A Study Of Manufacturing Process And Tensile Properties Of Co-Woven-Knitted Fabric Reinforced Composites

The tensile properties were studied for a self-developed novel type of biaxial weft knitted fabric, in which warp and weft yarns introduced the knitting structure are in a state of interweaving, and its reinforced composites in this paper. The fabric was named also co-woven-knitted (CWK) fabric because it includes both the knitting structure and the woven structure. The CWK fabric has comprehensive performances of both woven fabric and knitted fabric, because of combining knitted structure and woven structure into one. Compared with the traditional biaxial knitted fabric, the biggest difference is that the warp and weft yarns are weaving in the CWK fabric.In view of the close relationship between the CWK fabric performance and its structural parameters, depth research on the structure of the CWK fabric to determine the weaving process and design its weaving and feeding equipments on the flat knitting machine. According to the weaving process of the CWK fabric, a co-woven-knitted machine was reformed from a manual V-bed flat knitting machine to producing the CWK fabric, on which the CWK fabric was produced with polyester yarns. This demonstrated the validity of the weaving method and the feasibility of the CWK fabric. While the weaving, the process of the CWK fabric was analyzed and discussed, including the configuration and positions and motion cooperate of weaving parts, the establishment for the yarn guide devices, layout for the yarn guide path, all kinds of yarn tension, the precise control for warp position and adjustment of synchronization control for the heads. During the course of the actually machine operation, gradually improving and optimizing of weaving and knitting process and mechanical parts were carried.Based on the weaving process above, the CWK fabric was fabricated with high tenacity polyester yarn for knitting yarn, the same glass filaments for warp and weft. After many weaving experiment, the best knitting process was determined for high performance fiber CWK fabric. Tensile tests were conducted in the CWK fabric coursewise and walewise direction. It was found that the mechanical properties were quite different between the warp and weft direction. This will affect the application of the CWK fabric. The CWK fabric was fabricated again with re-selecting the warp, weft and knitting yarn, increasing linear density of warp, reducing linear density of the knitting yarn and decreasing the number inserted into fabric. After tensile properties were tested, it was found the mechanical properties are greatly improved in both directions. Mechanical behavior of the CWK fabrics is in between woven structure and knitted structure. Meanwhile, the axial mechanical properties of these fabrics can be adjusted by changing the density of warp and weft yarn to meet the application needs.In addition, the multi-layer biaxial weft knitted fabric (MBWK fabric) was produced using the same yarn, in the same depth of stitch cams for knitting yarn on the same flat knitting machine. The tensile tests were carried on the fabric for comparative study of the tensile properties with the CWK fabric. The results showed that axial tensile properties have great similarities between two kinds of biaxial weft knitted fabrics. During the course of drawing, the two fabrics have the same performance as that first by drawing on the direction of the warp or weft, in low elongation, bear high load, until the warp or wefts damage, then to the knitted fabric, at higher elongation, undertake low load, until the knitting structural damage. Comparative studies also found that, compared with the MBWK fabric, the buckling of warp and weft yarns in the CWK fabric played certain effect on the tensile strength of the CWK fabric. But the pre-coating draft function that the interweaving of the warp and weft yarns carried on the knitting yarn in CWK fabric is more outstanding. Thus, bind effect that the knitting yarn to the warp and weft yarns is better, the contacts between the warp layer and wefts layer is closer. This shows the performance of resisting delamination of the CWK fabric is better than that of the MBWK fabric.The tensile tests were carried on the composite material, which were made using the CWK fabric as reinforcement. The tensile properties of the composites showed good linearity performance. The tensile strength is mainly depends on the yarn density and the tensile strength of weft and warp yarns in the tensile direction. Differences between in the axial tensile mechanical properties can be decreased through the selecting and designing of the linear density and inserted density of the weft and warp yarns. Due to the bending of the weft and warp yarns, there has an inflection point in the tensile curves of the CWK fabric reinforced composites. And degree of the bending of warp yarn is higher than the weft yarn. Compared to ordinary knitted fabric reinforced composites, the CWK fabric reinforced composites have higher initial modulus and fracture strength. In the transverse stretching, fracture is generally along the wale direction. In the longitudinal tension, usually along the course direction was broken. In the 45°diagonal tension, broken is in course direction, and have minimum tensile strength and modulus, the highest tensile energy. Meanwhile, tensile tests were carried on the MBWK fabric reinforced composites for comparative study. The result showed that the axial performance of CWK fabric reinforced composites is basically the same as that of it. This is because that, in the course of the manufacture for composites, buckling of the weft and warp yarns in the MBWK fabric is increased, while buckling of the weft and warp yarns in the CWK fabric is reduced.Under certain assumptions, unit cell and representative volume element (RVE) were selected for the CWK fabric reinforced composites, established three-dimensional geometric model of the CWK fabric by the geometric analysis method, obtained length of warp, weft and knitting yarns according the geometric model. Theoretical predictions of the yarn length agree well with the measured test values, confirming the effective and practical of the model. The fiber volume fraction is calculated through the fabric structure parameters and the measured yarn length after forming. The experimental data and theoretical model are in good agreement, which further shows that effective and practical of the model. The total fiber volume fraction trends was discussed, under a given fabric tightness, with changing in the ratio of the knitting yarn to the inserted yarn density. This provides a basis of how to choose yarn linear density for a given fiber percentage content of CWK fabric reinforced composites.The 3D model of the composite is basically same as the CWK fabric after molding. Thus, According to the 3D model of the CWK fabric, the geometric shape and position of the inserted yarns and knitting yarns in the RVE can be determined. So the orientation of various yarn were determined. After molding three kinds of yarns and metrin can be seen as unidirectional fiber composite. Using the average volume analytical and the micromechanics analysis methods, obtained the local rigid/flexibility matrix of the warp, weft yarn, knitting yarn and the matrix. Then the overall material stiffness/flexibility matrix and the elastic constants were analysis using the mixed rate law, the equivalent stress and strain boundary conditions and the best factor combination. In order to realize the calculating of theoretical elastic constants for the CWK fabric reinforced composites, MATLAB language computing program is compiled. The results shows that theoretical predictions of elastic constants in good agreement with the experimental results.There has a more overall understanding to the mechanical properties of the CWK fabric and its reinforced composites through experimental tests and theoretical analysis and prediction. This provided a good foundation for more in-depth study and design of the CWK fabric and its composites in structure and properties. At the same time, the analysis method can be used as a reference for the analysis of other similar materials.