Zero Poisson's Ratio Cellular Honeycombs for Flex Skins Undergoing One-Dimensional Morphing

Cellular honeycomb cores with overlying flexible face sheets have been proposed for use as flex skins for morphing aircraft. The cellular cores, which provide underlying support to the face sheets for carrying aerodynamic loads, must have low in-plane stiffness and high in-plane strain capability. For one-dimensional morphing applications such as span-, chord-, or camber-change, restraining the Poisson's contraction (or bulging) that a conventional cellular honeycomb core would otherwise experience in the non-morphing direction results in a substantial increase in the effective modulus in the morphing direction. To overcome this problem, this article develops zero Poisson's ratio hybrid and accordion cellular honeycombs. Cellular Material Theory is extended, and analytical solutions for the mechanical properties and global strains of the hybrid and accordion cellular honeycombs are developed. The analytical results show excellent agreement with ANSYS finite element results. Comparing the properties shows that the hybrid and accordion honeycombs proposed have generally similar in-plane axial stiffness and strain capabilities to conventional honeycombs when the latter are unrestrained in the non-morphing direction. However, with the zero Poisson's ratio of the hybrid and accordion honeycombs, it is observed that the axial stiffness in the morphing direction will not increase when the skins are restrained in the non-morphing direction. The zero Poisson's ratio of the accordion and hybrid cellular honeycombs is not helpful from an out-of-plane load carrying ability standpoint. However, the out-of-plane load carrying ability of the accordion honeycombs can be superior to those of conventional honeycombs if the 'continuous fibers' are sufficiently thick, leading to a very large modulus in the non-morphing direction. The effective out-of-plane stiffness of hybrid cellular honeycombs, on the other hand, is poorer than conventional cellular honeycombs.