Fatigue Life Recovery in Corroded Aluminium Alloys Using Bonded Composite Reinforcements

Bonded composite reinforcements, such as boron/epoxy or carbon/epoxy, are becoming widely used to repair fatigue or stress corrosion cracks in aluminium alloy aircraft components. They also have considerable potential as repairs for corrosion damage, although the non-discrete nature of corrosion damage is a considerable complication. This paper describes studies on bonded repairs of two types of typical corrosion damage, both of which cause a dramatic reduction in fatigue life: a) relatively severe exfoliation and b) relatively minor pitting. Both studies are based on earlier Australian experimental studies on the structural significance of corrosion. In the studies described in this paper, other than the standard surface treatment used to attach the reinforcement no attempt was made to remove the corrosion damage. It is concluded that bonded repairs can provide useful fatigue life recovery even with serious exfoliation damage; however, such repairs should be regarded only as a temporary measure. For permanent and far more effective repairs most of the damage should be removed (ground-out) prior to reinforcement. In contrast, with minor pitting, greater than full life recovery is feasible. This observation also leads to the conclusion that bonded repairs will be tolerant of minor pitting inadvertently left after grind-out. Strain reduction by the reinforcement, as expected, is the major contributor to life extension. However, in the case of minor pitting environmental isolation by the adhesive is probably highly beneficial. Residual stresses in the metal resulting from the composite reinforcement causes some loss in patching efficiency.