A Study Of The Fabrication And Mechanical Properties Of The Weft-Knitted Structural Reinforced Composites

Weft knitted fabrics are characterized by superior extensibility, conformability and high productivity, and the composites reinforced by weft-knitting fabrics possess many advantages such as better formability and impact resistance, lower manufacturing costs and so on. Compared with other textile structural reinforced composites, investigations on the weft knitted fabric reinforced composites are limited. Although more investigations were focused on the laid-in weft knitted reinforced composites and theoretical analyses of the mechanical properties of the plain weft structural composites based on the geometrical models, investigations on the mechanical properties of the composites produced with a big variation of weft-knitted structures and influences of different loop structures have still been limited. Therefore, this thesis focuses its target on fabrication of the composites reinforced by various weft knitted structures and their mechanical properties, analyzing the influences of the different loop structures and parameters such as loop length, float and tuck on the mechanical properties of the reinforced composites. It is expected that the results obtained from this thesis could be taken as reference in the application of the weft knitted structures for composite reinforcement.The thesis mainly includes three parts:(1) Production of the weft knitted structures on an electronic flat knitting machine. In order to investigate the influences of the different loop structures and parameters, mainly loop length, float, tuck on the mechanical properties of the knitted reinforced composites, eight different weft-knitted structures: jersey, rib, fleecing structure, semi-Cardigan, Cardigan, interlock and double plain-tuck structure as well as interlocks with five different loop lengths were chosen and knitted on an electronic flat knitting machine. Through preliminary experiments, the knitting parameters for the above knitted structures were determined and optimized. By using given these parameters, the above structures were successfully produced with glass fiber filaments.(2) Fabrication of the weft-knitted reinforced composites. The thesis was aimed at rigid composites, therefore, unsaturated polyester was chosen as matrix to fabricate the composites with eight weft-knitted structures. The fabrication process used was VARTM. By selecting appropriate parameters, the weft knitted structural reinforced composites were successfully fabricated and thus the specimens were ready for testing and analyzing the mechanical properties in the next step.(3) Tests and analyses of the mechanical properties of the weft knitted reinforced composites fabricated. Mechanical properties of composites involve several aspects. Taking in consideration on workload and depth of study, only two typical tests, i.e., tensile and flexural tests were carried out. The tensile properties of the reinforced composites were particularly investigated in more detail, analyzing how different loop structures and loop length affect tensile properties of the reinforced composites. Besides, the fracture behaviors of the composites were also discussed. The results obtained will be very helpful in the better understanding of mechanical properties of the weft knitted reinforced composites.The work has achieved its original objective, i.e., producing out different weft knitted structural reinforced composites and investigating their mechanical properties. Furthermore, for the problems met in different periods of experiments, suggestions and methods for improvement were given and these could be referenced in the future studies.