Microstructure and unit-cell geometry of four-step three-dimensional rectangular braided composites

This article investigates the microstructure of 3D four-directional rectangular braided composites produced by the four-step 1 × 1 procedure. The planar and spatial yarn paths are analyzed systematically based on 3D braiding process. Three different types of microstructural unit cell models are established with respect to the interior, surface, and corner regions of a braided preform. These models truly simulate the microstructure of the braided composites. Furthermore, they are oriented in the same reference frame of the preform cross-section, which coincides with the actual configuration of composites and suits for the analysis of mechanical properties. The mathematical relationships among the structural parameters, such as braiding dimensions, fiber orientation, braiding pitch, the yarn packing factor, fiber volume fraction, are deduced. The calculated results are in good agreement with the measured values of the geometric characteristics of braided composite samples.