Influence of thermal spikes on the ageing of adhesively bonded structures - A dielectric study

Dielectric measurements are reported on epoxy bonded aluminium joints exposed to moisture at 75 degrees C. The observed changes in the dielectric spectrum that occur with time of exposure are correlated with the variation in the mechanical strength. Certain joints were subjected to periodic cooling to -20 degrees C to simulate the shock of aircraft flight. The effects of dehydration were examined for joints that had been exposed to moisture for a prolonged period of time. The data indicate that the initial hydration process lowers the glass transition temperature and the stress released creates micro-voids. Further ageing leads to hydroxide formation in the interfacial layer. Freezing of water during the thermal spike experiments induces cracking in the adhesive, and leads to greater water uptake. Plasticisation of the resin appears to be to a large extent reversible and dehydration allows some recovery of the bond strength. The shock cooling of the joints has only a relatively small effect on the ageing of the joints but does, however, produce differences in the dielectric data. This study illustrates the power of the dielectric technique for the assessment of ageing in adhesively bonded structures.