Composites
Carbon | Year: 2014

In order to improve their mechanical properties, carbon open-cell foams of two different pore sizes were infiltrated with pyrocarbon by chemical vapour deposition at reduced pressure and using pure propane as precursor. The optimal conditions in terms of deposition rate and uniformity in coating thickness, structure and anisotropy were first investigated. Foam specimens were infiltrated at various stages, with two pyrocarbons of distinct microtextures and their morphology, relative density and geometrical features were evaluated. Compressive crushing tests were conducted to determine the influence of the pore size, the pyrocarbon type and the relative density on the mechanical properties of the pyrocarbon-infiltrated foams. They retain their non-brittle and dissipating behaviour up to relative densities of 0.15. The stiffness, crushing strength and dissipated energy increase significantly with the relative density. The crushing behaviour of the pyrocarbon-foam specimens can be essentially explained using simple structural models and failure mechanisms, according to the Gibson & Ashby's approach for brittle cellular solids. © 2013 Elsevier Ltd. All rights reserved.


Marcin L.,ONERA | Maire J.-F.,ONERA | Carrere N.,ONERA | Martin E.,CNRS Laboratory for Thermostructural Composites
International Journal of Damage Mechanics | Year: 2011

The aim of this article is to propose a macroscopic damage model, which describes the nonlinear behavior observed on woven composites with ceramic matrix. The model is built within a thermodynamic framework with internal variables. First of all, the efficiency of the model to describe the mechanical behavior of carbon fiber-reinforced ceramic matrix composites is outlined. Then, the predictive capability of the model is evaluated with the help of an alternate torsion test. © The Author(s), 2010.


Loseille O.,CNRS Laboratory for Thermostructural Composites | Lamon J.,CNRS Laboratory for Thermostructural Composites
Advanced Materials Research | Year: 2010

Previous works have shown that ceramic matrix composites are sensitive to delayed failure during fatigue in oxidizing environments. The phenomenon of slow crack growth has been deeply investigated on single fibers and multifilament tows in previous papers. The present paper proposes a multiscale model of failure driven by slow crack growth in fibers, for 2D woven composites under a constant load. The model is based on the delayed failure of longitudinal tows. Additional phenomena involved in the failure of tows have been identified using fractographic examination of 2D woven SiC/SiC composite testspecimens after fatigue tests at high temperatures. Stochastic features including random load sharing, fiber overloading, fiber characteristics and fiber arrangement within the tows have been introduced using appropriate density functions. Rupture time predictions are compared to experimental data. © (2010) Trans Tech Publications, Switzerland.


Vignoles G.L.,CNRS Laboratory for Thermostructural Composites
International Journal of Heat and Mass Transfer | Year: 2016

Heat transfer properties from ambient up to extremely high temperatures are a key feature of advanced thermal protection and thermal exchange materials - like ceramic foams or fiber assemblies. Because of their porous nature, heat transfer rests not only on conduction in opaque solids and on convection in pores, but also on radiation trough pores. The precise knowledge of the thermal behavior of these materials in these conditions is an issue. In a "virtual material" framework, we present a computational simulation tool for heat transfer in such materials, combining solid-phase conduction and linearized radiative transfer in open or closed radiating cavities with opaque interfaces. The software is suited to working in large 3D blocks as produced e.g. by X-ray CMT or by image synthesis. An original Monte-Carlo mixed random walks scheme accounting for both diffusion and radiation is presented and validated. The application to a real image of a fibrous medium is described and discussed, principally in terms of the influence of the diffusion/radiation ratio on the effective (large-scale) diffusivity tensor. © 2015 Elsevier Ltd.

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