On the fatigue durability of clad 7075-T6 aluminium alloy bonded joints representative of aircraft repair

The fatigue durability of bonded joints representative of repairs to aircraft structure with and without the presence of a clad layer was investigated by testing aluminium alloy 7075-T6 double lap shear joint specimens. This was done by changing the bonding interface of the outer strap. The joint geometry, central adherend material, adhesive and surface preparation method were all kept the same. On two of the specimen types, the strap material was clad 7075-T6, with one type fabricated with the clad layer left on prior to surface preparation, and the other with the clad layer removed. On the last specimen type, the strap material was unclad 7075-T6. The test results showed that the fatigue durability was lowest when the clad layer was left in-situ, followed closely by those with the clad layer removed. The unclad specimens achieved a fatigue life one order of magnitude greater than those with the clad layer physically removed. Under constant amplitude loading, adhesive fatigue cracking was observed at the location of peak load transfer, which progressed to the interface. Analysis showed that the cracking caused a substrate stress concentration which may have caused the clad fracture. Further analysis, supported by test observations, showed that once a small notch had formed at the interface, damage progression through the outer strap was rapid.