BENDING AND IMPACT PROPERTIES OF BIAXIAL WEFT KNITTED COMPOSITES

In order to improve the mechanical properties of weft knitted fabric, straight yarns both in weft and warp directions can be integrated. These types of reinforcements are called biaxial weft knitted structures. Biaxial weft knitted fabrics include weft and warp yarn layers, which are held together by a stitching yarn system. Reinforcing yarns, e.g. glass or aramid fibers, can be used within all yarn systems. They provide necessary strength and stiffness of the fabric [1]. The present work concentrates on the mechanical properties of biaxial weft knitted composites. Mainly the effect of stitch yarn type such as aramid, glass and nylon and biaxial, warp and weft, yarn type such as aramid and glass were investigated. Six plies composite panels were fabricated by hand lay-up lamination method. Volume fraction (Vf) of all composites were kept constant. After production of composites, bending and impact tests had been conducted. GF-GF-NY composites exhibited superior flexural property than other tested composites. It was found that GF-GF-AR composites have higher impact properties (total energy and maximum load) than other tested composites. Better mechanical properties can be achieved by using aramid stitch yarn with combination glass warp and glass weft yarns. A better interfacial bonding of aramid stitch yarn seemed to be cause of this result. After impact test, fracture aspects of reverse side of weft knitted composites were analyzed. The crack propagation was explained by visual inspection. Moreover, impact damage for tested materials was evaluated by cross-sectional observations. For the better understanding of the fracture behavior, schematic drawings of all kind of specimens were drawn. To analyze the damage behavior of tested specimens, the damage lengths were measured. By this observation, the tendency between impact energy and crack length was showed.