Trois approches cohérentes pour modéliser la multifissuration des composites 1D

International audienceIl existe de nombreux modèles pour décrire la fissuration des composites 1D, avec, pour la plupart, les points communs suivants : la mécanique est simplifiée en 1D, le comportement des matériaux (fibres et matrice) est élastique, l'interphase est souvent modélisée par une contrainte de cisaillement. Le point délicat de ces modèles consiste à gérer le caractère aléatoire de la fissuration, souvent décrit par un modèle deWeibull. Pour ce faire, plusieurs stratégies ont été déployées qui peuvent se classer en trois approches : la première approche (CL) discrétise le composite, et tire aléatoirement des contraintes limites suivant une statistique de Weibull ; la seconde approche (PF) détermine de manière aléatoire les différentes fissures de manière séquentielle ; la troisième approche (AC) modélise un composite infini, et propose des formules analytiques pour décrire la statistique des fragments de composites. Nous montrons, dans le cas simplifié du micro-composite, que ces trois approches conduisent à des résultats équivalents

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

A procedure for identifying the plastic behaviour of single crystals from the local response of polycrystals

The overall and local tensile responses of an α iron multicrystalline sample are investigated in order to derive the plastic constitutive equations for the constituent single crystals. The macroscopic stress–strain curve and some statistical characteristics of the strain field measured on the sample surface are compared with their simulated counterparts. The optimal values of the material parameters of four types of hardening laws are derived by a minimization procedure. The best results are obtained with a nonlinear anisotropic law which uses the dislocation densities on the slip systems. This procedure is then validated on a fine-grained polycrystalline sample of a similar material by using the measured displacement field on the edge of a selected area as boundary conditions for finite element method (FEM) computation. The resulting optimal material parameters for the single crystal are found to be consistent with the values available in the literature, and the whole simulated strain fields as well as the evolution of the crystallographic texture, is compared satisfactorily with the experimental data

Caractérisation et modélisation du comportement mécanique de matériaux composites SiC/SiC

National audienceLes composites SiC/SiC sont envisagés comme matériaux de coeur des réacteurs nucléaires de 4e génération. Leur comportement mécanique à été étudié par le biais d'essais biaxés, ayant permis la construction et l'identification d'un modèle d'endommagement ainsi que son implantation dans un code éléments finis

Multiscale approach of mechanical behaviour of SiC/SiC composites: Elastic behaviour at the scale of the tow

SiC/SiC composites are candidates for structural applications at elevated temperatures in the context of the development of the 4th generation of nuclear reactors. A multiscale approach is under development to construct a predictive modelling of their complex mechanical behaviour due to their heterogeneous microstructure. This approach is based on two scale transitions: from the fibres/matrix microstructure to the tow and from the tow to the woven composite, each scale presenting a significant residual porosity. This paper focuses on the first scale transition and on the modelling of the elastic behaviour of the tow at room temperature. A microstructural investigation of several tows in a 2D SiC/SiC specimen has been conducted using scanning electron microscopy to get statistical data on microstructural characteristics by image analysis in order to generate a virtual microstructure. The elastic problem of homogenisation is numerically solved by means of finite element techniques. The simulations performed on various volumes show noticeable fluctuations of the apparent behaviour: so separation of length scales is not satisfied in this material. Nevertheless, this problem is neglected in a first approximation and the homogeneous equivalent behaviour is evaluated by averaging the apparent behaviours of several volume elements – smaller than the Representative Volume Element (RVE) – called Statistical Volume Elements (SVEs). Finally, influence of porosity and pores’ morphology is quantified