sandwich beam

A sandwich beam is a structural member made of two relatively thin outer layers, called skins, bonded to a much thicker inner core. The skins are typically made of strong, stiff material such as steel, aluminium, or composite laminate, while the core is usually low-density material such as foam, balsa wood, or a honeycomb structure. The core sits perpendicular to the skins and does not directly carry bending loads itself, but instead maintains the distance between the skins, which is critical to the beam's resistance to bending.

The efficiency of a sandwich beam comes from mechanics of composite sections. Bending strength depends on the second moment of inertia, which increases with the distance of material from the neutral axis. By placing stiff material far from the centre and filling the middle with lightweight material, engineers achieve high strength-to-weight ratios. A 200 mm deep sandwich beam with 5 mm steel skins and a foam core will bend far less than a solid steel beam of equal weight, making sandwich construction valuable in aerospace, marine, and automotive applications where weight reduction matters.

Common core materials include phenolic foam, polyurethane foam, rigid polyisocyanurate (PIR), and aluminium honeycomb. Cores are selected by density, shear strength, and thermal properties. A honeycomb core might be 40 to 100 kg/m³, while rigid foam ranges from 30 to 150 kg/m³. The skin-to-core bond is critical; failure often begins as delamination where skins separate from the core, typically caused by impact, moisture ingress, or excessive shear stress at edges or around cutouts.

Applications and limitations

Sandwich beams appear in wind turbine blades, aircraft fuselage panels, composite flooring systems, and high-performance racing boats. In buildings, sandwich panel systems are used for curtain walling and modular flooring. However, sandwich construction is expensive compared to solid sections, and the manufacturing process requires controlled bonding conditions and quality control. Core crushing, thermal expansion mismatch between skins and core, and moisture absorption of certain foams can reduce service life if not managed.

The term reflects the layered structure: skins are the bread, the core is the filling. Sandwich beams are distinct from composite laminates, which are made of multiple thin plies of similar material stacked together, and from built-up beams such as welded I-beams, which use discrete flanges and webs rather than continuous core material.