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At the heart of knitting physics

Despite its innocuous appearance, a knit has amazing physical properties : while the thread that composes it does not stretch, the fabric remains deformable and can extend over several times its size. Researchers from the Physics Department have proposed a simple model linking the flexible properties of the wire to the very low stiffness of the fabric, paving the way for the development of innovative materials.

Beyond its traditional millennial appearance, a knit is a material with amazing properties that make it a key player in modern manufacturing technologies. Indeed, if the wire that composes it does not stretch, the knit can drap any object, and has an extraordinary elasticity in comparison with more traditional materials such as woven textiles. How do these properties emerge from Jersey’s customary tracery ? How to reconcile them with the inextensibility of the simple thread of wool or cotton that composes it ?

Physicists from the Laboratoire de Physique Statistitque (ENS / CNRS / PSL) have answered this question in a publication in the journal Physical Review X as part of a project aimed at rationalizing the behavior of mechanical "metamaterials"* made of elastic threads or plates - another example is that of Structured materials like origamis on which this team has made significant progress.
During this study, they made model knitting samples, in collaboration with the National School of Decorative Arts, and observed them from every angle, capturing in particular with great precision the trajectories of all the meshes during their controlled deformation. From these measurements, they proposed a simple mechanism linking the flexibility of the yarn to the very low stiffness of the fabric. Assuming then the inextensible thread, therefore of constant length, and the topology of the network of stitches (one on two stitch upside down), they have managed to explain the forms that these systems adopt when they are subjected to mechanical stresses, as well as their very low stiffness.

These works are part of a wider reflection on the "functionalization" of everyday matter by means of elementary excitations, which are the knots or interlacing for the wires, and the folds for the plates, thus opening the way. the development of innovative materials based on the reappropriation of popular techniques and their empirical values.

*ANR Project JCJC METAMAT "Statistical mechanics of mechanical and topological metamaterials", launched in 2014 and ending in late 2018.

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