Description of Sandwich Structures
Basics
Sandwich construction is a structural panel concept that consists in its simplest form of two relatively thin, parallel face sheets bonded to and separated by a relatively thick, lightweight core. The core supports the face sheets against buckling and resists out-of-plane shear loads. The core must have high shear strength and compression stiffness. (Read more about the theory behind sandwich structures here)
Composite sandwich construction is most often fabricated using autoclave cure, press cure, or vacuum bag cure. Skin laminates may be pre-cured and subsequently bonded to the core, co-cured to the core in one operation, or a combination of the two methods. Examples of honeycomb structures are wing spoilers, fairings, ailerons, flaps, nacelles, floorboards, and rudders.
Properties
Sandwich construction has high bending stiffness at minimal weight in comparison to aluminum and composite laminate construction. Most honeycombs are anisotropic;
that is, properties are directional. The figure illustrates the advantages of using a honeycomb construction. Increasing the core thickness greatly increases the stiffness of the honeycomb construction, while the weight increase is minimal. Due to the high stiffness of a honeycomb construction, it is not necessary to use external stiffeners,
such as stringers and frames.
Facing Materials
Most honeycomb structures in aircraft construction are used in combination with aluminum, fiberglass, Kevlar®, or carbon fiber face sheets. Carbon fiber face sheets cannot be used with the aluminum honeycomb core material, because it causes the aluminum to corrode. Titanium and steel cores are used for specialty applications in high-temperature environments. The face sheets of many components, such as spoilers and flight controls, are very thin-sometimes only 3 or 4 plies. Field reports have indicated that these face sheets do not have a good impact resistance.
Core Types
Honeycomb Core
Each honeycomb material provides certain properties and has specific benefits. The most common core material used for structures containing honeycomb core is aramid paper (Nomex® or Korex®). Fiberglass or aluminum cores are used for higher strength applications.
Kraft paper—relatively low strength, good insulating properties, available in large quantities, and has a low cost.
Thermoplastics—good insulating properties, good energy absorption and/or redirection, smooth cell walls, moisture and chemical resistance, environmentally compatible, aesthetically pleasing, and have a relatively low cost.
Aluminum—best strength-to-weight ratio and energy absorption, has good heat transfer properties, electromagnetic shielding properties, has smooth and thin cell walls, and has a relatively low cost.
Steel—good heat transfer properties, electromagnetic shielding properties, and heat resistance.
Specialty metals (titanium)—relatively high strength to weight ratio, good heat transfer properties, chemical resistance, and heat resistant to very high temperatures.
Aramid paper—flame resistant, fire retardant, good insulating properties, low dielectric properties, and good formability.
Fiberglass—tailorable shear properties by a layup, low dielectric properties, good insulating properties, and good formability.
Carbon—good dimensional stability and retention, high-temperature property retention, high stiffness, very low coefficient of thermal expansion, tailorable thermal conductivity, relatively high shear modulus, and very expensive.
Ceramics—heat resistant to very high temperatures, good insulating properties, is available in very small cell sizes, and very expensive.
The standard and most common core shape, especially for aerospace applications, is the honeycomb core. Its name comes from the resemblance the hexagonal cells have of honeycombs. The cells are made by bonding stacked sheets at specific locations and intervals, then the sheets are expanded. As the sheets pull apart from each other, the bonded sections keep the sheets together, while the unbonded sections spread and create the hexagons. The direction parallel to the sheets (where the bonding occurred) is called the ribbon direction.
Bisected hexagonal core has another sheet of material cutting across each hexagon. Bisected hexagonal hon