A typical sandwich construction consists of two strong and relatively thin outer sheets or faces separated by but adhered to a layer of a less-dense and low-cost core material, which has lower strength and lower stiffness. The faces bear most of the in-plane loading and transverse bending stresses. The core separates the faces and resist deformation perpendicular to the face while provides a certain degree of shear rigidity along plane that are perpendicular to the faces. The face materials could be metals (titanium, steel, aluminum alloys), plywood or a fiber-reinforced composite.The core could be foamed plastics, synthetic rubbers, balsa wood, honeycomb, truss,etc.. The sandwich architecture provides exceptional flexural stiffness compared to monocoque structures while reducing weight and cost.

Images from Materials Science and Engineering by Callister (6edition)

Behavior of Sandwich Structures of Isotropic and Composite Materials