Elastic constants of a graphite-magnesium composite

This study contains three components: measurement, modeling, and inverse modeling to get the fiber elastic constants. The studied composite consisted of 70 vol % continuous uniaxial graphite fibers in a magnesium matrix. By ultrasonic-velocity methods, we measured the composite’s complete orthotropic-symmetry (nine-independent-component) elastic-constant tensor: Cij in Voigt contracted notation. Approximately, the composite shows transverse-isotropic symmetry. For a model, we used a wave-scattering method in the long-wavelength limit. The model requires two inputs: the two isotropic matrix elastic constants and the five anisotropic fiber elastic constants. We measured the first and guessed the second based on graphite-fiber elastic constants reported by others. This guess gave good measurement-model agreement only for C11, C22, and C33. Especially, the shear moduli Cii (i=4,5,6) agreed poorly (a 20% difference). Using inverse modeling—calculating fiber properties from measured matrix properties and measured composite properties—we estimated the anisotropic fiber elastic constants. For the fiber we predict a Young-modulus anisotropy of 12 and a principal Poisson ratio range of 0.03–0.37.