The morphological effect of carbon fibers on the thermal conductive composites

Abstract

The locally-exact homogenization theory (LEHT) with thermal conductive capability is developed to investigate the morphological effect of carbon/graphite fibers on the effective and localized thermal responses of periodic composites. Based on the orientations of basal planes, the material properties of carbon fibers can be transversely isotropic, radially orthotropic or circumferentially orthotropic, possibly influencing the microscopic behavior of thermal conductive composites. By taking fiber-fiber interactions into account, repeating unit cells (RUCs) of hexagonal and rectangular geometries are considered with large fiber volume fractions. The efficiency and stability of the LEHT are guaranteed by solving the complete internal eigenvalue functions, imposing point-wise continuity conditions, as well as implementing the generalized variational principle. It is demonstrated that the present theory exhibits good agreement with the independently developed Eshelby solutions and Hashin's formula. The morphological effects are also tested by generating effective coefficients over a wide range of fiber volume fractions and recovering the local thermal field concentrations within the composite microstructures. The results clearly indicate that even if the homogenized properties are not significantly affected by the morphologies of carbon fibers by satisfying the replacement scheme, the large heat-flux gradients within the orthotropic fibers could still lead to the split of carbon reinforcement.