Introduction

Three-dimensional textile preforms are fully integrated continuous-fiber assemblies with multi-axial in-plane and out-of-plane fiber orientations (Chou, McCullough and Pipes 1986; Ko 1989a). Composites reinforced with three-dimensional preforms exhibit several distinct advantages which are not realized in traditional laminates. First, because of the out-of-plane orientation of some fibers, three-dimensional preforms provide enhanced stiffness and strength in the thickness direction. Second, the fully integrated nature of fiber arrangement in three-dimensional preforms eliminates the inter-laminar surfaces characteristic of laminated composites. The superior damage tolerance of three-dimensional textile composites based upon polymer, metal and ceramic matrices has been demonstrated in impact and fracture resistance. Third, the technology of textile preforming provides the unique opportunity of near-net-shape design and manufacturing of composite components and, hence, minimizes the need for cutting and joining the parts. The potential of reducing manufacturing costs for special applications is high. The overall challenges and opportunities in three-dimensional textile structural composites are very fascinating.

Three-dimensional textile preforms can be categorized according to their manufacturing techniques. These include braiding, weaving, knitting and stitching, as shown in Fig. 7.1.

There are three basic braiding techniques for forming three-dimensional preforms, namely 2-step, 4-step and solid braidings. In the case of 2-step braiding, the axial yarns are stationary and the braider yarns move among the axials. Thus, the axial yarns are responsible for the high stiffness and strength in the longitudinal direction and relatively low Poisson contraction. A high degree of flexibility in manufacturing can be achieved in 2-step braiding by varying the material and geometric parameters of the axial and braider yarns.