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A sandwich panel is a structural element consisting of thin outer layers of a solid material held together by a core of another material in a similar way as bread slices form a sandwich around a filling. Weight, strength, durability, and cost are all factors in the choice of materials. Sandwich panels can provide greater strength and resist stresses with a lighter overall weight than a single solid material. The strength of the unit depends to a large degree on the strength of the adhesive bond between the elements and the thickness of the core. These units can be designed for walls, roofs, or floors of buildings and are used in the aviation industry.
The outer layers of a sandwich panel are sometimes made of sheet aluminum, plastic, or solid wood veneer, but are often made of oriented strand board for building construction. This type of board is a composite from slivers of wood bonded with resin in a deliberate pattern. The skins are chosen depending on where the panels will be used.
The layers are fixed with adhesive and pressure to a core of material that insulates and stiffens the panel. Cores are made from a wide variety of materials. The core is often an expanded foam product such as polystyrene or polyurethane. Other cores include aluminum honeycomb and balsa wood. A sandwich panel may also be fire-retardant through treated polyurethane or mineral fiber such as Nomex® in the core.
The design of a sandwich panel is intended to withstand considerable stress. When a panel bends, the skin on one side stretches and the skin on the other side compresses. The core material holding the unit together helps to maintain the rigidity of the panel, which increases substantially with additional thickness. A quality adhesive is essential to the integrity of the panels because it resists the tendency of the skins to slide off in different directions when the unit is stressed.
Completed panels are tested for load bearing capability, impact resistance, and compressive and tensile strength. Testing also evaluates the tendency of the skins to peel away from the core. A sandwich panel intended for aviation applications is tested for fire and smoke release and must meet strict standards.
Units made for the construction industry are sometimes referred to as structural insulating panels (SIPs). Buildings erected using SIPs can be more airtight and better insulating than those using standard frame construction. The main advantage of sandwich panels for the aviation industry is the reduction of weight which translates to fuel savings. Structural units designed for aviation use combine stability and strength with light weight to improve aircraft efficiency.