Static and dynamic characterisation of the Zero Poisson's radio fish cells metamaterial

Morphing structures provide the ultimate aerodynamic efficiency for the aircraft wings during different phases of flight. Achieving a continuously deformed and smooth aerodynamic surface requires a sophisticated morphing skin that provides the necessary strain capacity, prevents stiffness augmentation under actuation, sustains aerodynamic loads, and presents anisotropic behaviour. Cellular metamaterials with zero Poisson’s ratio (ZPR) are able to satisfy the morphing requirements when used as a core in the sandwich morphing skin. Therefore, a new zero Poisson’s ratio metamaterial called “Fish Cells” with compliance in two planar directions is presented that supports the 2D morphing applications. The ZPR behaviour of the Fish Cells metamaterial is studied and proved analytically, numerically, and experimentally. The stress-strain curves of the Fish Cell metamaterial is studied experimentally and effect of geometrical details on the ZPR performance is investigated. In addition, the in-plane and out of plane homogenized equivalent elastic modulus of the Fish Cells are derived. Effect of Poisson’s ratio on dynamic characteristics of cellular structures is studied in two parts. First, hexagonal topology is considered that achieves negative to positive range of Poisson’s ratios by tuning the geometry. The modal properties of such metastructures are studied analytically, numerically, and experimentally. As a result, it is shown that modal properties including the damping ratio, natural frequencies and mode shapes can be tuned by tailoring the Poisson’s ratio. Second, the dynamic analysis of the Fish Cells metamaterial is considered where effect of gradient properties and pre-stress on the modal parameters are studied. It is found that under morphing strains, the ZPR is a crucial factor to save the stability of the cellular core. In addition, it is shown that Fish Cell geometry enables gradient properties that help in tuning modal properties while the global ZPR behaviour is maintained.