Energy balance and assessment of the pressure build-up around a bolt fastener due to sparking during a lightning impact

Abstract

This work is devoted to the calculation of the energy balance associated with the formation of an electric arc between the bolt shank and an inner structural part of the fuselage during a lightning strike. Assessment of the pressure build-up in the confined volume around the bolt fastener has also been performed. This pressure rise comes from the temperature increase and from the mass density increase (melting and vaporisation of materials). Previous electrical measurements performed by Airbus Group during a lightning test campaign have been used to calculate the total available electrical energy. The energies necessary for melting and vaporisation of bolt and rib are derived from thermodynamic properties of aluminium and titanium. A numerical code has been developed to determine the chemical composition (under the local thermodynamic equilibrium [LTE] assumption) and the internal energy of the plasma for air-Al/Ti mixtures. Plasma and material radiation losses and heat conduction losses have also been evaluated. Finally, an analytical model has been implemented to determine the overpressure as a function of the deposited electrical energy, the energy involved in the arc formation, the energy necessary for melting and the plasma composition and mass density. With this approach, maximum pressure values are in the range 200–330 bars.