Study On The In-Plane Mechanical Properties Of The Non-woven Composites Stitched By Plain Stitch Pattern

Since the early 1980's, the stitching technology which was applied to manufacturing 3D composites has achieved great success. Compared with the traditional composite laminates,through-the-thickness stitching can effectively enhance the interlaminar fracture toughness and impact damage tolerance. Meanwhile, the production cost of the stitched composites is much lower than that of 3D woven or braid composites.However,while the interlaminar properties are largely improved, the in-plane properties are damaged due to the breakage, misalignment and kinking of the in-plane fibers.The research on the in-plane properties of stitched composites is becoming more and more important.Based on this, this paper carried out the following work:In the experiments, non-woven fabrics were used as layer, materials, and 4 layers of Kevlar mats were stitched together into a preform. During the process of stitching, we selected 2 sewing yarns,5 stitching densities and 2 stitch patterns and then injected vinyl ester resin into the stitched structure by using Vacuum Assist Resin Transfer Molding (VARTM) technology. After composites were finished, the cured composites planes were cut into the sizes of tensile or bending property test specimen.The specimen for tensile property tests were stitched with the modified locked stitch pattern and the plain stitch pattern respectively. We kept the stitching density and sewing yarn parameters invariable. Compared with composites stitched by the modified locked stitch pattern, the tensile strength of the composites stitched by plain stitch pattern increased by about 7%.The tensile elastic modulus was improved by 2.5%.The bending strength was raised by 30.58%,and the bending modulus was enhanced by 13.8%.The plain stitch pattern as a new stitch pattern was proven to be a potential one.The effects of stitching parameters on the bending properties of composites stitched by the plain stitch pattern were investigated. The stitching parameters included stitching density, the stitch pitch distance, the stitch line spacing, the sewing yarn materials and the yarn fineness.lt was revealed that the higher the stitching density was, the more the bending property was damaged. The bigger the sewing yarn diameter was, the lower the bending strength was. What's more, when the stitching densities were identical, the stitch line spacing was much more influential than the stitch pitch distance. However, the sewing yarn materials almost had no effect on the bending properties.The finite element analysis software was used to modal the process of the stretching and bending. The contour plots of stitched composites were compared with those of unstitched ones. By this way, the reason why the tensile and bending properties of stitched composites decreased was explained. At last,considering the deficiencies of this research, we came up with some prospects about the equipment for plain stitching, the non-woven fabric composites and a number of data from various properties of stitched composites.