Design and performance of novel aircraft structures with folded composite cores

This chapter focuses on novel sandwich structures with open cellular composite cores manufactured by folding thin sheet base materials into a three-dimensional structure. Folded composite cores or ‘foldcore’ manufactured from resin impregnated aramid paper have similar densities and mechanical properties to Nomex honeycomb. Furthermore, they allow ventilation to prevent moisture build-up due to their open cell design and can be manufactured cost-efficiently in a continuous process. In aircraft fuselages, a sandwich design concept could yield significant weight savings compared to an aluminium reference fuselage, through increasing frame spacing and elimination of stringers. The foldcore concept is described with typical base sheet materials, core geometries, and fabrication technology suitable for continuous manufacturing processes. Folded core properties and design are discussed with test methods used for measuring thin base sheet properties and determining core through-thickness compression and shear failure modes and strength properties. Core design is based on micromechanics cell models used in FE methods for simulating progressive damage and collapse mechanisms to provide core properties for use in sandwich structural analysis and design of sandwich structures. Finally, the damage tolerance of sandwich panels with carbon fibre/ epoxy skins and aramid fibre/phenolic folded core is discussed, based on experimental investigations by drop tower and gas gun impact tests. Comparison between observed and computed failure behaviour for a range of impact load cases shows good agreement, indicating that the FE methods could provide the basis for design and certification of these advanced aircraft sandwich structures.