On the microstructure of three-dimensional braided preforms

This paper describes a study of tbe microstructure of 3D braided preforms produced by the four-step 1x1 method. An analytical approach is employed in conjunction with experimental investigation to establish the relationship between the braid structure and braiding parameters. Microscopic evidence of the microstructure of preforms reveals the different configurations of the yarns in the interior, surface and corner regions of a braided preform. On the basis of microscopic observations, three types of microstructural unit cell models are established for the three regions. By using these three unit cell models, the structural geometry of the preforms is analyzed and the mathematical relationships among the structural parameters, such as the yarn packing factor, fiber orientation, fiber volume fraction, braiding pitch, are derived. From the preforms to the finial composites, if the RTM process is utilized, the unit cell size and shape will be changed. The modification of the relationships among the structural parameters has been made. Good agreement has been obtained between the calculated and measured values of the geometric characteristics of braided composite samples. (Example material: T300 carbon fiber/epoxy resin.)