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Needle-punching is used as an alternative to expensive and sophisticated three-dimensional (3D) weaving processes to prepare a 3D composite. In this study, a 3D needled carbon–carbon (C/C) composite structure was examined using X-ray tomography and scanning electron microscopy (SEM). The effects of manufacturing porosities, needling diameter and needling density on the thermal conductivity of the composite were determined through multiscale finite-element modelling. The degradation of thermal conductivity caused by the manufacturing porosity was higher in the longitudinal direction than in the through-thickness direction. Moreover, it was found that the through-thickness thermal conductivity of the composites increased with increasing needling diameter and density.

Detailed thermal and mechanical finite element analyses of woven composites are computationally challenging due to the heterogeneous nature and the geometrical complexity of the composite. In this paper two finite element three-dimensional image-based models at different length scales are used to evaluate the thermal diffusivity and stiffness of a 2D carbon/carbon composite. The micro-scale model was developed from SEM micrographs of the carbon tow whereas the macro-scale model was derived from high resolution x-ray tomographic images of the composite. The micro-scale model predicts thermal conductivities and Young's modulus at the tow scale in the three orthogonal directions (x, y and z). The output results from the micro-scale model are then incorporated in the macro-scale model to obtain through-thickness thermal diffusivity and in-plane Young's modulus. The modelling results are in excellent agreement with the experimental results obtained from the laser flash and tensile tests and the deviations are within the bounds of numerical error of 5%.

The thermal properties of carbon/carbon composite are strongly affected by manufacturing porosity. This paper focuses on the characterisation of the manufacturing macro-porosity of a 2D carbon/carbon composite using X-ray computed tomography. The different types of manufacturing porosity were classified and quantified according to their size and location. Three types of macro-porosity were identified using computed tomography, namely trans-tow cracks, interfacial cracks and dry zones. A composite unit cell representing the three types of porosity was developed to model and investigate the effective transverse thermal transport properties (thermal conductivity and diffusivity) of the carbon/carbon composite. Finite element simulations and theoretical calculations were performed and compared with laser flash tests for validation. The influence of the stacking sequence of the laminates on porosity distribution and transverse thermal conductivity of the carbon/carbon composite was also investigated.

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