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Modern flat knitting machines using high performance yarns are able to knit fabrics including the reinforcement yarns arranged differently into knit structures. Due to their improved mechanical properties, composites made from multi-layer knit fabrics show great potential in lightweight applications. This paper reports on the development of flat knitted multi-layer textile preforms for high performance thermoplastic composites using hybrid yarns made of glass (GF) and polypropylene (PP) filaments. Such textile preforms with different reinforcements were used to consolidate into 2D thermoplastic composites. Moreover, the mechanical properties of these composites were studied.The mechanical properties of 2D composites were found to be greatly affected by different arrangements of reinforcement yarns. The integration of reinforcement yarns as biaxial inlays (warp and weft yarns) is found to be the best solution for knitting, whereas tuck stitch shaped and unidirectional arranged reinforcements offer also promising application possibilities.

Drape simulation of textiles is a field of research, which is known in the clothing sector for a long time. The ongoing development of high-performance composites made of textile reinforcements and matrix materials focus the interests on a serial production in many industrial sectors, such as aviation and automotive industries. Challenges occur mainly in the serial production technologies and in supplying concepts for the preform architecture and shape. Research aims on the acceleration of preform manufacturing and the reduction of expensive pretests. Numerical simulation models can help to improve the composite development chain with structure and process simulation. A special challenge in drape modeling is the bending behavior of textiles. This study introduces a novel approach for modeling single textile layers as laminates to gain a correct mechanical behavior, where all deformation mechanisms are uncoupled. The implementation in the finite element software LS-DYNA Õ is described. An algorithm is introduced which provides the membrane stiffness for each layer of a laminate to fit the measured cantilever bending stiffness of textiles in every bending direction and bending side. The calculated parameters for the laminate formulation result in the requested Downloaded from bending stiffness for the textile layer. The cantilever bending stiffness can be used directly for dimensioning the model.

Flat knitted spacer fabrics offer a strong potential for complex shape preforms, which could be used to manufacture composites with reduced waste and shorter production times. A reinforced spacer fabric made of individual surface layers and joined with connecting layers shows improved mechanical properties for lightweight applications such as textile-based sandwich preforms. We report the development of different flat knitted spacer fabrics from hybrid yarns consisting of glass filament (GF) and polypropylene (PP) filaments. Moreover, the mechanical properties of these reinforcement yarns and two-dimensional composites manufactured using various knit structures and knitting parameters were also studied. The composite structure with reinforcement yarn in weft direction is found to be the best in this regard.

The use of thermoplastic components with a complex three-dimensional (3D) shape, manufactured efficiently with thermo-presses, has been increased steadily. Flat knitting technology using reinforcing hybrid yarns in the horizontal and vertical direction is especially suited for producing near-net-shape or fully-fashion multilayer weft knitted fabrics – MLGs (abbreviated from the German word Mehrlagengestrick, meaning multilayer weft knitted fabric). The other advantages of manufacturing such MLGs, using flat knitting technology, are reduced waste and desired reinforcing fibre alignment to obtain improved mechanical properties for high-performance applications. Before knitting 3D shaped MLGs, it is necessary to transfer the 3D component geometry into a suitable two-dimensional (2D) pattern cut by implementing parting lines. The use of computer-aided design (CAD) programs enables an effective development of complex components preforms. The generated 2D pattern cuts are analyzed with the consideration of net-shape preforming processes on V-bed flat knitting machines. The development of a segmented take-down system for effective production of 3D MLG preforms is also discussed.