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.

On the fatigue response of a bonded repaired aerospace composite using thermography

“…A 5 mm circular notch was drilled (and repaired) in the middle of each substrate using a diamond drill as a typical damage scenario. Apart from a damage concept, the notch is characteristic of the methodology employed to arrest/retard the crack propagation by "stop drilling", in the case of aluminium [49] panels. Although a 5 mm notch may be small in size as a representative damage scenario in aircrafts, the strip configuration did not allow for the introduction of greater notch size.…”

Fatigue in Adhesively Bonded Joints: A Review

“…They observed that the locus of failure of the joints was highly temperature dependent, that is, failure in the composite substrate at low temperatures and failure in the adhesive at elevated temperatures. Aglan et al [181] evaluated the durability of composite repairs of aircraft skin grade Al with simulated flaws using mechanical fatigue and thermal cyclic aging. They found that thermal cyclic aging reduced the fatigue lifetime of the repaired structures due to the deterioration of the interface.…”

Degradation medium of agrocomplex - adhesive bonded joints interaction

“…Armstrong (1997) and Davis and Bond (1999) state that the main reason of the joint failure is the degradation of the boundary between adhesive and adherend. Studies of Gledhill and Kinloch (1974) and Aglan et al (1999) also showed that owing to ageing often the change in a way of joint failure occurs. The main change occurs from the cohesion failure (adhesive as well as adherend) to the failure in boundary, so called adhesive failure (Gledhill, Kinloch 1974;Aglan et al 1999).…”

“…Studies of Gledhill and Kinloch (1974) and Aglan et al (1999) also showed that owing to ageing often the change in a way of joint failure occurs. The main change occurs from the cohesion failure (adhesive as well as adherend) to the failure in boundary, so called adhesive failure (Gledhill, Kinloch 1974;Aglan et al 1999). Sargent (2005) and Gerald and Pethrick (2009) claim that changes of medium can have effect on both the way how the adhesive physical properties are changing in time and on the strength in the boundary between adhesive and adherend.…”