FASTENER HOLE REINFORCEMENT IN COMPOSITES USING COLD-EXPANDED INSERTS

Cold expansion, a technique widely used to increase the fatigue life o f holes in metal structures, has been adapted to composite materials a s a means to install metal inserts in fastener holes. Thin-walled cold expanded inserts (grommets) are used to reinforce fastener holes that may be sensitive to damage, for example, from repeated fastener insta llation and removal, or from lightning strike. Thicker inserts with a nut-anchoring feature are a cost-effective alternative to riveted nut plates currently used in composite-fastened assemblies. A comprehensiv e test program involving mechanically fastened joints in composites wi th adhesive bonded and cold expanded grommets showed that, in general, cold expanded grommets perform comparably to, or better than, bonded grommets. In the areas of installation costs and resistance to lightni ng strike damage, cold expanded grommets significantly out-performed b onded grommets. Work on cold expanded rivetless nut plates in carbon/e poxy has proven the viability of the concept.