Dielectric and mechanical studies of the durability of adhesively bonded aluminium structures subjected to temperature cycling. Part 2: Examination of the failure process and effects of drying

Abstract

Additional 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. Dielectric frequency domain measurements are discussed in terms of the effects on the adhesive and the oxide layer as a result of water ingress to the bond line. Initially, the moisture adsorption follows a pseudo-Fickian behaviour, and correlation with the mechanical properties would suggest that the initial loss in mechanical strength is consistent with plasticization of the adhesive layer. 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. X-ray diffraction measurements indicate that the hydroxide formed was predominantly of the boehmite [λ-A1O(OH)] form, but also with some of the diaspore [α-A1O(OH)] form. The change of the oxide to hydroxide changes the bond line characteristics and is clearly identifiable in the time domain reflectometry (TDR) measurements. Further analysis of the high-frequency dielectric data indicate that the changes in the adherend surface resistivity that occur in the latter stages of ageing may be indicative of the conversion of oxide to hydroxide. Comparison of the TDR measurements with ultrasonic and optical images indicated that the dramatic loss of shear and cleavage strength is coincident with the observation of defects having been created in the interfacial layer. The ability for a joint to recover on drying was investigated, and the data indicate that, depending on the distribution of the moisture in the bond, recovery of mechanical properties is or is not possible. In the initial stages, when moisture is in the adhesive, recovery is in principle possible. However, once moisture has changed the interfacial layer, recovery becomes more difficult.