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It would be possible in a woven hybrid to have one fibre running in the weft direction and the second fibre running in the warp direction, but it is more common to find alternating threads of each fibre in each warp/weft direction. Although hybrids are most commonly found in 0/90° woven fabrics, the principle is also used in 0/90° stitched, unidirectional and multiaxial fabrics. The most usual hybrid combinations are: Carbon / Aramid The high impact resistance and tensile strength of the aramid fibre combines with high the compressive and tensile strength of carbon. Both fibres have low density but relatively high cost. Aramid / Glass The low density, high impact resistance and tensile strength of aramid fibre combines with the good compressive and tensile strength of glass, coupled with its lower cost. Carbon / Glass Carbon fibre contributes high tensile compressive strength and stiffness and reduces the density, while glass reduces the cost. 5.3.4 Multiaxial Fabrics In recent years multiaxial fabrics have begun to find favour in the construction of com- posite components. These fabrics consist of one or more layers of long fibres held in place by a secondary non-structural stitching tread. The main fibres can be any of the structural fibres available in any combination. The stitching thread is usually polyester due to its combination of appropriate fibre properties (for binding the fabric together) and cost. The stitching process allows a variety of fibre orientations, beyond the sim- ple 0/90° of woven fabrics, to be combined into one fabric. Multiaxial fabrics have the following main characteristics: Advantages The two key improvements with stitched multiaxial fabrics over woven types are: (i) Better mechanical properties, primarily from the fact that the fibres are always straight and non-crimped, and that more orientations of fibre are available from the increased number of layers of fabric. (ii) Improved component build speed based on the fact that fabrics can be made thicker and with multiple fibre orientations so that fewer layers need to be included in the laminate sequence. Disadvantages Polyester fibre does not bond very well to some resin systems and so the stitching can be a starting point for wicking or other failure initiation. The fabric production process can also be slow and the cost of the machinery high. This, together with the fact that the more expensive, low tex fibres are required to get good surface coverage for the low weight fabrics, means the cost of good quality, stitched fabrics can be relatively high compared to wovens. Extremely heavy weight fabrics, while enabling large quantities of fibre to be incorporated rapidly into the component, can also be difficult to impregnate with resin without some automated process. Finally, the stitching process, unless carefully controlled as in the Gurit fabric styles, can bunch together the fibres, particularly in the 0° direction, creating resin-rich areas in the laminate. GTC-6-0417 - 37PDF Image | GUIDE TO COMPOSITES
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