Quasistatic Deformation and Failure Modes of Composite Square Honeycombs

Carbon fibre epoxy matrix composite honeycombs have been fabricated by slotting, assembling and adhesively bonding composite laminate sheets with various fibre architectures. Their out-of-plane compressive and in-plane shear responses were measured as a function of relative density, ratio of the cell height to width and the number of cells in the specimen. The measurements indicate that the response is relatively insensitive to the ratio of the cell height to cell width and number of cells in the specimen but is strongly dependent on the laminate type and fibre orientation. For example, the compressive strength of the honeycombs made from 0− 90 laminates with fibres aligned with the compression direction was greater than that of honeycombs made from a woven material with fibres at ±45. However, the shear strengths exhibited the opposite trend. These differences were attributed to a change in failure mode. In compression the honeycombs failed by either elastic buckling or plastic microbuckling while in shear the two main failure modes were elastic buckling or shear failure of the composite sheet material. Analytical models are derived for these collapse modes and used to predict the strength of the honeycomb structure. The predicted strengths are substantially higher than the measurements due to the presence of manufacturing imperfections in the honeycombs that are not accounted for in the analytical models. A limited finite element (FE) investigation is also reported to quantify the effects of imperfections on the compressive strength of the composite honeycombs. The measurements and analytical predictions reported here indicate that composite cellular materials such as honeycombs reside in a gap in the strength versus density material property space, providing new opportunities for lightweight, high strength structural design.