Evaluating Models for Metallic Honeycomb Structures

Sandwich panels with metallic honeycomb cores have been utilized in many industrial applications where structures with high specific modulus or stiffness to weight ratios are required, most notably in the aerospace industry. Sandwich panels with square honeycomb cores have been found to exhibit a slightly higher strength to weight ratio than their more common hexagonal counterparts, and are also easier to manufacture when high core densities are required. The majority of research on square honeycomb sandwich panels has been aimed at characterizing the structure's dynamic blast performance, as well as its response to out-of-plane shear, bending, and in-plane/out-of-plane compression. In this work, several analytical models for the failure of metallic square honeycomb structures under three-point-bending loading are evaluated. Failure modes of interest include face-buckling, face-yielding, core-buckling, and core-yielding. In contrast to traditional models, in which the moment is resisted entirely by the face sheets and the shear is resisted entirely by the core, several alternative analytical models are assessed. These account for the axial stress caused by the bending moment in the core as well as the face sheets, and for the shear in the face sheets. Accounting for the portion of the moment resisted by the core becomes especially important as the relative core density increases. Finite element analysis is used to verify and compare with the analytical models. Post-processing of the finite element models is focused on determining when local yielding and buckling occur in the structure, and on analyzing the effects of local failure on the global force-displacement behavior of the honeycomb structure. A parametric study with a range of honeycomb geometries is conducted in order to assess how accurately the analytical models predict failure. The intent is to develop simplified analytical models that reliably predict the onset of failure for a broad range of loading levels, geometries, and relative densities. These analytical models are then incorporated in structural optimization protocols to aid designers in selecting combinations of materials and geometries under competing performance constraints.