Abstract
Two-unit cells developed to predict the effective thermal conductivities of four-directional carbon/carbon composites with the finite element method are proposed in this paper. The smaller-size unit cell is formulated from the larger-size unit cell by two 180° rotational transformations. The temperature boundary conditions corresponding to the two-unit cells are derived, and the validity is verified by the temperature and heat flux distributions at specific positions of the larger-size unit cell and the smaller-size unit cell. The thermal conductivities of the carbon fiber bundles and carbon fiber rods are measured firstly. Then, combined with the properties of the matrix, the effective thermal conductivities of the four-directional carbon/carbon composites are numerically predicted. The results in transverse direction predicted by the larger-size unit cell and the smaller-size unit cell are both higher than experimental values, which are 5.8 to 6.2% and 7.3 to 8.2%, respectively. In longitudinal direction, the calculated thermal conductivities of the larger-size unit cell and the smaller-size unit cell are 6.8% and 6.2% higher than the experimental results, respectively. In addition, carbon fiber rods with different diameters are set to clarify the influence on the effective thermal conductivities of the four-directional carbon/carbon composites.
Highlights
The four-directional carbon/carbon (4D C/C) composites are lightweight and have good thermodynamic properties, since carbon fiber is used as the reinforcement
Carbon fiber rods with different diameters are set to clarify the influence on the effective thermal conductivities of the four-directional carbon/carbon composites
The results show that the finite elements method (FEM) is effective for the performance calculation of 3D braided composites, and the meso-structure of 3D braided composites can be considered [22,23,24]
Summary
The four-directional carbon/carbon (4D C/C) composites are lightweight and have good thermodynamic properties, since carbon fiber is used as the reinforcement. Gou [21] used multi-size unit cells to predict the effective thermal conductivities of carbon fiber braided composites. The thermal conductivities of the unit cells are predicted by the calculated results of the carbon fiber rods, carbon fiber bundles and the matrix, and were used to evaluate the thermal conductivities of the 4D C/C composite. Constructing smaller-scale unit cells is the key to simplify the prediction of the effective thermal conductivities of composites in finite element analysis that can substitute the structure of larger-scale composites [15]. A smaller unit cell is proposed to predict the thermal conductivities of 4D C/C composites using rotational symmetry in this work It was used in finite element analysis to save computing time and computing resources.
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