Emerging technologies demand the use of materials with superior properties e.g. high strength-to-weight ratios and excellent functionalities. As such, carbon fibres are extensively being applied in composite materials in various emerging technologies. Carbon fibres possess excellent properties that reinforce them for adoption in new technological advances that demand such characteristics. In order to ascertain their use, experimental characterization and micromechanics techniques have been evolved to aid probe the effective behavior of carbon fibre-reinforced composites. Without loss of generality, the carbon fibers’ material properties are assumed as transversely isotropic. However, it is already known that this is not exactly true. The material properties of carbon fibers are determined by the underlying morphological microstructures, such as the graphic crystals and crystal directions. Unfortunately, to date, reports concerning the morphological effect of carbon fibers on the effective mechanical properties of composites remain scarce in the literature with only a few exceptions. Specifically, very little work has been conducted on the morphological effect of the carbon fibers on the thermal conductive properties of composites.
The real difficulty pertains predicting the thermal response of composites in the transverse direction. In addition, existing literature has shown that the classical micromechanics approach cannot recover the localized field distributions while the numerical techniques require large-scale mesh discretization. To address these issues, researchers from Zhejiang, China: Dr. Guannan Wang, Ms. Mengyuan Gao and Prof. Bo Yang, in collaboration with Dr. Qiang Chen at the Ecole Nationale Supérieure d’Arts et Métiers in France, proposed to extend the recently developed mechanical version of the locally-exact homogenization theory (LEHT) to investigate the effective and localized thermal behavior of periodic composites reinforced with transversely isotropic, radially orthotropic or circumferentially orthotropic fibers. Their work is currently published in the research journal, International Journal of Heat and Mass Transfer.
The researchers employed the Trefftz concept so as to obtain the internal complete functions for the fibrous and matrix domains by solving the heat conduction governing equations. The unknown coefficients were solved through point-wise satisfaction of interfacial continuities and generalized balanced variational principle for temperature/heat-flux boundary conditions. After obtaining the internal fields, the effective thermal conductivities were obtained through homogenized Fourier law. Additionally, the convergence of the numerical solutions was tested by generating thermal responses with different harmonic terms. The effect of fiber morphologies on the effective and localized responses was finally demonstrated for composites with large fiber volume fractions, illustrating the significant fiber-fiber interaction that cannot be neglected at high volume fractions, See Figure below.
The authors reported that their theory exhibited good agreement with the independently developed Eshelby solutions and Hashin’s formula. Specifically, it was established that morphology significantly affects the microstructural behavior of carbon fiber-reinforced composites. Overall, results presented clearly indicated that even if the homogenized properties were 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.
In summary, the study developed LEHT with thermal conductive capability to investigate the morphological effect of carbon/graphite fibers on the effective and localized thermal responses of periodic composites. Remarkably, the presented theory was validated against the independently developed Eshelby solutions and Hashin’s formula, where was performed well. In a statement to Advances in Engineering, Dr. Qiang Chen highlighted that the developed theoretical derivations could be expanded to the functionally graded materials with microstructural morphological features and temperature gradients.
Guannan Wang, Mengyuan Gao, Bo Yang, Qiang Chen. The morphological effect of carbon fibers on the thermal conductive composites. International Journal of Heat and Mass Transfer, volume 152 (2020) 119477.