Durability assessment of composite repairs bonded to aircraft structures

Abstract

The durability or long-term performance of composite repairs of aircraft skin grade aluminum with simulated flaws was evaluated. Mechanical tension/tension fatigue and thermal cyclic aging were used, in addition to monotonic tensile loading, to assess the durability of the repairs. Thermal wave imaging was employed to inspect the initial quality of the repairs and to quantify the size of the interfacial disbond associated with the mechanical fatigue and the combined mechanical fatigue and thermal cyclic aging. Boron/epoxy patches with different numbers of plies were installed on aluminum substrates with simulated damage in the form of side-cracks with stop-drill holes. It was found that a patch with more than 4 plies resulted in the residual strength of the pre-cracked substrate to that of the uncracked aluminum. A considerable increase in the fatigue lifetime and a decrease in the crack growth rate were also observed as the number of plies increased. Both the energy release rate and the hysteresis (damage) energy for substrates repaired with 4- and 6-ply patches showed a slight decrease, indicative of the crack arrest nature of the boron/epoxy patches. This crack arrest phenomenon was captured using thermal wave imaging. The size of the interfacial disbond was found to taper down as the crack propagated from the stop-drill holes. Thermal cyclic aging according to ASTM D 1183-70 reduced the fatigue lifetime of the repaired structures due to the deterioration of the interface. Thermal wave imaging also showed that the pre-cracked structures repaired with 4-ply patches exhibited more disbond than those with 6-ply patches after being exposed to the same number of thermal cycles and fatigue-tested until complete separation of the aluminum substrate. Boron/epoxy repairs with 6 plies proved to be very effective in retarding multi-site damage emanating from rivet holes. More than an order of magnitude increase in the fatigue lifetime between the unrepaired and repaired structures was found. Thermal wave imaging also showed clearly that the propagation of multi-site damage from rivet holes was associated with interfacial disbond between the boron/ epoxy doublers and the aluminum substrate. The integrity of the boron/epoxy doubler installed over rivets was evaluated using thermal wave imaging. It was found that the doubler was intact after exposing the joint to severe fatigue loading which resulted in failure of the substrates, rather than disbond or delamination of the repair.