| Fiber-reinforced Composites have gained rapid progress, owning to its nearly perfect performance and relatively low cost. And it is widely used in many fields, such as aviation and transportation etc. Material technology, information technology and resource technology will be the three most important technologies in the future.The current trend in the composite material industry is to expand the use of composites from secondary non-load-bearing to primary load-bearing structural applications in aerospace, automobile, marine, civil engineering and other industries. This requires a significant improvement of the damage tolerance and reliability of composites. For thin-medium thickness structures, an attractive family of textile preforms, MWK fabrics, has the potential to meet the demand for structural composites. The MWK fabric preforms consist of warp (90° ), weft (0° ), and bias (±θ) insertion yarns held together by a basic warp knit stitch through the thickness of the fabric.All layers of insertion yarns are placed in perfect order and show the uniformity of the non-crimped parallel yarns. The stitch yarn can afford the through-the-thickness reinforcement, and therefore, provide a significant increase in damage tolerance, as well as the structural integrity and out-of-plane strength of the preform. The commonly used basic stitch is chain stitch or tricot stitch, however, another basic stitch, double loop pillar stitch, can also be used as basic stitch of MWK fabric and proved to be a much better basic stitch.The MWK fabric preforms can be produced in a single step. Besides its good ease of handling and production economics, the MWK preforms also provide the conformability to complex shapes, the flexibility in the principal yarn directions, and the improved through-the-thickness strength.The mechanical properties of MWK composites, especially in out-of-plane strength and impact behavior, may be superior to those of conventional woven laminated composites due to the elimination of fiber crimp and to the presence of the through-thickness reinforcing stitch loops. However, there is not any research on the impact energy absorptioncapability of MWK structure, especially the effect of MWK structure parameters on the energy absorption capability, as well as impact behavior.As the use of MWK in automotives and other industrial applications increases, the need to determine the impact energy absorption property and impact behavior of MWK materials becomes more and more important to ensure the stability and safety of the designed structure. Low-velocity impact is one of the most subtle threats to composite materials. Due to the weak bonds between the materials, even small energies imparted by out-of-plane loads can result in hardly detectable damages, causing considerable strength losses.For this purpose, the energy absorption capability of MWK structure and its reinforced composite plates subject to low velocity impact was examined experimentally firstly in this study. The low velocity impact testing devices can be divided into pendulum type (Izod,Charpy) and the drop weight type. Both kinds of experiments were carried out in order to get the comparable results.Then based on the analysis of basic MWK stitches, the geometry characteristics and micromechanical properties of MWK reinforced structures were studied. A new geometric model and micromechanical model of MWK structure of double loop pillar stitch were established and examined experimentally. The mechanical properties of MWK structure, i.e. the global effective stiffiiess matrix of MWK reinforced structure, were analyzed and predicted theoretically. The analysis results are found to be in good agreement with experimental data.The impact behavior and energy absorption characteristics were then research with one of the common used FEM tools-ANSYS 5.6. The analysis was carried out based on the LS-DYNA explicit transient finite element method and the global stiffness matrix, which were calculated theoretically in the previous parts. A n...
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