Multiscale Modeling of Random Microstructures in SiC/SiC Ceramic Matrix Composites within MAC/GMC Framework

Abstract

Advanced ceramic matrix composites (CMCs) are being aggressively pursued as potential candidates for many aerospace applications as they offer a unique combination of high temperature strength, creep resistance, low density, high thermal conductivity and low thermal expansion. Low fiber volume fraction composite systems like CMCs inherently have more microstructure variations (location of fiber centers – disordered/random packing, coating thickness etc.) than higher volume fraction polymer matrix composite (PMC) systems. They rarely resemble the ordered packing (square or rectangular packing) that is generally assumed with periodic boundary conditions employed for computational expediency thus raising the question of how should one model such systems effectively. Prior work on PMCs has shown that random microstructures can be well approximated by an ordered repeating unit cell (RUC) with hexagonal fiber packing or an RUC containing multiple randomly located fibers; both with periodic boundary conditions. In order to keep a proper balance between accuracy and efficiency when performing multiscale composite analyses, it is preferable to limit the number of fibers in the RUC to just a few. NASA’s in-house micromechanicsbased code MAC/GMC provides a framework to analyze such RUCs for the overall composite behavior. This framework embodies two basic micromechanics models, 1) Generalized Method of Cells (GMC) and 2) High Fidelity Generalized Method of Cells (HFGMC), that provide closed-form constitutive equations for composite effective properties, composite response, as well as local (microscale) stress and strain fields in the constituents of the composite. In this paper, the influence of micromechanics idealization (GMC and HFGMC), for both ordered and disordered microstructures, on the unidirectional and laminated composite effective properties, proportional limit stress (PLS) and fatigue life, will be examined for a typical ceramic matrix composite (CMC) with a weak interface.