Research On The Structures And Properties Of Warp Knitted Fabric Reinforced Composites

Warp knitted fabrics possess a number of changes in the structures, so they do have great advantages on the property design of composites. The biaxial and the multi-axial warp knitted fabrics are generally used for the preforms of reinforced composites. The composites reinforced by warp knitted fabrics knitted with the high modulus yarns are researched seldom and reported less. Because of the severely bent yarns in the warp knit structures, the utilization of fiber stiffness and strength is limited. In addition, damage inflicted on the fibers during the knitting process and relatively low fiber volume fractions also degrade mechanical properties of warp knitted fabric composites. But, they do have several obvious advantages over the other fiber performs such as woven and braided fabric reinforced composites, including mass productivity, low fabrication cost, superior resistance to impact, and high ability to conform to complicated contours. Therefore, it is necessary to reduce fiber damage during warp knitting by researching the knittability of high-modulus such as glass fiber yarns. Further, it is also very important to study structures and mechanical properties of warp knitted fabric composites from both the experimental and theoretical aspects and to build theoretical models for predicting stiffness and strength of them for their widespread application.The main research work done in this dissertation includes the following:In Chapter One, the research progress on the structures and mechanical properties of knitted composites were reviewed in a more detail review, and the existent problems in current research were analyzed. The purpose, content and significance of this research project were explained.In Chapter Two, the warp knitting performances of glass-fiber yarns were analyzed by the means of tests. Several problems on the warping processes were discussed. The forty types of single and double faced warp knitted glass fabrics were knitted with 1 to 3 guide bars on a Raschel latch needle warp knitting machine. The knitting technology of them was studied. Some structures of these fabrics referred to the composite for the first time.Tensile strengths of single glass fiber yarns and loop strengths of them around the knitting needle hook were tested and were compared with polyester filament and polyester/cotton yarn. The testing results show that the values of loop strengths decrease with decrease of diameter of the needle hook and the glass fiber yarn rupture at the needle hook. In addition, the larger the inclining and wrapping angles of glass fiber yarns through guide needle pinhole are, the lower bend strengths of them are. Although glass fiber yarns possess higher tensile strength than general yarns, because of its brittleness, they easily break after subjected to bending and plying. In the warping process on the Z321 warper, the breakage of yarns often occurred on the position of eyelet reed and front reed. The small knots were adopted for connecting breaking yarns was suitable. It was also important to adjust the yarn tension balancing, warping speed and other parameters. The conclusions are followed through researching knitted technologies on the RMDU6 Raschel warp knitting machine. The larger the curvature and the wrap angle of glass-fiber yarns bended at the hook of needle and the pinhole of guide bar needle were, the easier the rupture of the yarns was. The technological parameter adjustment was important during the warping processes. In course of the stitch formation, the collapse of yarns occurred largely at the moment of landing as knitting two guide bar fabrics. The needles hook of narrower width had better be used. In the precondition of stitch formation ensured smoothly, the tension of warping yarns ought to be decreased. The fabric surface was fuzzed when the fabric of high density was knitted. To get the double needle bar fabrics of close structure was difficult, and it is significant to prevent that the needles on the front needle bar could not lap the yarns of front guide bar.In Chapter Three, this thesis described the characteristics of unsaturated polyester resins and analyzed the molding process of the warp knitted fabric composites. The resin casting body, the laminates of unidirectional glass-fiber reinforced composites and warp knitted fabrics/unsaturated polyester resin composites were prepared by hand lay up method. The basic parameters of them were tested. The results show that the composite specimens have the lower fiber volume fraction.In Chapter Four, the tensile properties were tested to the resin and composites, and the influencing factors in vertical, horizontal and diagonal stress-strain curves, elastic modulus, fracture strengths of the various warp knitted composites were studied. The data of test showed, the unsaturated polyester resin casting was of low tensile strength, low modulus, strain. Its stress-strain curve was nonlinear, and there was no yield point. Unidirectional composite possessed high tensile strength, high modulus, small strain, and the tensile stress-strain curve was basically linear. All the tensile stress-strain curves of warp knitted fabric composites were nonlinear; their tensile properties displayed significantly anisotropic characteristics. Their stiffness and strength related to the number of yarns bearing loads in specimen width and fiber volume fraction. A universal rule existed in single guide bar tricot, cord and satin fabric composites was, tensile strength and modulus along the 0°direction (course direction) were the highest, along the 45°direction (diagonal direction) the second, along the 90°direction (wale Direction) the lowest, the strain along the 45°direction was minimum. The studies on tensile tests of two and three guide bar fabric and double-faced fabric composites shown that the damage of glass fiber yarns in knitting process would reduce the mechanical properties of composites. The reinforced yarns in composites broke asynchronously made vertical tensile curve of some composites have a yield stage. The pillar stitches added to single guide bar fabrics could improve the vertical tensile properties of the composites. Using the local inlay guide bar the transverse tensile strength and modulus of the composites could be raised. The above studies shown, that change of the fiber orientation in composite occurred by altering the warp knitted fabric structures, parameters and fiber volume fraction and so on, to make composites presenting different strength, modulus and anisotropic characteristics. Thus, the mechanical properties of warp knitted fabric composites can be designed according to requirement.In Chapter Five, the flexural strength and modulus of 32 kinds of warp knitted composites in three directions were tested and analyzed. The results showed that the distribution of the flexural strength and modulus was basically consistent compared with the tensile test results. They also depended on the structures of knitted fabrics, yarn orientation, number of yarns bearing load, fiber volume fraction and so on. Flexural mechanical properties also were orthogonal anisotropy. The degree of their anisotropy was similar or slightly smaller than in tension. The damage of fibers had little influence on the flexural mechanical properties of the composites. Debonding between interfaces of the fiber and the resin largely affected them. Double-faced warp knitted fabric composites revealed good flexural mechanical properties.In Chapter Six, using the geometric method, based on the actual form of the loop and making a reasonable assumption, the geometry of tricot fabrics was established. By contrasting, the difference of loop length between the geometric models and the six actual fabrics of different densities were almost within 3%. This indicated that assumptions and methods of the loop geometric model established were correct. The three-dimensional geometric model of tricot loop constructed by Pro/E software was accorded with the actual loop shape.In Chapter Seven, Pro/E and the Algor finite element analysis (FEA) software were used to establish a FEA model of tricot fabric composites. This model consisted of a representative volume element (RVE) of composites. The general procedure using FEA includes (1) modeling the geometry, (2) dividing the composite structure into a number of unit cells and analyzing the mechanical properties of a unit cell using FEM and (3) reconstructing the entire reinforcement geometry by assembling the unit cells for predicting mechanical properties of textile composites. In Meshing process, the model was divided into hexahedral and tetrahedral shape of the unit. The type of analysis was selected as'nonlinear MES', unit type as'block'. The loops of the fabric were regarded as unidirectional composites, and the units of resin were defined as'using Von Mises curve'. Through the finite element analysis and calculation, the stress and strain distribution, stress-strain curves and elastic modulus were obtained as the FEA model was tensioned along the course and wale direction of the fabric. Stress and strain contour nephogram displayed, it was the loops consisting of glass fiber yarns in composites to bear the main tensile loads. Predicted stress-strain curves and elastic modulus were in reasonable agreement with experimental results. The results show that the finite element analysis model and the tensile properties of the theoretical prediction were correct.The final chapter, the paper's findings and conclusions were summarized, and the future research direction on warp knitted fabric composites pointed out.