📖 The lesson
What a composite is
A composite combines two or more materials that stay distinct but work together. In aircraft it almost always means reinforcing fibres (which carry the load) embedded in a matrix (usually resin, which binds the fibres, shapes the part and transfers load between fibres). Fibre-reinforced plastics are named by their fibre: GFRP (glass), AFRP (aramid), CFRP (carbon).
- Advantages: high strength- and stiffness-to-weight; excellent fatigue and corrosion resistance; complex shapes moulded in one piece (fewer fasteners); tailored strength in chosen directions; smooth aerodynamic surfaces.
- Disadvantages: damage can be hidden inside (delamination, barely-visible impact damage); low electrical conductivity needs added lightning-strike protection (metal mesh or foil); absorbs moisture; sensitive to heat; repairs need special materials, clean conditions and controlled curing; expensive; dust and resins are health hazards.
Fibres
- Glass — E-glass (electrical grade, the common, cheap type) and S-glass (higher strength). Heavier than other fibres, but transparent to radar — radomes, fairings, floor panels, light-aircraft structure.
- Aramid (Kevlar) — yellow, very light and tough, with high tensile strength and outstanding impact resistance, but poor compressive strength; it absorbs moisture and is difficult to cut and drill (special tools).
- Carbon / graphite — black, the stiffest and strongest, used for primary structure. Brittle under impact and electrically conductive: in contact with aluminium it causes galvanic corrosion, so a glass-fibre isolation ply, sealant and titanium or stainless fasteners are used.
- Boron (very stiff, used in some repairs and military aircraft) and ceramic fibres (high temperature).
Matrix resins
- Thermosetting resins cure irreversibly by chemical reaction (usually with heat): epoxy (the main structural resin), polyester (cheap, light aircraft and non-structural), phenolic (fire-resistant — cabin interiors), bismaleimide (high temperature). Once cured they cannot be re-melted.
- Thermoplastic resins (such as PEEK) soften when heated and can be reformed and welded.
- Prepreg — fabric pre-impregnated with a measured amount of resin. Stored frozen (around −18 °C) with a limited out-time at room temperature and a shelf life; cured with heat and pressure (oven, hot bonder or autoclave).
- Wet lay-up — dry fabric impregnated with mixed two-part resin at the job; common in field repairs.
Fabric forms and orientation
- Unidirectional (tape) — all fibres in one direction: maximum strength along that axis.
- Woven (bidirectional) fabric — plain weave (over one, under one: stable, easy to handle) and satin weaves (fibres float over several threads: drapes over curves, fewer crimps, stronger).
- Warp runs along the roll, fill/weft across it, bias at 45°. The selvage edge is the woven edge that stops fraying.
- A quasi-isotropic lay-up (0°, ±45°, 90°) gives similar strength in all directions. Repairs reproduce the original ply count and orientation.
Sandwich structures
Cores include Nomex (aramid-paper honeycomb — the most common), aluminium honeycomb (strong but corrodes if water gets in), fibreglass honeycomb, foam (polyurethane, PVC, polymethacrylimide) and balsa wood. Edges are closed out with filler or sealant to keep moisture out.
Transparent plastics
- Acrylic (PMMA — Perspex, Plexiglas) — cabin windows and light-aircraft canopies: clear, scratches easily, prone to crazing (fine surface cracks) from solvents and stress.
- Polycarbonate — far tougher (impact resistant), but scratches and is attacked by many solvents.
- Clean with plenty of water and mild soap, using a soft cloth; never use solvents such as acetone or thinners; protect with paper or film during work.
Rubbers and sealants
| Elastomer | Character | Typical use |
|---|---|---|
| Natural rubber | elastic, but attacked by oil, fuel and ozone | shock cords, some tyres |
| Buna-N (nitrile) | resists petroleum fuels and oils | fuel and oil hoses, seals |
| Butyl | resists phosphate-ester hydraulic fluid (Skydrol) | Skydrol system seals and hoses |
| Neoprene | resists ozone, weather and some oils | seals, grommets, weatherstrip |
| Silicone | very wide temperature range, poor fuel resistance | high-temperature seals, ducts |
| Fluorocarbon (Viton) | high temperature, fuel and chemical resistance | engine and fuel seals |
- Sealants: two-part polysulphide (fuel-tank and pressure sealing, fay-surface and fillet sealing), one-part silicone RTV, and polyurethane types. They have a work life (pot life), tack-free and cure times, and a shelf life — always check batch dates.
- Rubber parts age: check cure dates and storage (cool, dark, away from ozone, fuel and electric motors).