Morphing aircraft wings require flexible skins that can undergo large strains, have low in-plane stiffness and very high out-of-plane flexural bending stiffness. The large strain capability is especially important for gross morphing applications such as span change where the skins may be required to undergo axial strains of the order of 50% or greater. Low in-plane stiffness allows morphing to be accomplished at a reasonable energy cost while high bending stiffness ensures that skin sections between supports do not suffer from significant out-of-plane deformation under aerodynamic pressure loads. One solution proposed is to use sandwiched skins with flexible face-sheets and cellular cores. The cellular cores can be designed to be high-strain capable, have low axial stiffness and high bending stiffness. For some morphing applications (for example, wing span change or chord or camber change), the required deformation is mostly one-dimensional. In such a case, cellular cores with zero Poisson's ratios, which do not display contraction (or bulge) perpendicular to the morphing direction are desired. Restraining the Poisson's contraction (or bulge) of a "conventional" cellular honeycomb results in the effective axial stiffness in the morphing direction increasing by over an order of magnitude. This paper proposes "hybrid" and "accordion" cellular honeycombs, where regular cells (with positive cell angle) and auxetic cells (with negative cell angle) are combined so as to provide large strain capability in one direction (the morphing direction) together with zero Poisson's ratio. Cellular material theory is extended to allow for the analysis of such hybrid and accordion cellular honeycombs, and the results are validated using the Finite Element code ANSYS. Thereafter, the properties and behavior of the hybrid and accordion zero Poisson's ratio cellular honeycombs are thoroughly examined vis-a-vis conventional cellular honeycombs which have single cell-type.
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