What Are Sandwich Structures?

Sandwich structures are layered composites in which two thin, stiff face sheets are bonded to the top and bottom of a thick, lightweight core, producing a panel with very high bending stiffness relative to its mass. The configuration draws an engineering analogy to an I-beam, with the face sheets acting like flanges that carry in-plane tensile and compressive loads and the core acting like the web that resists transverse shear and keeps the face sheets separated. Because bending stiffness scales with the cube of the separation between the load-bearing skins, a sandwich panel can be many times stiffer than a monolithic sheet of the same mass, which is why the format is ubiquitous in aerospace, marine, wind-energy, and transportation structures.

Typical face sheet materials include aluminum alloys, carbon fiber reinforced polymer, glass fiber reinforced polymer, and thin steel. Cores range from aluminum and aramid honeycomb to closed-cell polymer foams, balsa wood, corrugated metal, and additively manufactured lattices. The earliest widespread use was in the plywood and balsa-cored de Havilland Mosquito in World War II, and the architecture has since become a standard building block of composite design.

Honeycomb Cores

Honeycomb cores are the most common sandwich core in weight-critical applications. The hexagonal cell geometry provides a very high specific compressive and shear stiffness in the through-thickness direction while using only a small fraction of the volume for solid material. Aluminum honeycomb dominates rigid aerospace secondary structures, while aramid-paper honeycomb such as Nomex is preferred where fire resistance, dielectric behavior, or corrosion immunity matter, for example in aircraft interior panels, radomes, and rotor blades. NASA research on honeycomb core sandwich panels has characterized their compression-after-impact performance with glass and carbon fiber facesheets, informing damage tolerance rules for commercial airframes.

Face Sheet and Core Mechanics

The mechanics of a sandwich panel are treated by a first-order extension of classical beam theory, in which the total bending moment is carried by axial stresses in the face sheets and the transverse shear is carried almost entirely by the core. Standard assumptions are that the face sheets are thin relative to the core and that the core does not contribute to bending stiffness. The Wikipedia-free ASTM C393 beam flexure standard is the industry reference for measuring core shear strength through three- or four-point bending of a full sandwich beam. Companion standards, including ASTM C273 for flatwise shear and ASTM D7249 for face sheet flexural properties, complete the mechanical characterization package.

Sheet Materials and Manufacturing

Sheet materials for face sheets are usually delivered as prepreg composite plies, metal sheet stock, or thermoplastic skins, and are joined to the core with structural film adhesive, co-cured resin, or thermal welding. Vacuum bag autoclave cure, resin infusion, and press bonding are the dominant manufacturing routes. The review of composite sandwich structure in aeronautic applications in Composites Part C surveys these process families and their relation to face-sheet quality, bond line integrity, and damage tolerance requirements. Process control of the adhesive bond is especially critical because face-sheet disbond is a common service failure mode.

Applications

Sandwich structures have applications in a range of disciplines, including:

• Aircraft secondary structure such as floors, fairings, flaps, and radomes
• Helicopter rotor blades and tail booms
• Launch vehicle payload fairings and satellite buses
• Wind turbine blade shells and shear webs
• Marine hulls, decks, and superstructures
• High-speed rail carbodies and automotive floors
• Architectural and refrigerated building panels