Auxetic materials, as a class of metamaterials, are of great interest due to their unique mechanical properties. auxetic materials exhibit positive transverse strain and therefore a negative Poisson's ratio under longitudinal strain. The rotation of a geometric member made from these materials can lead to elongation in the lateral direction in response to an applied load, resulting in a negative Poisson's ratio. Such a geometry can be further incorporated into fabrics. However, fabrics contain pores of various sizes through which fluids or smaller particles can infiltrate, thereby reducing the performance of the auxetic fabric structure. In the present research, the Poisson's ratio of resin-impregnated plain weft-knitted fabrics with completely blocked pores was investigated for the first time. Weft-knitting is widely recognized as the most significant and attractive technology in the textile industry. In this design, hollow elliptical and rhombic patterns were developed in the fabric, with predefined major-to-minor diameter ratios, arranged orthogonally to each other, and the pore pattern was finally filled with resin. A square network is established by these holes, which rotates under strain, inducing diameter elongation in the normal direction to the applied load. This research was performed in two steps at macro and micro scales. Macro-scale studies were conducted using the finite-element method (FEM) in Abaqus software 6.14 to analyze the effects of different parameters on the establishment of a negative Poisson's ratio. The parameters studied in the macro-scale analyses included the geometrical shape of the pattern developed in the fabric (e.g., major diameter and spacing of the patterns), anisotropy and Poisson's ratio of the composite (i.e., resin-impregnated yarn and the resin around the yarn), and most importantly, the resin-to-composite Young's modulus ratio. Micro-scale studies included micromechanical analyses to eva‎luate the effects of knitting geometry, yarn diameter, and thickness on the Young's modulus of the composite. According to the studies and analysis, the obtained results, the resin-to-composite Young's modulus ratio had the largest effect on the Poisson's ratio. In other words, a higher Young's modulus for the composite facilitated the rotation of square-shaped patterns within the structure, pushing the Poisson's ratio of the structure towards negative values. It is worth mentioning that when the Young's modulus of the composite increased to excessively high levels, the Poisson's ratio no longer depended on the resin-to-composite Young's modulus ratio, and the effect of geometry (i.e., major diameter of the ellipses/rhombs and their spacing) became dominant. By investigating composite anisotropy, an optimal point was identified for each particular geometry where the Poisson's ratio was minimal. Generally, at this optimal point, the Young's modulus in the 1-direction was twice as large as the Young's modulus in the 2-direction, with the exact position of the optimal point being controlled by the composite geometry. By considering these parameters, it is possible to achieve a non-porous auxetic composite that it's reinforced with plain weft-knitted fibers.

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محمد جواد عبقري

مهدي جوان بخت , محمد سيلاني