Non destructive testing for environmental degradation of adhesive joints

Abstract

This project has been looking at ultrasonic methods as a way of monitoring the degradation of adhesive joints during exposure to a hot wet environment. We have used a range of specimens, both two and three layered. All the specimens have been aluminum epoxy systems, covering various common pre-treatments, with each specimen having a variety of different edge conditions. We have been using normal and oblique incidence scanning to monitor for interfacial changes in the joints. The oblique incidence method has been used to excite oblique shear waves in the aluminum, which previous work at Imperial College has identified as being the most sensitive to the properties of the interlayer, that being the micro-composite layer formed between the epoxy and the aluminum oxide. So far the majority of changes have been seen at edges where the interface is open to attack, and the changes seen have been as a result of local debonding. No precursors to disbonding have been detected. However some specimens have shown signs of change well removed from any edge. Ultrasonically undetected changes have been observed, and these results will be presented and discussed.