Tensile Properties Of Biaxial Weft-Knitted Fabric Reinforced Thermoplastic Composites

Over the recent years, there has been an increasing interest on the use of thermoplastic matrices for the manufacturing of fiber reinforced composites. High ultimate strain, short molding cycle, recyclability and re-processibility are some of the driving factors for their increasing uses. The use of hot-pressing process to fabricate thermoplastic composites from textile preform made of commingled yarns has been proven to be a simple and cost-effective way. Using this way, composites with required mechanical properties for engineering applications can be projected and fabricated with a high flexibility in fiber kind selection and their proportions as well as laying-up method of fabrics.Biaxial weft knitted fabric (BWKF) is a kind of reinforcing structure in which the warp and weft yarns are inserted into a weft basic knitted structure. Compared with common knitted structures, it has higher fiber contents and less voids. Using mechanical properties of straight yarns, it is possible to fabricate composites with high mechanical performance.The objective of this work was to investigate the mechnical properties of BWKF reinforced thermoplastic composites. The BWKF perform was firstly made of high performance fiber/PP commingle yarns as inserted weft and warp yarns, and then the composites were fabricated through hot-processing process. The tensile properties of BWKF composites were experimentally investigated and their elastic constants were analyzed and calculated using micromechanical analysis method. The tensile strengths were theoretically predicted using Monte Carlo method in combination with shear-lag theory. The influences of fabric structural parameters on the tensile mechanical properties were both experimentally and theoretically analyzed.The work was firstly focused on the fabrication processes and mechanical testing of BWKF thermoplastic composites made with different linear densities of glass yarns and different kinds of yarns such as glass, carbon and armid commingled with PP yarns. Based on the analysis of experimental data, the behaviors of BWKF composites under tensile testing were discussed. The influences of fiber contents on initial modulus and failure strengths of the composites materials were analyzed. At the same time, different failure modes and mechanisms were also analyzed.In order to analyze the mechanical properties of the composites, a 3D geometrical model of the BWKF structure was established, in which shapes of knitting yarn, inserted weft and warp yarns in a unicell were theoretically described and the relations between geometrical parameters and length of each kind of yarn in a unicell were derived. The factors which influence the thickness of fabricated composites and fiber volume fractions were also discussed. A solid basis was established for the theoretical analysis of influences of fabric structure on the mechanical properties of the composites.By analyzing shape change of each kind of yarn in a unicell after hot forming process, the established model was modified to get better fit on real geometrical form of the fabric. Using bridge model and according to yarn orientation change, the overall stiffness/compliance matrices of each sub-element were analyzed and assembled, and final overall stiffness/compliance matrices and elastic constants were obtained. Due to the complexity of BWKF structure, the average volume method was used during micromechanical analysis. The representative volume element (RVE) was firstly subdivided into a number of UD composites with arbitrary yarn orientation, and then the local stiffness/compliance matrices of UD composites were analyzed with application of bridging micromechanics model and assembled under iso-stress and iso-stress assumptions for knitting yarn and inserted yarn respectively. Finally the overall stiffness/compliance matrices were obtained by assembling all the sub-elements based on best fit constant in combination of iso-strain and iso-stress assumptions under the rule of mixture. The predicted stiffnesses had close agreements with experimental data.Concerning predicting the tensile strengths of BWKF composites, Monte Carlo simulations with the 2D shear-lag model were firstly conducted to obtain the ultimate tensile strength of UD composites as a function of fiber tows strengths and interfacial properties, and then the strengths of BWKF composites were predicted by adding the effects of stitch yarn. Good agreements were obtained between the predicted results and experimental ones, and feasibility and validity of the theoretical method were proven.Through an experimental and theoretical investigation, a better and comprehensive understanding on the mechanical properties of BWKF composites has been obtained. The methods used in calculating and predicting the mechanical properties in this paper will be useful for the design of such kind of composites. At the same time, these methods could be used as reference ways for similar composite deign.