Broadband radar absorption and mechanical behaviors of bendable over-expanded honeycomb panels

When over-expanded, the hexagonal honeycomb (HH) turns to bendable quadrilateral honeycomb. The over-expanded honeycomb (OH) is anisotropic. In-plane crushing behaviors of the OH were revealed by experiments. Compressed along arm-chair direction, the OH has three deformation modes, turning from quadrilateral honeycomb, re-entrant hexagonal honeycomb to triangular honeycomb. The Poisson's ratio turns from zero, negative to positive. Compressed along the beeline direction, the OH is crushed at shear buckling band. The equivalent strength and modulus change with the compression direction and consistently predicted by structural models. Coated with carbon powder based absorbing material, the OH turns to a multifunctional material. The microwave absorbing behavior changes with the relative density and the thickness of the OH. Oblique incidence promotes the microwave absorption and it is suggested that the incident angle should not be larger than 45°. Pasting GFRC skin, the OH-core sandwich panel still has excellent radar absorption but it will be decreased when the skin is thickening. Through the research, radar absorbing OH has the capabilities of load bearing and electromagnetic energy absorbing. Furthermore, the OH is bendable along arm-chair direction and convenient to make cylindrical sandwich shell.