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recently have been introduced. The structural integrity of the fibrous preform is derived mainly from inter-fiber friction. The strength translation efficiency, or the function of fiber strength translated to the fibrous assembly of the reinforcement system, is quite low.

The second category of fiber architecture is the continuous filament or unidirectional (0°) system. This architecture, which has the highest level of fiber continuity and linearity, has the highest level of property translation efficiency and is very suitable for filament-wound and angle-ply-tape layup structures. The drawback of this fiber architecture is its intra- and interlaminar weakness because of the lack of in-plane and out-of-plane yarn interlacings.

A third category of fiber reinforcement is the planar interlaced and interlooped system. Although the intra-laminar failure problem associated with the continuous filament system is addressed with this fiber architecture, the matrix strength limits the interlaminar strength because of the lack of through-the-thickness fiber reinforcement.

The fully integrated system forms the fourth category of fiber architecture wherein the fibers are oriented in various in-plane and out-of-plane directions. With the continuous filament yam. a 3-D network of yam bundles is formed integrally. The most attractive feature of the integrated structure is the additional reinforcement in the through-the-thickness direction that makes the composite virtually delamination-free. Another interesting aspect of many of the fully integrated structures such as 3-D woven, knits, and braids is their ability to assume complex structural shapes.

Besides having a strong influence on the directional contribution of fiber properties to the composite, fiber architecture also dictates the ease of matrix placement in composite processing. For this review, we concentrate on the linear, planar, and 3-D fiber assemblies. Figure 1 provides an overview of these three categories of fiber architecture showing the sub-groups of each class. A detailed description of the classes of fiber architecture follows.

Linear Fibrous Assemblies

Yams and ravings are linear fibrous assemblies composed of discrete or continuous fibers. The basic differences between ravings and yams are listed:

1. Rovings are large fiber bundles with little or no twist.

2. Yams are finer fiber bundles with some twist.

Yams that are composed of discrete fibers are called staple yams; yams having continuous fibers are continuous-filament yams. The majority of high-performance yams are continuous-filament yams having single or multiple strands. For example, Avco SCS fibers are available in single-strand and are referred to as monofilaments. The AS4 12-K yams composed of 12000 filaments are referred to as multifilaments. Figure 2 shows the geometric features of filament yams.

The technology for converting short fibers into yam assemblies is well established (ref. 2). For high-modulus fibers that exist in staple form (such as single crystal whiskers and asbestos), special yam formation techniques have been developed. For example, researchers at Los Alamos National Laboratory (fig. 3) are growing long SiC whiskers (75 mm to 100 mm in length by 3 lim to 10 lim in diameter) for spinning into staple yams (ref. 3). Similar methods may be applicable for the vapor-grown carbon fibers developed at the General Motors Research Laboratory

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Figure 1. Classification of fiber architecture.

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High-bulk filament yarn

Figure 2. Structural geometry of filament yarns.

(ref. 4). Through proper selection of whisker length and yam twist level, the pore structure and mechanical properties of the staple yarn can be engineered for weaving into 2-D or 3-D fabrics for CCC.

Motivated by the need for finer carbon yams, Heltra, a division of Courtaulds, has also developed a staple yam formation method using a stretch breaking process. Compensating for the inevitable reduction in tensile strength, the protruded fibers on the transverse direction of the staple yams (as shown schematically in fig. 4(a)), can potentially improve the through-the-thickness strength and shear resistance and also the toughness of the CCC. This improvement in properties is derived from the subtle interlocking of the protruding fiber creating a network of in-plane and out-of-plane crack arrestors. An example of a yam manufacturing process that produces protruding fibers in the transverse direction is the family of hispiduous yarns created by the chenille process (fig. 4(b)).

Fabric Preforms

A fabric is defined as an integrated fibrous structure produced by fiber entanglement of yam interlacing, interlooping, intertwining, or multiaxial placement. Fiber felts, composed of fabrics formed directly from fibers, are an example of fiber-to-fabric structures. This research concentrates on yam-to-fabric structures. Table II compares the four basic yam-to-fabric formation techniques, and figure 5 shows examples of fiber architecture created by these techniques.

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Figure 3. Spinning of staple yarns from whiskers.

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Figure 4. Structural geometry of (a) spun and (b) hispiduous yarns.

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Biaxial High modulus Multilayer Triaxial Tubular Tubular braid Flat braid Flat braid
woven woven woven woven braid laid-in warp laid-in warp

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While the weaving, braiding, and knitting techniques can produce planar or 3-D structures, the nonwoven fabrics are mainly for 3-D systems. Yam orientation distribution and the number of yarn diameters in the thickness direction distinguish 2-D and 3-D fabrics. A 2-D fabric consists of two or three yam diameters in the thickness direction with fibers oriented in the x-y plane. A 3-D fabric, consisting

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