THE REINFORCING EFFECT OF COMPOSITE PATCH REPAIRS ON METALLIC AIRCRAFT STRUCTURES

The use of composite patch repairs on cracked metallic aircraft struct ures is an accepted technique for improving fatigue life and maintaini ng high structural efficiency. This study investigates various factors affecting such repairs including the selection of patch materials, th e use of room-temperature-cured vs. high-temperature-cured adhesives, and reduced temperature bonding vs. standard bonding for high-temperat ure adhesive repairs. In addition, the effect of stop-drilling at the crack tip has also been determined. The results show that both boron/e poxy and graphite/epoxy composite patches, bonded with either room-tem perature-cured or high-temperature-cured adhesives, attain sufficientl y high fatigue lives to meet the damage tolerance requirement. However , the boron/epoxy patch with high-temperature adhesives gave better fa tigue lives than the graphite/epoxy patch with room-temperature adhesi ves. In order to decrease the thermal residual stress in the structure around the bondline, the curing cycles of high-temperature adhesives were modified to temperatures lower than their standard curing cycles; this has been shown to increase repair efficiency. Stop-drilling at t he crack tip before patching repair reduced fatigue life by about 60%, and misalignment of the directional composite patch with the crack al so adversely affected the efficiency of the repair.