Study On The Influence Of Stitching Technique Of Bi-Axial Warp-Knitted Fabric On Mechanical Performance Of Composites

The problems with using the traditional fabric reinforced polymer laminates in structures are their low interlaminar fracture toughness and impact damage tolerance, so its application on the light structure is limited.The interlaminar fracture toughness and impact damage tolerance of fabric reinforced polymer laminates can be effectively improved by through-the-thickness stitching. Meanwhile, stitching can connect 3D fabrics into a complicated structure which cannot produce by the 3D woven or braid machine at one time. Stitching also can integrate 2D fabrics into a whole structure after design. Therefore, there is a large developing space on using stitching technology in composites.The axial warp-knitted fabric is become more and more useful in fabric reinforced composites because of its perfect mechanical performance. During the experiment we simply modified a normal industrial sewing machine to stitch 6 layers or 4 layers bi-axial warp-knitted fabrics. During the process of stitching, we selected 3 sewing yarns, 7 stitching densities and 2 seam patterns to stitch and then injected unsaturated polyester resin into stitched structure by using Vacuum Assist Resin Transfer Molding (VARTM) technology. After composites are finished, cut them into specimens we need.According to ASTM standard, we made the double cantilever beam (DCB) specimens with hinge which are used to test the composites' Mode I interlaminar fracture toughness. Through test we find that compared with unstitched composites, the value of stitched composites' Mode I interlaminar fracture toughness is improved a lot. Meanwhile, we tentatively tested the composites' Mode II interlaminar fracture toughness using the End Notched Flexure (ENF) specimens. Through analyzed the result we find that stitching can also improve composites' Mode II interlaminar fracture toughness. The process of interlaminar fracture is a behavior of acoustic emission. So we used the acoustic emission information to describe the process of crack opening displacement.In order to study the effect of stitching parameters on the mechanical properties of stitched composites, we tested the composites' Mode I interlaminar fracture toughness of many kinds of specimens with different stitching parameters. At the same time, we tested their tensile and bending strength. The results show that under the condition of one stitching parameter is variable and the other stitching parameters are invariable, the higher the stitching density is, the higher value of the Mode I interlaminar fracture toughness, but the tensile and bending strengths decrease with the increasing of the stitching density. The higher strength of sewing-yarn is, the higher value of the Mode I interlaminar fracture toughness, but the tensile and bending strengths decrease with the increasing of the diameter of sewing-yarn. Modified locked seam pattern is better than locked seam pattern. We can see that stitching can improves interlaminar fracture toughness but decreases tensile and bending strengths because stitching damage fiber. On the base of study, stitching technique can be optimized under the restriction of tensile and bending strengths.However, there are still some shortages in this dissertation that should be improved, so we give some suggestions to the future research at the end of this dissertation.