Fabrication and microgeometry of two-step braided composites incorporating pultruded rods

Abstract

The concept of using consolidated rods as axial reinforcements in two-step braided composites has been demonstrated in this paper. The composite fabrication consists of three major processes: the fabrication of unidirectional rods, 1 mm in diameter, by pultrusion, the formation of three-dimensional preforms by a modified two-step braiding set-up with the rods as axial reinforcements, and the impregnation of resin into the preforms by resin transfer molding, followed by a curing process to consolidate the composites. In comparison with conventional three-dimensional fabric composites, this approach (1) essentially eliminates both axial and braiding yarn crimp in the composite interior, (2) enhances fabric consistency, (3) provides fairly rigid fabrics, and (4) facilitates resin infiltration in resin transfer molding. The resulting composite geometries have been experimentally characterized. The mechanism of braider slip was found to dominate the resulting interlacing pattern. According to the micrographs, the rods were placed regularly within the fabrics with satisfactory compactness. As squeezed by the rods, the braiders are thin and close to rectangular. Two families of specimens with varying take-up distances and braider sizes have been made, and the corresponding pitch length, yarn content, rod gap, braider cross-section, and interlacing pattern have been correlated with these varying parameters. The processing characteristics and the inherent limitations of this method are discussed in detail.