Grommet Hole Reinforcement and Lightning Strike Protection in Composite Structural Assembly 2011-01-2610

Aircraft design and assembly challenges include provision of hole protection from lightning strike in fastened joints, especially in the vicinity of fuel tanks or electrical services. Installation of interference fit fasteners helps minimize the effect of lightning strike and also enhances the durability and load transfer ability of the joint. However, these are known to cause laminate damage during installation which can degrade the fastener system performance. Use of a pre-installed expanded grommet into the hole prior to installing interference fit fasteners has been shown to greatly enhance the effectiveness of the joint as well as the conductivity of electrical current into the structure to minimize arcing or plasma discharge at the fastener in the event of lightning strike.

The Fatigue Technology (FTI) GromEx_® system provides such hole protection for fastener installations into composites, allowing for the installation of interference fit fasteners without detrimental impact to the laminate. In a test program performed to evaluate the performance of the GromEx system under lightning strike testing, in accordance with the methods specified in SAE ARP 5416, it was found that interference fit fasteners, combined with the expanded grommets, eliminated gaps between the fastener and surrounding structure that could cause sparking during a lightning strike. These joints provide very low resistance between the fastening system and the structure and improved joint durability by more than four times. A description of the GromEx installation process, the structural benefits afforded by the GromEx system as well as the results of lightning strike simulated direct strike tests on complete tanks or tank Sections for a Zone 1A environment, are discussed in this paper.