Fabrication And Mechanical Properties Of The Multilayer Knitted Composites

In recent years,knitted fabrics have attracted great attention as reinforced structures which have a suitable liquid-molding property,improved impact resistance,good energy absorption properties and lower manufacture costs when compared to other textile composite reinforcements.Currently,there are many studies on single knitted reinforced composites.However,there is very little literature that focuses on the composites layered by different knitted reinforcements.The mechanical properties of the multilayer knitted composites were investigated in this thesis.Firstly,two different knitted structures,namely bi-axial warp knitted fabric and interlock fabric,made from basalt fiber yarns have been prepared.Then,these two knitted structures have been sewed together with selected basalt fiber yarns according to the given direction and order.After that,the composite materials were fabricated using those two kinds of preforms which are with and without sewing by vacuum assisted resin transfer molding technique.Finally,the tensile and flexural properties of multilayer knitted composites were tested and analyzed. Meanwhile,the energy absorption properties of the composites have been researched. The thesis mainly includes there parts:1.Fabrication of preforms.By adjusting suitable knitting parameters such as cam setting,feeding yarn's tension, take-down force and machine speed,interlock fabrics with good quality and aspects were successfully knitted on a Stoll computerized flat knitting machine.Bi-axial warp knitted fabrics were supplied by company.Two knitted structures were made up to four layer multilayer knitted structures,layered according to given direction and order,in parallel laid-up.Bi-axial warp knitted fabrics were layered on surface layer.This is good for spreading energy quickly.Interlock fabrics were layered on internal layer. This is good for energy absorption.Some of four layer fabrics were sewed together with basalt fiber yarn in improved chain lock stitch.2.Fabrication of multilayer knitted composites.The preforms with and without sewing were used to manufacture composites by vacuum assisted resin transfer molding process.Vinyl resin was used in trails.For immersed by resin completely,choosing suitable process parameters,demoulding fabrics were layered under sewing preforms.3.Tests and investigation of mechanical Properties of the multilayer knitted composites.Mechanical Properties of composites including several aspects,taking in consideration on workload and depth of study,only two typical tests,i.e.,flexural and tensile tests were carried out.The results showed that the numbers of load-bearing yarns are main reasons for influencing flexural tensile properties in the wale and course directions and the orientation and numbers of load-bearing yarns are main reasons for influencing tensile properties.The stitched composites have higher flexural and tensile strength and modulus per unit fiber volume percentage than those of no-stitched composites.The damage modes demonstrate that stitch plays a significant role against the delamination.The results of this thesis just preliminarily searched mechanical and energy absorption properties of the multilayer knitted composites and these could be referenced in the future studies.