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of three or more yarns in the thickness direction, is a fibrous network wherein yarns pass from surface to surface of the fabric in all three directions.

The key criteria for selecting the fiber architecture for structural composites are: the capability for in-plane multiaxial reinforcement, the through-the-thickness reinforcement, and the capability for formed-shape and/or net-shape manufacturing. Depending on the processing and end-use requirements, some or all of these features are required. In this section, the representative structural geometry of 2-D and 3-D fabrics is introduced.

Structural Geometry of 2-D Fabrics
Woven Fabrics

The interlacing of yams fabricates the hundreds of possible woven fabric combinations. From the in-plane fiber orientation, woven fabrics can be divided into biaxial and triaxial woven structures.

Biaxial weaves consist of 0° and 90° yarns interlaced in various repeating patterns or topological unit cells. The three basic weave geometries from which many other patterns evolve are the plain, twill, and satin weaves. A schematic diagram of various views of these three basic weaves is shown in figure 6. The frequency of yarn interlacing and the linearity of the yarn segments distinguish these three fabrics. The plain weave has the highest frequency of yam interlacing, whereas the satin weave has the least number of yam interlacing, with the twill weave somewhere in between. Accordingly, the plain weave has a higher level of structural integrity and greater ductility because of the crimp geometry produced by yam interlacings. On the other hand, the satin weave has the highest level of riber-to-fabric strength and modulus translation efficiency because of the low level of yam interlacing and yam linearity. The low level of yam integration in satin weave also allows freedom of yam mobility, which contributes to higher fiber packing density and. consequently, higher levels of fiber volume fraction.

Although cane weaving for cane chairs has existed for a long time, machinemade triaxially woven fabrics weren't available until Norris Dow's development of the triaxial weave in the early 1970's (ref. 5). The unique feature of triaxial weave is the 90±60° hexagonal yam orientation in one plane, resulting in a high level of in-plane shear resistance. High levels of isotropy and dimensional stability can be achieved with triaxial weave at low fiber volume fraction. Figures 7 and 8 show a schematic diagram of two triaxial weave geometries.

Knitted Fabrics

Knitted fabrics are interlooped structures wherein the knitting loops are either produced by the introduction of the knitting yam in the cross-machine direction (weft knit) or along the machine direction (warp knit). As shown in figures 9 and 10. knitting can produce a large number of stitch geometries. By controlling the

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Figure 6. Structural geometry of biaxially woven fabrics: (a) plain weave, (b) twill weave, and (c) satin weave.

Figure 7. Structural geometry of triaxially woven fabrics (basic weave).

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Figure 8. Structural geometry of triaxially woven fabrics (biplain weave).

stitch (loop) density, a wide range of pore geometry can be generated. Because of the nature of the interlooped structure, the maximum fiber packing density of knitted structures is lower than that of the woven fabrics. The severe bending of yarns during the knitting process also discourages converting ceramic yarns to knitted structures. However, if a creative combination of ceramic sewing threads or very fine yarns is used to form the stitches, knitted fabrics may be used effectively as a base structure for the incorporation of 0° and/or 90° yarns (fig. 11). The resulting preform is an integrated structure that combines the high conformability of the knitted base structure with a high level of directional reinforcement from the straight lay-in yarns.

Braided Fabrics

By intertwining three or more yarn systems together, braided fabrics can be produced in flat or tubular form (fig. 12). The bias interlacing nature of the braided fabrics makes them highly conformable, shear resistant, and tolerant to impact damage. To enhance reinforcement in the 0° direction, triaxial braiding can be used to introduce 0+0 yarns as shown in figure 13. Although braiding and filament winding bear some similarities, subtle differences exist between these two processes and the resulting structures. Table III provides a comparison of braiding and filament winding. In reference 6, the subject of braiding is treated in greater detail.

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Structural Geometry of 3-D Fabrics

The structural geometry of 3-D fabrics will be discussed according to the four basic methods of textile manufacturing: weaving, orthogonal nonwoven, knitting, and braiding.

Woven 3-D Fabrics

The 3-D woven fabrics are produced principally by the multiple-warp weaving method that has long been used to manufacture double cloth and triple cloths for bags, webbings, and carpets. Fabrics with as many as 17 layers have been successfully woven. By this weaving method, various fiber architectures can be produced including solid orthogonal panels (fig. 14(a)), variable-thickness solid

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