| Natural materials usually exhibit positive Poisson’s ratio; that is, such materials tend to contract or expand in the transverse direction if they are stretched or compressed, respectively, in the longitudinal direction. In recent decades, some materials with negative Poisson’s ratio (NPR) have been found, and named as auxetic materials by Evans et al in1991. The greater the absolute values of Poisson’s ratio of auxetic materials have, the higher the NPR effect. Compared with common materials, in addition to NPR, auxetic materials possess other unique properties, such as high shear stiffness, high fracture toughness, high resilience and energy, ultrasound, sound absorption capacity, etc. These excellent mechanical properties make auxetic materials good candidates for the applications in aerospace, environmental protection, biomedicine and other defense field. Hence, the development and study of auxetic materials are of great significance.The study aims to design and develop an innovative kind of three dimensional fabrics with negative Poisson’s ratio, and analyze their deformation mechanism under compression process and influences of structure parameters on the NPR effect. The fabric structrure is composed of three yarn systems, i.e., weft yarns, warp yarns and stitch yarns. The structure features include:1) the weft and warp yarns layers are not interlaced;2) the weft and warp layers are fully or partially arranged in each layer;3) the weft and warp yarns are bound by the stitch yarns through the fabric thickness direction. Two kinds of structures, structure A and structure B, were suggested and produced. In structure A, the weft yarns are fully arranged, and the warp yarns are partially arranged. In structure B, both of the weft and warp yarns are partially arranged. The relaxation rate of the two kinds of stuructures is defined to describe the initial state of the fabric at which the weft and warp yarns are crimped or completely straight. The structural parameters including radiuses of the weft and warp yarns, the spacing of two adjacent warp yarns in the same layer, the spacing of two adjacent weft yarns in the same layer, the Young’s modulus of the weft and warp yarns and structure relaxation rate, are the factors affecting auxetic effect of the fabric structures. The study includes four parts. An innovative three dimensional structure with NPR was first designed and developed. In the first part, a geometric model was established, and the effects of structural parameters on NPR of the fabrics were discussed based on the model established. Then, a machine to produce this kind of3D fabric structure was manufactured to produce. In the second part, fourteen fabric samples were manufactured manually and produced using the machine, respectively, and compressed using an Instron5566machine. The reasons for the fabric structure with axuetic effect under compression and the affecting trends of structural parameters on negative Poisson’s ratio were experimentally analyzed. In the third part, a2D analytical model was established based on the classical theory of elasticity and contact theory, and compared with the experimental results and the above mentioned geometric model. In the last part, a finite element model was built, and the simulation results were compared with the results from the compression tests and the2D analytical model.The main conclusions of the study are as follows:(1) The results of the geometric model confirm that the innovative structure has a negative Poisson’s ratio which increases with the compression strain. To obtain more obvious auxetic effect of the structure, the radius of the warp yarns and the spacing of two adjacent warp yarns should be increased,, and the radius of the weft yarns and structure relaxation rate should be not too high.(2) The innovative three-dimensional structure can only have NPR in one direction for both the structure A and structure B. The axuetic effect under compression results from a group of yarns with higher bending stiffness and alternately arranged. Comparative experiments between the fabrics produced by hand and the fabrics manufactured by machine show that all the samples have NPR. The fabrics with the same structural parameters having different structural relaxation rates lead to different Poisson’s ratios. The increase of radius of warp yarns and the spacing of two adjacent warp yarns in the same layer enhances the auxetic effect and this trend agrees well with the prediction of the2D geometric model.(3) The results from the2D analytical model showed that the theoretical Poisson’s ratio-compression strain curves had the same trends as the experiment results. Auxetic effect of the structure was more obvious with the increasing the compressive strain. But the NPR values obtained from this2D analytical model are slightly greater than the experiment results, and the force of unit cell is significantly greater than that from experiment. In addition, the results showed that increasing the radius and elastic modulus of the warp yarns, and decreasing the spacing of two adjacent warp yarns in the same layer and the radius and elastic modulus of the weft yarns have a significant and positive impact on the NPR ratio effect. Although the structural relaxation rate at the initial state had significantly improved the NPR performance of the fabric structure, it can affect the compressive strain rate of the fabric structure to a certain extent.(4) The finite element analysis showed that using the smallest unit cell of the structure to simulate Poisson ratio and stress in compression process was reasonable. Additionally, structure A and structure B with the same structural parameters have the same trends in the Poisson’s ratio-compressive strain and stress-compressive strain relationships. The influences of structural parameters on Poisson’s ratio in the finite element model had the same trend as the results of compression test and2D analytical model. |
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