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5.3 Fabric types In polymeric composite terms, a fabric is defined as a manufactured assembly of long fibres of carbon, aramid or glass, or a combination of these, to produce a flat sheet of one or more layers of fibres. These layers are held together either by mechanical interlocking of the fibres themselves or with a secondary material to bind these fibres together and hold them in place, giving the assembly sufficient integrity to be handled. Fabric types are categorised by the orientation of the fibres used, and by the various construction methods used to hold the fibres together. The four main fibre orientation categories are: Unidirectional, 0/90°, Multiaxial, and Other/random. These are described below. 5.3.1 Unidirectional Fabrics A unidirectional (UD) fabric is one in which the majority of fibres run in one direction only. A small amount of fibre or other material may run in other directions with the main intention being to hold the primary fibres in position, although the other fibres may also offer some structural properties. While some weavers of 0/90° fabrics term a fabric with only 75% of its weight in one direction as a unidirectional, at Gurit the unidirectional designation only applies to those fabrics with more than 90% of the fibre weight in one direction. Unidi- rectionals usually have their primary fibres in the 0° direction (along the roll – a warp UD) but can also have them at 90° to the roll length (a weft UD). True unidirectional fabrics offer the ability to place fibre in the component exactly where it is required, and in the optimum quantity (no more or less than required). As well as this, UD fibres are straight and uncrimped. This results in the highest possible fibre properties from a fabric in composite component construction. For mechanical prop- erties, unidirectional fabrics can only be improved on by prepreg unidirectional tape, where there is no secondary material at all holding the unidirectional fibres in place. In these prepreg products only the resin system holds the fibres in place. 5.3.1.1 Unidirectional Construction There are various methods of maintaining the primary fibres in position in a unidirectional including weaving, stitching, and bonding. As with other fabrics, the surface quality of a unidirectional fabric is determined by two main factors: the combination of tex and thread count of the primary fibre and the amount and type of the secondary fibre. The drape, surface smoothness and stability of a fabric are controlled primarily by the construction style, while the area weight, porosity and (to a lesser degree) wet out are determined by selecting the appropriate combination of fibre tex and numbers of fibres per cm. Warp or weft unidirectionals can be made by the stitching process (see information in the ‘Multiaxial’ section of this publication). However, in order to gain adequate stability, it is usually necessary to add a mat or tissue to the face of the fabric. GTC-6-0417 - 33PDF Image | GUIDE TO COMPOSITES
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