Micromechanical modeling of the elastic behavior of unidirectional CVI SiC/SiC composites

International audienceThe elastic behavior of SiC/SiC composite is investigated at the scale of the tow through a micromechanical modeling taking into account the heterogeneous nature of the microstructure. The paper focuses on the sensitivity of transverse properties to the residual porosity resulting from the matrix infiltration process. The full analysis is presented stepwise, starting from the microstructural characterization to the study of the impact of pore shape and volume fraction. Various Volume Elements (VEs) of a virtual microstructure are randomly generated. Their microstructural properties are validated with respect to an experimental characterization based on high definition SEM observations of real materials, using various statistical descriptors. The linear elastic homogenization is performed using finite elements calculations for several VE sizes and boundary conditions. Important fluctuations of the apparent behavior, even for large VEs, reveal that scales are not separated. Nevertheless, a homogeneous equivalent behavior is estimated by averaging apparent behaviors of several VEs smaller than the Representative Volume Element (RVE). Therefore, the impact of the irregular shape of the pores on the overall properties is highlighted by comparison to a simpler cylindrical porous microstructure. Finally, different matrix infiltration qualities are simulated by several matrix thicknesses. A small increase in porosity volume fraction is shown to potentially lead to an important fall of transverse elastic moduli together with high stress concentrations

Approche multi-échelle du comportement mécanique des matériaux composites SiC/SiC : comportement élastique à l'échelle du toron

National audienceUne approche multi-échelle a été entreprise afin d'obtenir un modèle prédictif du comportement mécanique des composites SiC/SiC. L'étude du comportement élastique à l'échelle du toron (microstructure poreuse et hétérogène), réalisée sur des microstructures générées à partir des résultats de la caractérisation microstructurale, met en évidence un problème de séparabilité des échelles. Néanmoins, une estimation du comportement homogène équivalent est proposée en première approximation

A phase field method to simulate crack nucleation and propagation in strongly heterogeneous materials from direct imaging of their microstructure

International audienceIn this work, crack initiation and propagation in 2D and 3D highly heterogeneous materials models, such as those obtained by micro-CT imagery of cementitious materials, is investigated for the first time by means of the phase field method. A shifted strain split operator algorithm is proposed to handle unilateral contact within cracks in a very efficient manner. The various advantages of the phase field method for voxel-based models are discussed. More specifically, we show that the resolution related to the initial image and thus to meshes for discretizing the same microstructure does not significantly affect the simulated crack path

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

In-situ X-ray microtomography characterization of damage in SiC/SiC minicomposites

International audienceThe purpose of the present study is to characterize matrix crack propagation and fiber breaking occurrences within SiC/SiC minicomposite in order to validate later on a multiscale damage model at the local scale. An in-situ X-ray microtomography tensile test was performed at the European Synchrotron Radiation Facility (ESRF, ID19 beamline) in order to obtain 3-dimensional (3D) images at six successive loading levels. Results reveal a slow and discontinuous propagation of matrix cracks, even after the occurrence of matrix crack saturation. A few fiber failures were also observed. However, radiographs of the whole length (14 mm) of the minicomposites under a load and after the failure were more appropriate to get statistical data about fiber breaking. Thus, observations before the ultimate failure revealed only a few fibers breaking homogenously along the minicomposite. In addition, an increase in fiber breaking density in the vicinity of the fatal matrix crack was observed after failure. These experimental results are discussed in regards to assumptions used in usual 1-dimensional (1D) models for minicomposites

On the choice of parameters in the phase field method for simulating crack initiation with experimental validation

International audienceThe phase field method is a versatile simulation framework for studying initiation and propagation of complex crack networks without dependence to the finite element mesh. In this paper, we discuss the influence of parameters in the method and provide experimental validations of crack initiation and propagation in plaster specimens. More specifically, we show by theoretical and experimental analyses that the regularization length should be interpreted as a material parameter, and identified experimentally as it. Qualitative and quantitative comparisons between numerical predictions and experimental data are provided. We show that the phase field method can predict accurately crack initiation and propagation in plaster specimens in compression with respect to experiments, when the material parameters, including the characteristic length are identified by other simple experimental tests

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

On the role of in-plane damage mechanisms on the macroscopic behavior of SiC/SiC composites from complementary 2D and 3D in-situ investigations

International audienceThe mechanical behavior of architectured SiC/SiC composites is driven by different damage mechanisms whose understanding is required for building micromechanics-based models able to reproduce and predict its complexity. The kinematics of the surface, precisely analyzed using DIC at the textile pattern scale, exhibit a fiber realignment unexplained by the cracks observed at the surface. The missing mechanism, tracked by tomography in-situ testing (SOLEIL synchrotron), appears to be in-plane microcracking which does not emerge at the free surface of the composite

A Method for Quantifying Back Flexion/Extension from Three Inertial Measurement Units Mounted on a Horse’sWithers, Thoracolumbar Region, and Pelvis

Back mobility is a criterion of well-being in a horse. Veterinarians visually assess the mobility of a horse’s back during a locomotor examination. Quantifying it with on-board technology could be a major breakthrough to help them. The aim of this study was to evaluate the accuracy of a method of quantifying the back mobility of horses from inertial measurement units (IMUs) compared to motion capture (MOCAP) as a gold standard. Reflective markers and IMUs were positioned on the withers, eighteenth thoracic vertebra, and pelvis of four sound horses. The horses performed a walk and trot in straight lines and performed a gallop in circles on a soft surface. The developed method, based on the three IMUs, consists of calculating the flexion/extension angle of the thoracolumbar region. The IMU method showed a mean bias of 0.8° (±1.5°) (mean (±SD)) and 0.8° (±1.4°), respectively, for the flexion and extension movements, all gaits combined, compared to the MOCAP method. The results of this study suggest that the developed method has a similar accuracy to that of MOCAP, opening up possibilities for easy measurements under field conditions. Future studies will need to examine the correlations between these biomechanical measures and clinicians’ visual assessment of back mobility defects