The I-beam idea, spread over a panel
When a beam bends, the material near the top and bottom surfaces is stretched or compressed the most, while the material near the middle carries little stress. That is why an I-beam puts most of its material in two flanges, separated by a thin web. A sandwich structure applies the same idea to a flat or curved panel. Two thin, stiff skins are placed far apart, with a thick, light core between them holding them at that distance.
The skins carry the bending loads as tension on one side and compression on the other. The core keeps the skins apart, resists shear, and supports the skins against buckling. Because the core is light, the panel as a whole can be very stiff in bending for its weight. This is a standard example of how a composite design exploits arrangement rather than raw material strength.
Why thickness matters so much
Bending stiffness depends strongly on thickness. Doubling the thickness of a solid plate makes it much stiffer in bending, far more than twice as stiff, while only doubling its weight. A sandwich panel makes use of this: a thicker core adds very little weight but pushes the skins farther apart, and bending stiffness rises rapidly with the separation. In practice, engineers can often get a panel that is several times stiffer than a solid skin-only plate for only a small increase in weight, though the actual ratio depends on materials and dimensions.
Skins
Skin materials include fiber-reinforced polymer laminates, aluminum sheet, and in some cases wood or steel. A carbon or glass skin can be tailored so that fibers lie along the main load directions. Skins need to be stiff and strong in the plane of the panel, resistant to local denting, and compatible with the adhesive that bonds them to the core. The selection of fibers draws on the usual trade-offs among glass, aramid and carbon fiber.
Core types
- Honeycomb. Thin walls form hexagonal or similar cells, usually with the cell axis perpendicular to the skins. Common materials are aluminum, aramid paper (often phenolic-coated) and thermoplastics. Honeycomb offers a very high stiffness-to-weight ratio and is common in aerospace panels such as floors, fairings and control surfaces.
- Foam. Closed-cell polymer foams such as PVC, polyurethane, PET or polyimide types. They are easy to shape, resist water absorption, and suit boats, wind blades and curved panels.
- Balsa and other wood cores. End-grain balsa has been used for many years in marine and wind energy structures because it is light and has good compressive strength. It needs to be sealed against moisture.
- Corrugated, truss and 3D-printed cores. Designs where the core geometry is engineered for specific loads, sometimes made by additive manufacturing.
- Syntactic and filled cores. Resin filled with hollow microspheres, used in thin applications or where compression strength is key.
Each core has its own range of density, shear strength, temperature capability and cost. The right choice depends on the load, environment and manufacturing method.
What the core must do
The core’s tasks are modest but critical:
- Carry the shear forces between the skins.
- Resist crushing under loads perpendicular to the panel, such as local impact or clamped fasteners.
- Hold the skins stable and prevent local buckling or wrinkling.
- Stay bonded to the skins through the service life.
Because the core is light, it is relatively weak and soft compared with the skins, so local loads need care.
The bond
A sandwich panel only works if the skins and core act as one. The adhesive bond (or the resin itself in a co-cured process) transfers shear between them. A poor bond, an unfilled area or a contaminated surface can leave a region where skin and core behave independently, and the panel loses much of its stiffness there. Process control is vital, and the details parallel those found in composite layup and curing methods. For honeycomb, only the thin cell edges touch the skin, so the adhesive must form fillets around them. Adhesive film or prepreg with a controlled flow is often used.
The general principle that bonding controls performance echoes the fiber-matrix interface, although here the interface is at a much larger scale.
Typical failure modes
Sandwich panels have their own specific ways of failing:
- Skin wrinkling. The compressed skin buckles locally into the core.
- Core shear failure. The core cracks or yields in shear when loads are high or the span is short.
- Skin-core debonding. The skin separates from the core, which is the sandwich equivalent of delamination.
- Indentation and local crushing. A concentrated load such as a dropped tool crushes the core under a thin skin.
- Impact damage. Thin skins have limited ability to spread impact energy, so damage can penetrate the skin and crush the core beneath. It may be hardly visible on the surface.
- Water ingress. Water entering honeycomb through damage or poorly sealed edges can freeze and expand, or corrode aluminum cores, and add weight.
These mirror many of the mechanisms in the general discussion of how composites fail, with extra sensitivity to core crushing and moisture.
Inspection and repair
Tap testing, ultrasonic inspection, thermography and other techniques are used to find disbonds and crushed core. Honeycomb panels are common targets for nondestructive evaluation because flaws can be both internal and hard to see. Repairs typically remove the damaged area, replace the core with a plug and bond a patch over it, using procedures specified for the particular panel.
Joining and edges
Attaching fasteners or fittings requires care, because the core cannot carry high bearing loads. Designers use potting compounds, solid inserts or denser core regions at attachment points so loads spread into the skins. Edges are normally closed out with a solid laminate or an edge member to protect the core from moisture and damage.
Where sandwich structures appear
Sandwich construction appears in aircraft interiors and control surfaces, satellite panels, high-speed trains, boats, wind turbine blades, building panels and sports equipment. Wherever stiffness for weight is a priority and the loads are mainly distributed bending, it is a strong candidate. Where loads are highly concentrated, repeated impact is likely or moisture exposure is severe, solid laminates or other designs may be more appropriate.