Module 6.3 Composite and non-metallic materials, wood and fabric

Knowledge levels

Item	A	B1 / B3	B2 / B2L
6.3.1(a) Characteristics, properties and identification of composites and non-metallics (other than wood); sealants and bonding agents	1	2	2
6.3.1(b) Detection of defects/deterioration	1	2	—
6.3.1(c) Repair and inspection procedures	—	2	1
6.3.2 Wooden structures	1	1	—
6.3.3 Fabric covering	—	1	—

What is a composite?

A composite combines two or more materials that keep their identities: fibres (reinforcement, carrying the load) in a matrix (resin, holding fibres in place and transferring load).

Fibre	Properties	Uses
Glass (E-glass, S-glass)	cheap, good strength, heavier than carbon, radio-transparent	fairings, radomes (with care), light aircraft
Carbon/graphite	very high strength and stiffness, light; electrically conductive; galvanically cathodic to aluminium	primary structure (787/A350 fuselage and wings), control surfaces
Aramid (Kevlar)	very tough, impact-resistant, light; poor in compression; absorbs moisture; hard to cut	fairings, helicopter panels, ballistic protection
Boron	very stiff, used in repairs and military structure	patch repairs
Ceramic	high-temperature	engine hot sections, heat shields

Matrix	Notes
Epoxy	most common thermoset; good strength and adhesion
Polyester, vinyl ester	cheaper, light aircraft
Phenolic	fire-resistant (cabin interiors)
Thermoplastics (PEEK, PEI)	can be re-melted; tough; growing use

Forms

• Unidirectional tape (all fibres one way), woven fabric (plain, satin, twill weaves), pre-preg (fibres pre-impregnated with resin, stored frozen, has shelf life and out-time limits), wet lay-up.
• Sandwich construction: two thin strong skins bonded to a light core (Nomex/aramid or aluminium honeycomb, foam, balsa) - very stiff and light. Floors, control surfaces, fairings.
• Ply orientation (0°, ±45°, 90°) is designed for the loads; repairs must restore it (warp direction is marked on fabric; the warp clock shows orientation).

Sealants and bonding agents

• Polysulphide sealants (fuel tanks, pressure cabin), silicone sealants (high temperature, firewalls, windows), polyurethane.
• Two-part sealants have pot life (working time), tack-free time and cure time; they must be mixed correctly and used before expiry.
• Adhesives: epoxy (structural bonding), film adhesives (cured with heat and pressure in an autoclave or with heat blankets), cyanoacrylate (non-structural).