Lightning Hazards Due to Composite Materials in Aircraft

The most important type of composite material at present in use in the aircraft industry is made up of carbon fibres in an epoxy resin matrix, or carbonfibre composite (CFC). This is rather poorly-conducting fibre1 (1.4 x 10'5fim) embedded in an insulating medium with the fibre volume usually about 60% of the whole. The resistivity of the composite below about 1MHz is independent of frequency and is between two and three orders of magnitude greater than that of aluminium alloy (5.2 x 10"®nm) if long fibres are used. This is the normal practice for aircraft work. Long fibres are used in 'tows' side by side, or sometimes woven to form a mat, coated with resin, and partially cured (pre-preg).. If chopped fibres are used the resistivity is about two orders of magnitude greater. Using long fibre tows in pre-preg form the direction of each lamina can be determined from the mechanical performance required. The resistance o f the whole material depends on the fibre type, the pattern of lay-up used, the fraction of carbon fibre in the final composite and on the manufacturing process. In unidirectional CFC with current parallel to the fibre direction the dc resistivity is 1.0 x 10” 5fim according to Gajda2 . Values of about 2.0 x 10"5fim have been obtained by Smithers3 and others for composites having a fibre volume-fraction'of 0.6 to 0.65. The resistivity with current flow perpendicular to the' fibre is several hundred times larger. In multi-ply laminates having a variety of fibre directions the behaviour of the bulk laminate is much more isotropic than the undirectional CFC.