Effects of heavy lightning strikes on pristine and repaired carbon composite structures

The effects of particularly heavy lightning strikes on representative carbon fibre reinforced plastics composite airframe structures, specifically with epoxy matrix systems, which are common within the aerospace industry, have been investigated in this study. The applied action integrals of the lightning strikes significantly exceed the requirements for airworthiness. All tests were performed with conventional prepreg materials and resin transfer moulding/non-crimp fabric materials with high lightning strike current ratings. The pristine panels exhibit major damage zones around the impact points. The results of non-destructive investigations show that the surface damage is predominantly superficial. Only small zones were considerably damaged where extensive repair was necessary. Carbon fibre reinforced plastics panels featuring repair patches were also investigated. Lightning strikes were placed above the scarf and the damage was analysed by various non-destructive investigation methods including micro-computed tomography. In contrast to the pristine panels, the repaired panels reveal different damage behaviour. The damaged zone on the surface was relatively small. In the tapered zone of the patch, electric flashovers between the patch and the base material were observed. Additional microscopy investigations show that these electric sparks also occur inside within the adhesive layers between the patch and the base material. After enhancing the electrical conductivity of the adhesive by adding carbon nanotubes, these difficult-to-detect electric sparks within the layers disappear.