Curling morphology of knitted fabrics: Structure and Mechanics

Knitted fabrics are two-dimensional-like structures formed by stitching one-dimensional yarn into three-dimensional curves. Plain stitch or stockinette stitch, one of the most fundamental knitting stitches, consists of periodic lattices of bent yarns, where three-dimensional (3D) curling behavior naturally emerges at the edges. The elasticity and geometry of knitted fabrics have been studied in previous studies, primarily based on 2D modeling. Still, the relation between 3D geometry and the mechanics of knitted fabrics has not been clarified so far. The curling behavior of knits is intricately related to the forces and moments acting on the yarns, geometry of the unit knitted loops, mechanical properties, and contacts, hence requiring a 3D analysis. Here, we show that the curling of plain knits emerges through the elasticity and geometry of the knitted loops, combining desktop-scale experiments and reduced elasticity-based simulations. We find that by changing the horizontal and vertical knitting numbers, three types of curl shapes emerge: side curl and top/bottom curl shapes, which are curled only horizontally and vertically, and double curl shape, in which both curl shapes appear together. We uncover that the knit is side-curled when the knitted loop is vertically elongated, while we observe double curl and then top/bottom curl as the loop becomes horizontally elongated. Furthermore, we find that this characteristic loop shape affects the mechanical properties of knitted fabrics. The fundamental mechanism of intricate shape deformation is clarified through the force and moment balance along yarn whose centerline shape is discretized through the B-spline curves, where elastic stretching, bending, and contact mechanics are taken into account. We reveal that the 3D structure of the single knitted loop plays a critical role in the curling behavior. Our results imply that the change in shape per a single knitted loop has the potential to control the 3D natural overall shape of knitted fabrics. The 3D curling behavior of knitted fabrics is useful for industrial applications such as composite materials, wearable devices, and actuators. Our findings could be applied in predicting or designing more complex 3D shapes made of knitted fabrics.