*5.1.2. Braided fabric structure*

Nishimoto et al. [113] investigated 2D circular biaxial braided fabrics. They used a step response model to examine the temporal change in braiding angle under unsteady-state conditions. An examination of the flow pattern during the consolidation revealed that the permeability of the fabric is determined by spaces between the fibers especially in the case of low braid angle. Permeability and porosity may result in a non-uniform flow pattern during liquid molding [114]. The effect of braiding angle on the mechanical properties of 2D biaxial braided composite was analyzed. It was shown that when the braiding angle is increased, the bending modulus and strength decreased [115]. Smallest braiding angle (approximately 15˚) resulted in the highest bending properties [116]. The mechanical properties of 2D biaxial 2×2 pattern braided fabric composites were studied by a 3D finite element micromechanics model and compared with equivalent 2×2 twill fabrics to analyze their fracture modes under various loading requirements [117]. The biaxial compressive strength properties of 2D triaxial braided cylinders were investigated. It was reported that the fiber waviness affects the axial compres‐ sion strength. The composites exhibited considerably higher compression and tension strength in the axial direction when compared to those in the braid direction [118]. Smith and Swanson [119] studied the response of 2D triaxial braided composites under compressive loading. It was shown that the laminated plate theory provided good stiffness predictions for low braid angle, whereas a fiber inclination model yielded close estimations for various braid angles. Tsai et al. [120] investigated the burst strengths of 2D biaxial and triaxial braided cylindrical composites. It was reported that the crack formation in biaxial braided composite starts in tow direction. In a triaxial fabric composite, on the other hand, the cracks first appear in the longitudinal direction.

Byun [121] developed an analytical approach to predict the geometric characteristics and mechanical properties of 2D triaxial braided textile composites. The model is based on the unit cell geometry of the braided structure. It was reported that the geometrical model can accurately predict some important properties such as the braid angle and fiber volume fraction. An averaging technique based on the engineering constants was used to calculate the stiffness properties of the composites. It was shown that the averaging technique yields more precise results when worked with small braid angles. It was also reported that the model gives more accurate results when the bundle size of the axial yarns is much larger in comparison with that of the braider yarns. Yan and Hoa [122] used an energy approach for predicting the mechanical behavior of 2D triaxially braided composites.
