Lightning strike damage resistance and tolerance of scarf-repaired mesh-protected carbon fiber composites

Carbon fiber/epoxy composite specimens are manufactured using liquid resin infusion and incorporate a copper wire mesh on the outer layer for lightning strike protection. The specimens are then painted in order to be representative of an aircraft skin. The specimens are subjected to a scarf repair, which removes a portion of the wire mesh and of the carbon fiber substrate. The bonded repair is performed to re-establish the structural and electrical integrity of the laminate. Purpose of the study is to evaluate the effect of repair procedure on the structural performance of the carbon/epoxy specimens following a lightning strike, and in particular it is aimed at comparing the two extreme cases where full electrical conductivity is re-established, and where the electrical conductivity is interrupted. To do so, the copper wire mesh is re-applied during the repair following two scenarios. The first, denoted as "good" repair, involves overlapping part of the repair mesh with the parent mesh surrounding the repair area, while the second, referred to as "poor" repair, involves applying a repair mesh that is shorter than the parent mesh, thereby leaving a gap in the electrical path. The repaired specimens are then subjected to simulated lightning strike at the location of the repair. The damage resistance characteristics of the repaired specimens are compared to the benchmark values of unprotected specimens (i.e. without copper mesh) and protected pristine specimens (i.e. without repair). Residual strength testing using four-point bend flexure is used to assess the damage tolerance behavior of the specimens. Results show that a "good" repair performs as well as the pristine protected specimen, while a "poor" repair performs equally or worse than a fully unprotected specimen. (C) 2011 Elsevier Ltd. All rights reserved.