Characterizing Jamming in Knitted Fabrics

When stretched, most knitted fabrics have a soft mechanical response determined by yarn properties and curvature of the stitches for low stress, which gradually stiffens for higher stress as the yarn is increasingly compressed. However, when knit fabrics have tight stitches, the yarn is highly compressed even when the fabric is unstretched. This creates an initially stiff response that then softens to the usual linear response, before stiffening again. This new stiff regime at low-stress is a result of geometric confinement of yarn and can be considered a fiber-analogue of "jamming." Once a significant enough force is applied such that the stitches collectively move apart and the confinement is overcome, the fabric can begin to stretch. This jamming can be seen in force-extension experiments on stockinette fabric when it exhibits an initially large tensile modulus at low stress. Utilizing computational simulations of knit fabrics, we investigate yarn and manufacturing parameters that affect the presence and intensity of jamming. To study the interplay between confinement and mechanics, we map the distribution of non-local contact interactions, known as the contact set. Changes in the location and geometry of the contact set provide a morphological indicator of jamming.