Lightning occurs during extreme weather conditions and can interact with aircraft during flight. Since aircraft are increasingly made of electrically resistive composites, rather than conductive metals, it is important to analyse the damage caused by a strike. Literature has shown lightning can be modelled as a plasma (an ionised gas) and has been idealised into four electric current waveforms of varying current and duration. Authors have almost exclusively modelled just one of the key lightning physics (plasma, thermal-electric, pressure or thermal-expansion), ignoring some important physical phenomena such as strain and heating rate effects, using assumed loading conditions on the specimen surface, neglecting the key lightning plasma behaviour. Within this work simulations were conducted to establish the likely damage to a composite from electric current Waveform B. A robust coupling framework was developed to link plasma and damage models using automated specimen meshing and loading methods based on the predicted output of the plasma model. The precision, run-time and flexibility of the proposed approach were demonstrated, with thermal damage predictions generated in approximately 33 hr.
Herein five distinct specimen designs have been compared. It was found that specimen representation had limited impact on plasma global structure, even with significant change in specimen properties (e.g. from copper to epoxy). However, noteworthy variation in the local specimen surface loading (pressure, velocity, current density and temperature) was witnessed with specimen property change, changing by up to 88%. Such variation in local specimen surface loading significantly varied the prediction of composite material thermal damage depth (up to 1200%) and surface damage area (up to 1314%). Moreover, this work, for the first time, provided predictions for the thermal damage suffered by composite specimens exposed to Waveform B. In addition the waveform parameters of `waveform peak', `rise time' and `time to reach the post-peak value' have been varied. It was found that peak current was the key parameter influencing plasma properties and specimen damage. A 10% increase in peak current magnitude (and resulting 21% increase in action integral) resulted in a 12% increase in plasma peak pressure, a 5% increase in specimen surface current density, and subsequently a 8.7% increase in thermal damage volume and a 15.2% increase in thermal damage depth. Overall action integral had the strongest correlation with four of the five considered damage measures while peak current had the strongest correlation with the other.
Finally a benchmarked progressive damage model incorporating strain and heating rate effects for the prediction of damage resulting from lightning strike is presented. A dynamic, coupled temperature-displacement, explicit model then predicted the material state due to the combined thermal and pressure loads and thermal-expansion. Unprotected specimen damage results were presented for two SAE lightning test Waveforms (B & A); with the results illustrating how thermal and mechanical damage behaviour varied with waveform duration and peak current. Considering all lightning physics the main damage modes for test Waveform B were thermal decomposition of the epoxy matrix (degradation and conversion to gaseous products) and delamination, resulting from thermal expansion.