Knittin of carbon and Dyneema® fibres to fit for contour sahpes in composites

University essay from Högskolan i Borås/Akademin för textil, teknik och ekonomi

Abstract:

Textile process and textile structures that are suitable for composites are carefully studied and chosen to have weft knitted fabrics. The aim of this research is to knit the carbon and Dyneema® fibres in circular weft knitting to fit contour shapes. Carbon/Dyneema® can also be knitted in warp knitting machines to get properties in multi axial direction. But the fabric was flat and can be used only for 2D shape products which are having less drapabiity. According to previous research, weft knitting is the best suitable for complex preforms. Before knitting these fibres properties were studied in order to avoid the damage to the carbon fibres. The carbon fibres have high bending rigidity, low resistance to friction and are very brittle. A small damage to the carbon fibre in knitting subsequently affect directly on the composite properties. The strongest manmade fibre manufactured till date is Dyneema® and these fibres could be used in composites due to its performance, properties and light weight. But, the Dyneema® fibres are expensive when compared to common polyester, so polyester fibres are used to compare the properties and cost performance ratio. The critical bending of the carbon fibres causes friction between the fibres and also between fibre and machine. This was considered carefully during the knitting of carbon fibres and the idea chosen is mentioned in this thesis. Between the two layers of Dyneema®/polyester, carbon fibres are laid circularly in unidirectional and in un-crimped condition. This makes the carbon yarn to possess good mechanical properties. The 2 layers of Dyneema®/polyester fibres exchange the loops at certain points to increase the inter-laminar strength and decrease the carbon fibre distortion. This structure helps to withstand external load. It is also lighter than the carbon composite with additional properties. This makes much more space in the future for the Dyneema® fibres in the 3D carbon composite manufacturing. The internal carbon fibres are fully covered by the Dyneema® fibres to withstand the external impact load and not to damage the carbon fibres. So the loop length, stitch density, fibre volume fractions are considered before knitting.

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