UC Santa Barbara

Ceramic matrix composites (CMCs) with woven fibers are of interest for aerospace applications due to their low density, high strength and melting temperature, as well as broad flexibility in placement of fiber tows to match anticipated stress fields. Even when placed according to design, tows experience non-uniformities in stresses and strains due to their waviness. The added complexity of constructing complex weaves can further lead to undesirable variability in the weave structure. The present study seeks to extend the current understanding of structure-property relations in woven CMCs.

It begins with the implementation of 3D digital image correlation (DIC) to characterize full-field strains and displacements with sub-tow spatial resolution. This enables the characterization of strain heterogeneities that develop in woven CMCs. 3D DIC is extended to characterize the variability in tow placement within 3D weaves. This variability exists at both short- and long-wavelengths. Short-wavelength variations are intrinsic to the weaving process and result in local variations in the packing density of the tows. Long-wavelength variations are attributed to shear deformation during handling after weaving. Discrete Fourier transforms of the intrinsic variations are used to provide a statistical representation of the weave structure. Using these statistics, along with the Monte Carlo method, virtual specimens are generated.

The effects of weave structure on surface strain evolution are investigated experimentally on a SiC/SiC CMC and through a meso-scale finite element model. Experimentally, strain elevations and failure initiation are observed on wavy segments of surface tows. Model predictions show similar features and indicate that strains are elevated in response to bending and straightening of the wavy tow segments. Tow straightening is accommodated by out-of-plane motion of the tows and can be suppressed to some extent by the underlying (sub-surface) plies. The constraint in the present material is low, because of porosity and micro-cracks present in the matrix-rich regions between the plies.

Additionally, the study addresses the intermediate temperature embrittlement phenomenon endemic to SiC/SiC CMCs, in both water vapor and dry air environments. Oxidant ingress occurs through existing porosity and micro-cracks in the matrix, prior to the formation of stress-induced matrix damage. This process is sluggish, reducing the oxidant activity within the composite, resulting in preferential oxidation of the Si-based constituents prior to the BN fiber coating. These findings are in contrast to the current understanding in the literature.