Séminaire: Emmanuel SIEFERT, chargé de recherche au LIPHY

Recent advances in mesoscale material architecture have enabled the design of structures exhibiting nonclassical mechanical responses, such as auxetic behavior, mode coupling (stretching–bending, stretching–twisting), and extreme stiffness-to-weight ratios. Inspired by the nonlinear bending of a carpenter’s measuring tape and the pronounced drooping of U-shaped petioles under water stress, we investigate the bending behavior of rectangular beams decorated with one or several longitudinal vertical fins. Upon bending, the longitudinal stretching of the fins couples with the beam’s curvature, inducing transverse compression that triggers fin buckling beyond a critical curvature. We rationalize this instability in terms of the governing geometric parameters. Fin buckling leads to an abrupt reduction in the beam’s moment of inertia, producing a non-monotonic moment–curvature relationship analogous to that of a carpenter’s tape. By placing pillars on the beam that engage only beyond a prescribed curvature, we induce curvature-dependent hardening. Combined with the fins, this enables the programming of rich, non-monotonic torque–curvature responses. Finally, we demonstrate that when incorporated into pneumatic bending actuators, the fins introduce a snap-through instability, resulting in rapid motion and a hysteretic mechanical response.