16.9 HONEYCOMB AND OTHER STRUCTURAL SANDWICH PANELS

Structural sandwich panels represent what is probably one of the most common applications for structural adhesives. Facing materials can be made to adhere to core materials, such as aluminum or paper honeycomb, to give a high composite strength-to-weight ratio.

Adhesives with filleting properties are required for honeycomb cores. A modified phenolic is often used with aluminum honeycomb for high strength, while a neoprene- or nitrile-based organic solvent type of adhesive is often used with impregnated paper honeycomb. Epoxy adhesives are also commonly used in the fabrication of honeycomb sandwich panels.

Facings of metal, paper, or plastic may be combined with a variety of solid-core materials. The adhesive must transmit shear stress from the facing to the core. It must also be able to resist the tendency of the thinner facing to buckle under design loads or under thermal stresses caused by variations in the panel components' coefficients of thermal expansion.

Adhesives for sandwich structures such as honeycomb and faced cores must have different properties than other adhesives. To achieve a good attachment to an open-cell core, such as honeycomb, the adhesive must have a unique combination of surface wetting and controlled flow during its early stages of cure. This controlled flow prevents the adhesive from flowing down the cell wall and leaving a low-strength top-skin attachment and an overweight bottom-skin attachment. The adhesive must resist being squeezed out from between faying (close-fitting) surfaces when excessive pressure is applied during cure to a local...