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.

This paper describes the splicing of an 8 × 8 484 cross-break track, based on the carrier arrangement characteristics (CAC) method for track splicing technology. The arrangement period of the carrier (T) in the track is obtained in order to discern which slot has a carrier in a period and which one does not. Six basic types of track unit are deconstructed, and a mathematical model is established to determine the possible state of the carrier in each slot of the track unit. Finally, the specific pattern of the carrier arrangement in the 484 cross-break track unit is obtained and verified using a three-dimensional (3D) rotary braiding machine with a square track disc. This provides a reference for solving the carrier arrangement of a 3D square rotary braiding machine.

This paper focuses on the thermomechanical properties of cylindrical short-fiber-reinforced composites. Under the premise of an equal average aspect ratio, a more systematic exploration was conducted to assess the relevance of short-fiber length characteristics to the thermomechanical of the composites. A random modeling algorithm for short fibers based on Monte Carlo theory is proposed to generate more realistic models of short-fiber composites. Finite-element simulations were combined to quantify the effective thermal conductivity, coefficient of thermal expansion, and elastic modulus of the composites models, providing comprehensive thermomechanical data for statistical evaluation and comparison of the results. The comparison between the simulation results and experimental results demonstrates the reliability of the short-fiber composites model. The analysis of the resulting data indicates that the effect of different aspect ratio distributions of short fibers on the thermomechanical properties of the composites is negligible. The maximum errors for the three thermomechanical properties are 1.65%, 0.52%, and 1.4%, respectively. This method provides an effective supplement for understanding the thermomechanical properties of short-fiber composites.

The method of braiding in three-dimensional (3D) array rotary braiding offers considerable adaptability, enabling the attainment of diverse mechanical characteristics through the braiding of assorted structural patterns. However, the relationship between the track form of 3D array rotary braided composites and the structure of braided fabrics has not been studied previously. The discussion centers on how the shape of the braiding track affects the structure of braided fabrics, with the study of the guide track’s configuration. The “oblique track” and the “484 cross–break” track are analyzed, and according to the collision problem between the carriers in the tracks, the law of restraint on the carriers’ arrangement in the tracks is proposed to ensure full occupation carrier positions in the four-slot horn gears. The forming principle of the braiding is further studied, and the braiding forming unit is confirmed. The results show that the interior unit cells have different topological characteristics in different braiding yarn track forms, and the yarn size, orientation, and fiber content of braided yarn are closely related to the braiding strength of braided yarn on the fabric.