Three composites materials, Glare 2 fiber metal laminates, graphite/epoxy (Gr/Ep) and carbon/carbon (C/C) have been tested mechanically under quasi-static loading in uniaxial and bending modes using uniform and notched specimens. Acoustic emission (AE) technique was utilized in tracking the damage accumulation profile during loading up to fracture in terms of AE counts rate and cumulative. In addition wavelet transforms was used to process AE signals in order to obtain both frequencies and time information on the main failure mechanism and the sequential events during fracture process. This was supported by light and electron microscopies characterizations. The mechanical and acoustical responses were examined with respect to orientation and temperature effects for the Glare 2, exposure temperature effect for the Gr/Ep and porosity degree effect for the C/C. Manifestly, the AE results demonstrate different damage build-up profiles and point to a transition in failure micro-mechanisms with respect to the influence of each parameter (temperature, orientation, and density) on the specific composite tested. For the Gr/Ep, the wavelet transforms indicate the sequence of events in the fracture process, from fiber breaks followed by debonding and ending with matrix cracking. In some cases, it has been observed that the damage accumulation profile in terms of AE resembled the dependency of the crack density versus the strain predicted by a micro-mechanical damage model.

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We used cube corner, Berkovich, cono-spherical, and Vickers indenters to measure the indentation modulus of highly oriented bulk pyrolytic carbon both normal to and parallel to the plane of elastic isotropy. We compared the measurements with elastic constants previously obtained using strain gage methods and ultrasound phase spectroscopy. While no method currently exists to extract the anisotropic elastic constants from the indentation modulus, the method of Delafargue and Ulm (DU) [17] was used to predict the indentation modulus from the known elastic constants. The indentation modulus normal to the plane of isotropy was ∼ 20% higher than the DU predictions and was independent of indenter type. The indentation modulus parallel to the plane of isotropy was 2-3 times lower than DU predictions, was depth dependent, and was lowest for the cube corner indenter. We attribute the low indentation modulus to nanobuckling of the graphite-like planes and the indenter type dependence to the impact of differing degree of transverse stress on the tendency toward nanobuckling.

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a b s t r a c tA multiscale modeling approach is utilized to evaluate the contribution of irregularly shaped threedimensional pores to the overall elastic properties of carbon/carbon composites. The degree of anisotropy of a carbon matrix depends on nanotexture, which is defined by manufacturing conditions. Elastic properties of the matrix are predicted assuming a Fisher distribution of orientations of graphene planes with respect to the pyrolytic carbon deposition direction. X-ray computed microtomography is employed to identify pores in a sample of carbon/carbon composite. The pores have highly irregular shapes so that micromechanical modeling based on the analytical solutions of elasticity becomes inapplicable. Thus, the cavity compliance tensor of an individual pore is found numerically by finite element method, and then used in a micromechanical modeling procedure. Examples of pores in isotropic and transversely isotropic pyrolytic carbon matrices are considered. The accuracy of pore approximation by ellipsoidal shapes is evaluated.