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

Abstract

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

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

“…[Castelier et al (2010)] showed that the three families of models tend to provide identical results as the composite become very long. All these modeling principles have been defined for the case of a microcomposite (containing a single fiber) and they are directly extended to minicomposites, neglecting the propagation of the matrix crack through the section of the minicomposite (despite experimentally observed [Chateau et al (2011)]).…”

“…First, some 1D models [Lissart and Lamon (1997), ] used the classical fiber bundle assumptions (developed by [Daniels (1945)]) according to which once broken, a fiber does not participate anymore to the load transfer throughout the whole composite, and as a consequence breaks only once. However, a fiber break characterization in SiC/SiC minicomposites (detailed in [Chateau et al (2011)]) recently showed that a fiber can break several times. So observations are rather in line with other family of approaches taking into account a sliding length around a break necessary for the fiber to recover back its previously carried stress.…”

Analysis of the damage initiation in a SiC/SiC composite tube from a direct comparison between large-scale numerical simulation and synchrotron X-ray micro-computed tomography

“…GPa and Poisson's ratio of 0.2 (values similar to those used in [56]). This is a reasonable assumption as long as the mechanical behavior remains in the elastic regime, for the following reasons.…”

“…Firstly, both SiC fibers and matrix are commonly considered as isotropic materials (see e.g. [16,56]). Although the SiC matrix exhibits an anisotropy resulting from the CVI process, the elastic modulus of -SiC varies moderately in different crystallographic directions (face-centered cubic system), hence the assumption of isotropic elastic property of SiC matrix can be admitted (Chapter 3 in [57]).…”

“…Another possible approach is to consider the tows as homogeneous media whose properties are determined by some analytical or numerical models [59]. However, the multiscale nature of SiC/SiC composites, and especially the distributions and shapes of micropores, make the scale separation impossible [56]: the size of the RVE of the uniaxial composite microstructure is larger than the size of the tow. As a result, it is believed that the approach followed herein is a better way to simulate the heterogeneities in the composites.…”