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

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

Identification of crystalline behavior on macroscopic response and local strain field analysis: application to alpha zirconium alloys

The purpose of this paper is to present an identification method of the crystalline behavior of a material from a mechanical test performed on a polycrystalline sample. Because of the lack of knowledge about its crystalline behavior, this method is applied to a Zirconium alloy. This identification is based on a finite element modeling of the microstructure, and the results are compared to both the macroscopic and the microscopic experimental results. On the microscopic scale, the plastic strains are obtained using a micro-extensometry technique and the crystalline orientation using an EBSD technique. In order to validate the method, an identification is performed with only two free parameters: the evolutions of the macroscopic and microscopic errors appear to be regular and exhibit a well-defined minimum so that the parameters can be clearly identified

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

Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

Intragranular localization induced by softening crystal plasticity analysis of slip and kink bands localization modes from high resolutionFFT-simulations results

International audienceWe investigate the ability of local continuum crystal plasticity theory to simulate intense slip localization at incipient plasticity observed experimentally in metals exhibiting softening mechanisms. A generic strain softening model is implemented within a massively parallel FFT solver framework to study intragranular strain localization throughout high resolution polycrystalline simulations. It is coupled to a systematic analysis strain localization modes: Equivalent plastic strain and lattice rotation fields are processed to create binary maps of slip and kink bands populations, estimate their volume fraction and mean strain level. High resolution simulations show the formation of an intragranular localization band network. The associated localization maps are used to identify accurately slip and kink bands populations and highlight the distinct evolution of kink bands, influenced by lattice rotation. Results highlight that the analysis of the nature of localization bands in numerical studies is fundamental to asses the validity of polycrystalline simulations. Indeed, it is evidenced that selection between slip or kink localization modes is only due to grain to grain incompatibilities as these two localization modes are equivalent in classical crystal plasticity models. As a result they predict the formation of a large amount of kink bands in contradiction with experimental observations of softening metals. We show that this holds for complex physics based models too. Hence, the use of classical crystal plasticity for strain localization simulation should be reconsidered in order to predict realistic localization modes