Effect of Corrosion on the Integrity of Adhesively Bonded Composite Patch Repairs of Damaged Aircraft Structures Using Acoustic Emission Monitoring

The aging and integrity of aircraft structures continues to be a matter of great concern especially for the U.S. Air Force. With the increasing average age of commercial aircraft fleet, there exists a critical need for non-destructive testing and evaluation (NDT&E) techniques for monitoring corrosion and detecting critical flaws both cost effectively and timely. Corrosion is a major maintenance issue, especially for aging military aircraft. The Air Force recently estimated that corrosion maintenance costs exceed $2.2 billion per year. Given this, it is desirable to reduce the amount of ‘find it - fix it’ corrosion maintenance activities. One method is to introduce damage tolerance approach for corrosion detection. This paper presents the results of the investigation on the effect of corrosion on the behavior of adhesively bonded composite patch repairs of cracked 2024-T3 clad aluminum aircraft structures. The cracked 2024-T3 clad aluminum panels are repaired with adhesively bonded octagonal and elliptical single sided boron/epoxy composite patches, using AF 163-2K adhesive (3M Inc.). Two types of accelerated corrosion tests, Continuous salt fog test (B117) and Prohesion test (Modified salt fog spray test - G85) were conducted on the masked specimens in Q-Fog accelerated corrosion chamber. The patched specimens were subjected to varying amount of exposure in the corrosive chamber ranging from 2 to 6 weeks, and later tension tests were conducted on Instron Satec universal testing machine to evaluate the effect of corrosion on the strength and integrity of the patch repair. Four broad band acoustic emission (AE) sensors were attached to the test specimen and used to monitor crack propagation and fracture of the repaired panels exposed in the corrosive environments for different lengths of period. AE sensor responses were studied for the entire load history during the tension testing using the four broad band AE sensors. Crack propagation gages were used to study the events corresponding to crack initiation and propagation. Results in the time domain and frequency domain obtained for the specimens are presented. A comparative AE analysis of events from the specimen exposed to accelerated corrosion tests: Continuous salt fog test (B117), and Prohesion (G85) is presented. It was observed that AE events increased with increase in the specimen exposure time in the corrosion chamber.

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