Spotlight on Cardiff research: How lightning strikes are hazardous to composite aircraft

By Rabia Abid

With the on-going quest towards more lightweight aircraft and better fuel efficiency, the demand is greater from the aerospace industry to replace conventional materials with composite equivalents. Owing to their high strength to weight ratio, carbon-fibre-reinforced plastics (CFRPs) are already well established and the 20% increase in fuel efficiency that they offer has resulted in substantial fuel savings. The anisotropic electrical conductivity of CFRPs, which is high along the fibres but minimal across them, stems from the arrangement of the different components, with conducting carbon fibres in different orientations reinforced with epoxy to fulfil mechanical strength requirements. This leads to complex electrical phenomena under different external conditions. Anisotropic electrical conductivity also accounts for the unpredictable behaviour of CFRPs under lightning strikes and the inevitable damage that arises.

The resistivity of CFRP materials is 1000 times higher than that of aluminium, such that a CRFP sample will dissipate 1000 times more energy than an equivalent one made of aluminium for the same lighting current magnitude. The risk of lightning-induced damage is, therefore, much greater for CFRPs as the electrical charge is less readily dissipated. This damage may involve thermal decomposition of the epoxy layers, melting and burning, de-lamination, and vaporisation of the resin. A thin protective metal layer (typically 20-μm thick) is commonly coated on CFRPs to dissipate lightning current and to prevent its penetration into the material. However, this layer also makes the material and, hence, the aircraft heavier. A key objective is, therefore, to design CFRP components and structures with effective lightning protection without compromising their other attractive properties for the aerospace industry, principal among which is their lightweight nature.

Understanding the electrical behaviour of these materials is indispensable in this context and involves characterising their electrical properties (primarily their electrical resistance) under different modes of energisation. Ultimately, a detailed study of effect of lightning currents is required. The research in Morgan-Botti lightning Lab under this studentship comprised electrical characterization of carbon fibre composites under various current energisations, ac, dc and impulse. The work was supervised by Professor Manu Haddad, Director of MBLL and Dr. Huw Griffiths, Reader in High Voltage Engineering, at Cardiff School of Engineering. This studentship investigated the damage effect of lightning current by measuring the residual mechanical strength in carbon fibre composites post lightning current under different levels. Important observations were that low-level lightning currents up to 350A do not cause any significant damage. However, lightning currents in the kA range can do where mechanical strength got adversely affected. These outcomes aid the mechanical design requirements for future composite aircraft for development of composite structures and components better protected against lightning strikes.

Computational methods were also developed through ARCAA in Cardiff University, where Thomas Green helped out to solve large CFRP flat panels under lightning strikes using supercomputer, Raven.

Moreover, the research is funded by the School of Engineering and a small contribution by Airbus Group Innovations. The experimental campaigns undertaken involved work in Cardiff High Voltage Lab, and Morgan Botti Lightning Lab, where the lightning chamber was used to strike carbon composites samples under lightning strikes, up to 70kA as in the above picture, this was coordinated and designed by Dr. David Clark, Lecturer in High Voltage Engineering and Mr. Chris Stones, Manager of Morgan Botti Lightning Lab. Technical help from Mechanical group, mainly Dr. Mark Eaton, Ian King, and Paul Malpas, was also sought throughout the studentship. In addition, Mr. Chris Dunscombe in Cardiff Physics Lab devised methods of best electrical contact to CFRP under low and high current for the studentship completed.

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