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Cold expansion, a technique widely used to increase the fatigue life of holes in metal structures, has been adapted to composite materials as 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 installation 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 comprehensive test program involving mechanically fastened joints in composites with 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 lightning strike damage, cold expanded grommets significantly out-performed bonded grommets. Work on cold expanded rivetless nut plates in carbon/epoxy has proven the viability of the concept.
Assistant engineering manager, Fatigue Technology, Inc., Seattle, WA
Stock #: CTR10478J