A sandwich structure is made up of a low density core material that is layered between thin rigid skins bonded to each side. This results in a structure that is strong and stiff but also lightweight. Most of the bending stresses in a structure are carried by the material on the outer faces, so the inner region of the structure can be replaced with a low density material to minimize weight while still maintaining much of the strength and stiffness found in a solid structure.
Sandwich structures appear in many everyday examples where mechanical performance and low weight are desired. Probably the most common example of this is cardboard. Cardboard is a sandwich structure made from a corrugated paper core that is faced on both sides with paper skins. The sandwich construction of cardboard gives it stiffness, and this is especially evident when one skin of the sandwich is peeled away leaving paper that is floppy on its own.
While sandwich structures can take many shapes including compound curves such as a dome, this article will focus primarily on flat sandwich panel composite structures.
Composite sandwich panels are an appealing construction approach in aerospace applications where strength, stiffness, and low weight are all desired simultaneously. Aerospace sandwich panels use low density core materials such as rigid foam, honeycomb, or balsa while their skins are typically made from materials that are strong and stiff such as carbon fiber composite or aluminum. Many examples of this type of panel construction can be found in the DarkAero 1. The ribs and shear webs in the wings and tail of the aircraft are made from panels constructed with an aramid honeycomb core that is faced on both sides with carbon fiber skins. The fuselage bulkheads in the DarkAero 1 prototype are made up of sandwich panels constructed from aluminum honeycomb core material faced with carbon fiber and glass fiber skins.
The strength, stiffness, and low weight of sandwich panels are natural reasons to use them in aircraft structures, but there are some other advantages that are not as obvious.
One of the biggest advantages of creating structures from flat sandwich panel material is manufacturing efficiency. Bulk flat panels can be fabricated generically and then used as a raw stock material to create individual components using a CNC router. While the process of creating the panel stock may involve manual labor, the process of cutting parts from the stock can be automated to achieve excellent manufacturing efficiency.
The alternative would be to create these same parts one at a time from dedicated molds. This process would require manual labor throughout the construction process of each component rather than just during the creation of the raw stock material.
Flat panels can also be folded to enable even greater design flexibility. Panel folding is accomplished by cutting a groove through the skin and core on one side of a panel while leaving the skin on the other side intact. This locally reduces the stiffness of the panel at the groove, which allows the groove to act as a fold line while the rest of the panel remains rigid. This technique can be used to create 3-dimensional folded structures such as boxes or prism shapes.
Structures in the DarkAero 1 that were created with this method include the armrests, wheel wells, and seat bulkheads. The designs of these parts went through several iterations, so building them from folded panels allowed the designs to be rapidly refined without building new molds for each design revision.
When many parts are cut from a single bulk panel, they are uniform and have minimal variation in their weight and mechanical properties. This makes quality control much easier to achieve, because it is inherent to the manufacturing process. The alternative would be to manufacture each part individually from its own dedicated mold, which would introduce potential sources of defects and variation between the parts. Although good quality control is still achievable when making parts individually, it definitely requires more attention.