Study On The Mechanical Properties Of Weft Knitted Fabric Reinforced Composites

Weft knitted fabrics possess superior extensibility, conformability and high productivity, so as a reinforcement, they can bring composites many advantages, such as better moldability and impact resistance, lower manufacture costs and so on. Compared with other textile structure composites, the researches on the weft-knitted-fabric reinforced composites are limited. This thesis aims at affording theoretical and experimental basis for design, manufacture and required mechanical properties of weft-knitted-fabric composites.By experimental work and theoretical analysis, the author investigated the knittability of glass fiber weft knitted fabrics on flat knitting machine, obtained the appropriate knitting parameters of the fabrics with higher tightness factor, and produced several fabrics with different stitches and densities. In addition, a three-dimensionally-fashioned tube structure from glass fiber yarn was fabricated on a computerized flat knitting machine, and the key technique and optimized parameters for the shaping of tube were explored. The composite laminates from glass fiber weft knitted fabrics and unsaturated polyester and epoxy were made by hand lay-up method.Tensile and flexural tests were carried out in the wale and course directions of composite specimens to examine their stress-strain curve (or deflection-stress curve) , initial elastic modulus and ultimate strength, and compare the differences of the mechanical performance parameters and the anisotropy of the materials. The effects of fabric orientation, stitch structure, loop shape and geometrical parameters on the tensile and flexural properties are analyzed. The author finds that changing stitch design and optimizing structure parameters can raise the course-direction modulus and strength and decrease the mechanical performance difference between the wale and course directions.Based on experimental study, finite element method and Algor software were used to analyze and calculate the tensile properties of two typical knitted fabric composites, i.e. plain fabric and rib fabric reinforced epoxy composites. Assuming that the composites are homogeneous and continuous on macrostructure, a representative volume element (RVU) including a stitch repeat and resin was taken to setup analysis model. Loop and resin were firstly modeled and meshed respectively, and then the two parts were merge as a RVU, which ensures that the nodes on the interface coincide. The loop model consists of several pieces of 3D geometrical curves. The two neighbor yarn shells are not intersected to avoid that the nodes inside the two yarn pieces do not coincide.Besides modeling, it is also important to specify the characteristics of element material. According to the principle of elastic-plastic mechanics, the author defined theepoxy resin showing nonlinear stress-strain relationship as nonlinear brick Von Mises curve with isotropic hardening element material, and the glass fiber twist yarn immersed with resin as nonlinear brick orthotropic element material. By the transform between the local and global coordinate systems, the engineering elastic constants of the twist-yarn unidirectional composite were calculated. The results of finite element analyses show that the predicted tensile stress-strain curves and elastic moduli are in reasonable good agreement with experimental curves and data.During the finite element analyses of the flexural performances of plain fabric and rib fabric reinforced epoxy composites, the model was simplified and a RVU in tensile analysis was treated as a element in bend model. In fact, weft knitted fabrics represent anisotropic properties and the mechanical performance tests of plain and rib fabric composites display nonlinear stress-strain relationships. However it is not easy to specify their materials properties precisely, so two kinds of element definitions were selected to make a comparison. Finite element analyses and calculations show that if the characteristics of element material is defined as nonlinear brick Von Mises c...