In the present study, the failure modes of a hexagonal cell honeycomb core under in-plane loading are investigated. It is shown that, under tensile loading, the node bond failure and ribbon fracture are the main causes of the honeycomb core failure when the core is loaded along the transverse and ribbon directions, respectively. However, under compression loading, the ribbon fracture dominates the failure mode along both directions. The development of a failure envelope for a honeycomb core material is demonstrated using a combination of experiments and simulations. The failureenvelope is cast in terms of the core normal strains along the principal material directions. The failure envelope can be used as a design tool to determine the suitability of the core material for sandwich structural applications.