Assessment of adhesively bonded aluminium joints by high frequency dielectric measurements

The most significant environmental factor in the degradation of adhesively bonded aluminium joints is moisture ingress. Water degrades the adhesive by plasticisation and hydrolysis, leading to failure with in the adhesive, it hydrates the oxide layer disrupting the adhesive/metal interface leading to interfacial failure. Since water is a polar material dielectric measurements are an effective means of detecting water at low concentrations. Dielectric techniques which use the aluminium adherends as the electrodes are capable of non-destructive measurement within a joint without the use of embedded sensors and if the frequencies employed are high enough (~GHz) then spatial information (~cms) on the water distribution should be available. In addition measurements over a large frequency spectrum can yield information on the interaction of the water within the adhesive, important in deducing the mechanisms by which the water diffuses through the adhesive and the interface.