Unidirectional weave

Unidirectional weave The number of threads is considerably higher in one direction than in the other (unidirectional warp fabric or unidirectional weft fabric). The threads are parallel and simply held together. 

As noted earlier, CVl is nsed primarily to form ceramic-fiber-reinforced ceramic matrix composites. The most common of these combinations is SiC fiber/SiC matrix composites. One commercially available product has a two-dimensional 0/90 layup of plain weave fabric and fiber volume fraction of about 40%. This same composite can be fabricated with unidirectional fibers and with 45° architectures. The most commonly used SiC fiber for the preforms is Nicalon , the mechanical properties for which were provided earlier in Section 5.4.2.7. A number of other carbide and nitride fibers are also available, including Si3N4, BN, and TiC. Preform geometries can be tailored to the application in order to maximize strength and toughness in the direction of maximnm stresses. The reactions used to form the matrix are similar to those used in CVD processes (cf. Section 7.2.4) and those described previously in Eq. (3.105). 

The constant k is called the Kozeny constant and it attains a value of 0.7 for well-ordered reinforcements with uniformly distributed fibers, (e.g. unidirectional prepreg) [17]. For commonly used fabrics in LCM (e.g., continuous strand mat or weaves) R2/4k should be seen as an adjustable model parameter with only weak coupling to the fiber or fiber bundle diameter. 

Another form is the glass cloth, composed of rovings. Weaving can take place in various patterns, varying from practically unidirectional to square weaves with equal properties in the two main directions. 

Anzelotti G, Nicoletto G, Riva E. Mesomechanic strain analysis of twiU-weave composite lamina imder unidirectional in-plane tension. Compos Part A Appl Sci Manuf 2008 39(8) 1294-301. 

Manufacturers of composite structures have traditionally used prepreg tape to manufacture structural components. Fibres are initially combined into unidirectional tows (bundles) of fibres combined into fabrics, e.g. by weaving or knitting. The vast majority of the tows employed in woven, braided or knitted reinforcements comprise low twist or untwisted continuous filament yams. Three-dimensional technical textiles can be produced by weaving [5], knitting [6] and braiding [7] or as non-crimp fabrics. 

A comparison between unidirectionally reinforced and plain-weave CFRP epoxy laminates (IM7/8552) for quasi-static mode I, mode II and mixed mode I/H [83] found similar initiation values, but for the woven CFRPs significantly higher propagation values of delamination resistance were observed. 

Figure 2.7 Examples of types of weaves (a) plain, (b) twill, (c) satin, (d) unidirectional, (e) gauze, (f) bidirectional deformable pattern, and (g) knit...

Property Unit PP60 1/1 balance weave PP75 1/1 PET 65 1/1 balanced weave PP604/1 PP75 unidirectional TRE/PP SMC... 

Many combinations of resins and reinforcement types and weaves available for specific structural applications. Directional strength properties can be varied from unidirectional to orthotropic by choice of reinforcement type and laminate fabrication method. 

There is another method of producing 45° stitched fabrics by weaving a weft unidirectional fabric and then pulling the fiber on a special machine to introduce a 45° skew (Figure 21.35). The process does introduce a measure of crimp when the fiber is skewed, but heavy tows can be used, keeping costs down. A quadraxial lay-up could consist of ( 45°, 0°, 90°, —45°), where the 0° and 90° can be a conventional woven fabric used with the skewed 45° and —45° layers and after assembly, the three layers are then stitched together. The disadvantage is that the woven layer will introduce a measure of crimp. 

Triaxial weave originated in the 1970s and feature a 90 60° hexagonal planar arrangement. A more popular construction is a unidirectional and — layer, with either a weft (90°) or a warp (0°) unidirectional layer. All three layers are normally combined by stitchbonding into a single fabric, which gives better drapeability than an adhesive bonded fabric. 

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

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