Elastic moduli of a 2.5D Cf/SiC composite: experimental and theoretical estimates

This study concerns the characterisation of the elastic properties of a long-fibre-reinforced ceramic-matrix composite. Seven of the nine independent elastic constants of a woven 2.5 D carbon-fibre reinforced SiC ceramic matrix have been measured by an ultrasonic technique associated with a numerical optimisation process. The elastic moduli are recovered by minimising the square deviation between measured and theoretical velocities. The ultrasonic measurements are discussed with the approximation of continuum mechanics, in regard to the wavelength and the size of the microstructural details of the material. For comparison, an estimate of the elastic moduli in all directions is performed with an Eshelby-based model, assuming the composite as a two-dimensional tow-reinforced matrix containing voids. The effective moduli for the tow composite structure are estimated from a first homogenisation step. Volume fractions of tow and matrix cracks slightly opened are taken into account in the microstructural description of the composite from experimental data on mean crack orientations, quantities and shapes. The specific effect of neglecting waviness is estimated from finite-element calculations. In the limit of the uncertainties on the phase moduli, the estimates of elastic moduli are in agreement with the available measured ones. As relevant, estimates are thus provided for the missing measurements.