Flexible Skins for Morphing Aircraft Using Cellular Honeycomb Cores

Abstract

This study presents and examines the concept of flexible skins for morphing aircraft applications comprising of a cellular honeycomb core covered by a compliant face-sheet. The overall properties of the flexible skins are then largely governed by the characteristics of the cellular honeycomb core, which are in turn dependent on the cell parameters. The results of this study showed that the cellular cores could easily undergo global strains over 10 times greater than the virgin material of which they were built. The in-plane stiffness of the cellular cores is generally several orders of magnitude lower than the virgin material. Using cores that are thicker than isotropic sheet skins, the out-of-plane stiffness can be many times greater than the sheet skin for comparable mass (due to porosity of the cellular core). In general, honeycomb cores with positive cell angles (as opposed to auxetic cores) produce a higher out-of-plane stiffness. For cellular cores made from high-strain capable materials and undergoing large strains, geometric and material non-linearities need to be considered. When the cores are stretched along the principle axes they geometrically stiffen, thereby reducing the maximum global strains achievable. When material softening is considered, the forces required to deform the cellular core to large global strains are reduced.