Study of auxetic fabrics based on rotating square geometry

Abstract

A novel auxetic fabric design concept is proposed in this research based on the typical rotating square structure, combining a flexible and stretchable base with a rigid auxetic frame. Two approaches are adopted to produce this fabric structure. The first one is to use the warp-knitting jacquard technique. Fabrics with different base structures and the same rotating square frame pattern are fabricated, for auxetic effect analysis through one-off and repeating tensile tests. The second one is to use the laminating technique. A low-elastic woven fabric and a high-elastic knitted fabric are selected respectively as the frame material and the base material. Laminated fabrics are fabricated by cutting the frame material into the designed auxetic rotating square structure and then attaching it to the base material with a fusible adhesive membrane. Effects of the geometric parameters of the frame, the orientation of the base, as well as the number of laminate layers, on the auxetic behavior of the laminated fabrics are investigated. Finite element analysis is also conducted on the 2-layer laminated fabric structure to simulate its deformation behavior. The effect of the elastic modulus difference between the frame and the base on the auxetic performance of the laminated structure was investigated. It proves feasible to produce the auxetic composite fabric structure by using either the warp-knitting jacquard technique or the laminating technique. In the warp-knitted jacquard fabrics, auxetic behavior is achieved only when chain stitches are used as the base structure and when stretched wale-directionally. Comparatively, the laminating approach appears more efficient and flexible in realizing the composite fabric design. It is found the auxetic property of the 2-layer laminated fabrics is affected by the geometric parameters of the frame, including the initial intersection angle, and the square size. An initial angle not larger than 45° is more preferable to obtain a better auxetic effect. Either the frame of too large or too small squares is disadvantageous to the auxetic property of the 2-layer laminated fabrics, thus an appropriate square size should be selected. The orientation of the base fabric affects the auxetic property of the 2-layer laminated fabrics because the base has different tensile properties in its two principal directions. Better auxetic behavior is shown when the courses of the base fabric are oriented perpendicular to the tensile direction, because the course-directional initial modulus of the base is lower, allowing it to expand laterally more easily. One of the major problems that limit the auxetic performance of the 2-layer laminated fabrics is out-of-plane deformation, which can be largely avoided in 3-layer laminates. It is also concluded that the model established based on the through-the-thickness segmentation can simulate the deformation of the 2-layer laminated fabric with good agreement. The modulus difference between the frame and the base is crucial to the generation of auxetic behavior in the 2-layer laminated fabrics. The larger the modulus difference, the better the auxetic property shown in the 2-layer laminated fabrics.