Laminated, Sandwich and Monolithic Structures

Laminated plastics consist of layers of synthetic resin-impregnated fibres (or other coated fillers), which are bonded together to form a single laminate or sheet of composite material. Plastic laminates are used to ‘face’ other structural materials, in order to:

• Provide a more durable surface to a softer material.
• Enhance the surface appearance (colour, porosity, smoothness etc.).
• Increase the strength and rigidity of many non-metallic structures.
• Produce other desirable surface characteristics, such as acid/corrosion resistance, non-conductivity, non-magnetisability or the ease of keeping a surface clean is required.

To provide a light-weight structure, which possesses strength and rigidity, one of several structural materials is sandwiched between two laminated composites. The sandwiched material (the core) may be made of a solid material, such as wood or a series of thin corrugations of a material, which are joined and placed end-on (in the form of the cells of a honeycomb) within the laminates.

Where wood is used as the core material, it usually consists of low-density balsa wood, which has been cut across the grain and sandwiched between two layers of reinforced resin (or a metal). This construction makes an extremely light, yet strong material, which can be used as floor panels, wall panels and occasionally, aircraft skins.

The cellular core, which are used for laminated honeycomb material may be made from resin-impregnated paper or from one of the many fibre cloths. The core is shaped and bonded between two face sheets of resin-impregnated cloth. The finished sandwich structure is very rigid and has a high strength-to-weight ratio. It is transparent to electromagnetic (radar/radio) waves, making it ideal for radome of all kinds.

Metal honeycomb cores are also sandwiched between two face sheets of fibre-reinforced resins. On other occasions, the metal honeycombs may be found sandwiched between sheets of light alloy, stainless steel or titanium. This type of core is referred to as ‘metal-faced honeycomb’ and is used where abrasion and heat-resistance is important or when sound-absorption qualities are desired.

In monolithic structures, angle sections (‘Top Hat’, ‘U’, ‘I’ and ‘Z’), frames ribs and stringers are fashioned from similar materials to the outer layers of the sandwich structure. These are then covered with the appropriate surface ‘skin’, before the stronger, metallic spars and hinges are attached. Such a structure can save many kilograms in the weight of the flying control surfaces or the fin structure of a large aircraft.