Dielectric and mechanical studies of the durability of adhesively bonded aluminium structures subjected to temperature cycling. part 1: examination of moisture absorption

Dielectric measurements and mechanical tests on adhesively bonded joints are reported for bonds having been aged by complete immersion in water at 50 and 65 °C and by changing between these temperatures. Such joints have direct application to aircraft structures and other important engineering areas. The dielectric frequency domain measurements at 3 MHz are used to monitor moisture ingress into the adhesive bond. Initially the rate of change in the permittivity follows pseudo-Fickian behaviour consistent with simple moisture diffusion. In all the cases studied, acceleration in the rate of increase in the permittivity was observed consistent with the moisture converting the interfacial oxide layer to hydroxide. Time domain reflectometry (TDR) measurements indicate that physical changes occur in the bond line. Shear and cleavage measurements were performed at regular intervals and indicate a loss of mechanical properties of the bond. Comparison of the TDR measurements with ultrasonic and optical images indicated that the dramatic loss of mechanical properties is coincident with the observation of defects having been created in the interfacial layer. A correlation between certain of the mechanical properties and the change in the dielectric permittivity is observed