OUT-OF-PLANE COMPRESSION STRENGTH OF HIERARCHICAL SANDWICH HONEYCOMB CORE

Expanded honeycomb cores from aluminum and phenol resin impregnated aramid paper (Nomex^®) can be considered the state-of-the-art in the field of lightweight cellular cores for sandwich construction. Their exceptional weight specific out-of-plane compression and shear strength are the key performance characteristics which contribute to the success of honeycombs as sandwich core material. Structural hierarchy has been introduced to the sandwich concept to further improve the weight specific mechanical properties. Hierarchical structuring prevents microstructural buckling enabling significantly higher compression strength than the first order structures from which they are derived.

Structural hierarchy can be considered at different levels, either for the face sheets with a thin sandwich panel replacing the single face sheet or in the honeycomb core replacing monolithic cell walls with thin sandwich cell walls. A layered combination of sandwich constructions with sandwich face sheets on a hierarchical core with large cell size prevents wrinkling and dimpling failure modes. Sandwich cell walls are especially efficient for honeycomb cores with a large cell size enabling a highly buckling resistant microstructure at very low densities. Such hierarchical sandwich honeycomb cores are proposed as a new class of honeycomb core materials and are protected by patents granted to the company EconCore. The out-of-plane compressive strength of the hierarchical sandwich honeycombs is investigated using finite element modeling, validated through physical testing and compared to conventional honeycomb cores.

The best compressive strength to core density ratio is observed in the Al 46 configuration with compressive strength of 3.9 MPa and a core density of 46 kg/m³. This is a large improvement with respect to both the classical aluminum honeycombs and the Nomex^® honeycombs of various cell sizes. Hierarchical sandwich honeycomb cores and sandwich structures with those cores and sandwich skins open many options for future development and focused research.