Bonded composite repair of fatigue-cracked primary aircraft structure

Repairs based on adhesively bonded fibre-composite patches or reinforcements are more structurally efficient and much less damaging to the parent structure than standard repairs based on mechanically fastened metallic patches. As a result of the high reinforcing efficiency of bonded patches "live" fatigue cracks can be successfully repaired. However, when such repairs are applied to primary structure a conservative certification approach is often taken in which Ilo credit is given to the patch system for slowing crack growth or restoring residual strength. Thus, cracks approaching critical size cannot be repaired and inspection intervals must be based on the predicted growth behaviour of the unpatched crack. To allow credit to be given to the patch the need is to demonstrate either (a) that the likelihood of patch loss is acceptably low or (b) that its loss can be immediately detected. Two approaches are discussed: the first approach which addresses (a) is based on a demonstrated ability to predict the patch system's fatigue behaviour and to assure its environmental durability. The second approach that addresses (b) is based on the "smart patch" concept in which the patch system monitors its own health. Crown Copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.