Figure 11. Weft- and warp-insertion knits: (a) weft knit, (b) warp-insertion weft knit, and (c) and (d) weft and warp insertion.

panels (fig. 14(b)), core structures simulating a box beam (fig. 14(c)), or truss-like structures (fig. 14(d)). Furthermore, by properly manipulating the warp yarns, as exemplified by the angle interlock structure (fig. 14(e)), the through-the-thickness yarns can be organized into a diagonal pattern. To address the inherent lack of in-plane reinforcement in the bias direction, new progress is being made in triaxial weaving technology by Dow et al. (ref. 7) to produce multilayer triaxial fabrics, as shown in figure 15.

Orthogonal Nonwoven Fabrics

Aerospace companies such as General Electric (ref. 8) pioneered nonwoven 3-D fabric technology, which was developed further by Fiber Materials Incorporated (ref. 9). Recent progress in automating the nonwoven 3-D fabric manufacturing process was made in France by Aerospatiale (refs. 10 and 11), Brochier (ref. 12), and SEP (ref. 13) and in Japan by Fukuta of the Research Institute for Polymers and Textiles (refs. 14 and 15).

The structural geometries resulting from the various processing techniques are shown in figure 15 (ref. 16). Figures 16(a) and 16(b) show the single bundle XYZ fabrics in a rectangular and cylindrical shape. Figure 16(c) demonstrates the multiple-yarn bundle possibilities in the various directions; figure 16(d) shows the multidirectional reinforcement in the 3-D structure plane. Although most of the orthogonal nonwoven 3-D structures consist of linear yarn reinforcements in all of the directions, introduction of the planar yarns in a nonlinear manner, as shown in figures 16(e), 16(f), and 16(g), can result in either an open lattice structure or a flexible and conformable structure.

Figure 14. Structural geometry of 3-D woven fabrics: (a) solid orthogonal panels, (b) variable-thickness solid panels, (c) core structures, (d) trusslike structures, and (e) angle interlock structure.

Knitted 3-D Fabrics

The knitted 3-D fabrics are produced either by the weft knitting or warp knitting process. An example of a weft knit is the near-net shape structure knitted under computer control by the Pressure Foot® process (ref. 17) (fig. 17). In a collapsed form, this preform has been used for carbon-carbon aircraft brakes.

The unique feature of the weft-knit structures is their conformability (ref. 18). When additional reinforcement is needed in the 0° and 90° directions, linear laid-in yarns can be placed inside the knitting loops as shown in figure 18. The most undesirable structural reinforcement feature of weft-knit structures is their bulkiness, which leads to the lowest packing density or lowest level of maximum fiber-volume fraction compared with other fabric preforms. Although the weft-knitted structures have applications in limited areas, the multiaxial warpknit (MWK) 3-D structures are more promising, and they have undergone more development in recent years (refs. 19 and 20).

Figure 16. Complex structural shapes by 3-D braiding: (a) rectangular shape, (b) cylindrical shape, (c) multiple-yarn bundle possibilities, (d) multidirectional reinforcement in plane of three-dimensional structure, and (e), (f), and (g) planes moving in nonlinear structure.