Shear driven propagation of solitons in metamaterials

Solitary waves steadily propagate in a system without changing their shape and are rarely observed in designed structures. We engineered a metamaterial using a network of nonuniform beams that can induce a localized kink under a pre-compression. By applying a shear deformation to the pre-compressed structure, the induced kink propagates, like a solitary wave, perpendicular to the direction of shear deformation. The shear-induced solitary wave is reversible since by changing the direction of shear, the kink travels in the inverse direction. The soliton in this structure appears because of multi-stable beams, where transitions between different states occur through an unstable intermediate state by shearing the system. Therefore, the kink propagates by the sequential snapping of beams that leads to oscillatory shear and normal force responses. We observe that the solitary waves for odd- and even-layer structures have an opposite phase. Localized kinks trigger surprising mechanical properties of the structure with negative shear and normal (Poynting) moduli. Our results provide an insight into harnessing mechanical instabilities for programming the shear and normal force responses of materials.