Failure mode maps for foam core sandwich beams

Sandwich panels can fail in several ways. The faces and core can yield plastically or fracture depending on the nature of the materials from which they are made; the compressive face can buckle locally or "wrinkle", and the bond between the faces and core can fracture, causing delamination. The critical failure mode, which occurs at the lowest load, depends in part on the properties of the face and core materials and, in part, on the design of the beam. Here, we develop equations describing the load at which failure occurs for each possible failure mode for a sandwich beam with face and core materials that yield plastically. We then develop a failure mode map, with axes of core relative density and the ratio of face thickness to span length, which, for a given loading configuration and set of face and unfoamed solid core materials, shows the dominant failure mode for every possible beam design. Tests on sandwich beams with aluminum faces and rigid polyurethane foam cores show that the equations and map describe failure well. The map can then be used to design the minimum-weight sandwich beam for a given strength. Similar failure mode maps can be developed for sandwich beams made from face and core materials that fracture and for sandwich plates.