Study On Resin Flowing Behavior And Impregnation In Biaxial Warp Knitting Stitched Preform And Mechanical Propertie Of Its Composites

Research on stitched composites started at the end of 1970s, the initial aim was for aviation and spaceflight, with the purpose of replacing material agglutinating and rivet in the fighter plane. Investigation results indicates that, compared with traditional fiber reinforced polymer laminates, stitched composites are characterized with obvious improvements on the anti-impact strength and damage tolerance properties, and Mode I and Mode II interlaminar fracture toughness are improved greatly. Connection strength under single or repeated load is enhanced, and the interlaminate mechanical properties are improved consequently. Normally, this kind of composites can be made by using RFI, VARTM and RTM, their feature is that fibers are impregnated in mould. In order to produce high-performance composites, resin impregnation is a vital factor which influences void content and final properties of composite, besides the strict request of resin matrix and fibres. Research shows that 1% hole (void) in the composites, results in 30% of bending intensity loss as well as 3% of drawing strength. Moreover interlaminar shearing strength decreases 7% results from the 1% increasing porosity rate when less than 4%. Therefore it is quite important to study the impregnation and flowing behavior of resin in preforms to obtain lower porosity content composites.At first, Necessary mechanical property tests for sewing threads were performed, such as drawing, bend-drawing and abrasive, as well as some sewing practice with the threads, and the suitable sewing thread was found for this study. The reasons for different sewing properties among these threads were obtained. Moreover a fully drawing test was carried out on the bi-axial warp knitting fabric, including fibre tow, ground yarn and fabric.Different stitched preform were prepared on industrial sartorius, with changing the stitching parameters such as thread density, needle gauge, sewing distance, laminate thickness, sewing direction etc. Some stitched composites were made with VARTM technique. Resin flowing behavior in the preform was recorded in detail, and results indicates that stitching influences resin flowing in preform as well as other stitching parameters with the same condition, which can be obtained in this paper.In addition, fully testing for stitched composites mechanical properties were conducted, including tension, bending, compressing, and impact and compressing after impact. Result showed that stitching results in negative effect on drawing, bending properties, however it can improve the strength of compressing after impact(CAI).