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.

Shape memory polymer-based hybrid honeycomb structures with zero Poisson’s ratio and variable stiffness

“…The rationale behind the use of cellular or honeycomb materials is their remarkable lightweight characteristics and tailorable mechanical performances, which are directly dependent upon the topology and size of the unit cell [1]. Flexible honeycomb structures have also been recently proposed as a promising solution for morphing skins, the latter being a critical technology for the design of morphing airframes [2,3]. Different honeycomb configurations result in different in-plane Poisson's ratios, which lead to multiple deformed shapes when bent out-of-plane [4,5].…”

“…These features (absence of transverse in-plane deformation and cylindrical bending) make ZPR cellular structures an interesting material platform for one-dimensional morphing, the build-up of cylindrical sandwich panels, or morphing structures that undergo cylindrical bending deformations [18,22]. Honeycomb structures with ZPR property have been also used in biomedical scaffolds [23] and one-dimensional spanwise morphing flexible skins [2,3,21]. For all the aforementioned honeycomb structures, the out-of-plane flatwise stiffness inevitably decreases when the out-of-plane flexibility increases when the thickness of the cell wall is minimized.…”

Design and mechanical testing of a variable stiffness morphing trailing edge flap

“…Unit cell geometry definition of the honeycomb core substructure (Ai et al, 2015) Due to their high anisotropy in mechanical properties, cellular solids and honeycomb core have received significant attention from the morphing research community (Gibson and Ashby 1997;Olympio et al, 2009Olympio et al, , 2010Bubert et al, 2010) and particular interest is given to the zero Poisson's ratio (ZPR) honeycomb core which has no anticlastic effect when subject to bending in one direction. A honeycomb core of zero Poisson's ratio (Bubert et al, 2010) is adopted in this study.…”

One Dimensional Morphing Structures for Advanced Aircraft

“…Morphing sandwich structures capable of high global strains have also been investigated (Joo et al, 2009;Bubert et. al., 2010;Olympio et al, 2010). However, suitable improvements over these structures, such as anisotropic fiber reinforcement and a better developed substructure for out-of-plane reinforcement, are desired for a fully functional morphing skin.…”