Stitched fabrics

The increases in through-the-thickness reinforcement achieved by NCFs have been demonstrated by a number of authors. For example. Backhouse et aV compared the ease of delaminating polyester stitched 0/ 45 carbon fibre NCF with equivalent carbon fibre/epoxy UD laminates. There were large increases, some 140%, in the measured 

For in-plane properties of NCF composites, there is sufficient similarity to UD materials to enable similar analyses to be used when approximate values are required (although Hogg et suggest that the properties of NCF composites may exceed the in-plane properties of UD equivalents). The interested reader is encouraged to consult the book edited by Lomov for a discussion of approaches to modelling the effect of the stitching on the mechanical properties and damage accumulation in composites based on NCFs. 

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The technologies of stitching various product types for stitching fabrics can be summarised as follows ... 

Producing a stitched fabric by the simuitaneous stitch process [34]... 

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. 

Key words matrix, strength, stiffness, composite, carbon fibers, Kevlar fibers, glass fibers, unidirectional fabrics, woven fabrics, stitched fabrics. 

Stitched fabrics represent one of the possible forms of 3D fabrics. These dry fabrics are manufactured by simple stitching of layers of fabrics together to allow for better handlability, and enhance the out-of-plane properties of the composite. The yarn used for stitching has to be ductile enough to sustain the stitching forces and the very small radius of curvature at the end of the stitch path. Typically a polymeric yarn such as para-aramid, nylon, or high density polyethylene is used to stitch fabrics. 

Responding to the challenge of achieving optimum fiber architectures, material producers have devised innovative braided, woven and nonwoven, and stitched fabrics that are being used today even in bridge decks, beams, and other primary structural elements. 

The above fibre reinforcements are available in several forms that include almost parallel bundles of continuous filaments, either untwisted (rovings) or twisted (yarns), and short fibres (chopped) with a length of 3 mm to 50 mm (Keller, 2003). For use in pultrusion, fibre reinforcements can be worked to obtain textile products with several reinforcing directions. There are, therefore, several products available, either with randomly oriented fibres, which can be short (chopped strand mat) or continuous (continuous strand mat), or with oriented reinforcements (such as woven and non-woven fabrics, stitched fabrics, grids and meshes), which can be biaxial (0°/90° or -i-45°/-45°) or triaxial (0°/-i-45°/ 5°), the latter being considerably more expensive and less widely used in pultrusion. All these forms can be further combined to make complex textile products with continuous oriented fibres, together with randomly oriented short or continuous fibres. Figure 9.1 shows examples of forms of fibre reinforcement. 

Flat Knit Fabric n (1) A fabric made on a flat-knitting machine, as distinguished from tubular fabrics made on a circular-knitting machine. While tricot and mil-anese warp-knit fabrics (non-run) are knit in flat form, the trade uses the term flat-knit fabric to refer to weft-knits fabrics made on a flat machine, rather than warp-knit fabrics. (2) A term used in the underwear trade for plain stitch fabrics made on a circular-knitting machine. These fabrics have a flat surface and are often called flat-knit fabrics to differentiate them from ribbed-knit or Swiss rib fabrics. In this case, the term refers to the texture, not the type of machine on which the fabric was knit (Kadolph SJJ, Langford AL (2001) Textiles. Pearson Education, New York Vincenti R (ed) (1994) Elsevier s textile dictionary. Elsevier Science and Technology Books, New York). 

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