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

Abstract

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.

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

“…The characteristics of the dicyandiamide-cured epoxy adhesive and the processes used for the fabrication of the porous oxide coated aluminium have been described in reference [1]. The bonding of the aluminium and epoxy was carried out using an autoclave, and the procedure was also discussed in Part 1.…”

“…Lap shear and cleavage measurements were performed according to the relevant ASTM specification [15][16] (see reference [1]). The mean shear strength was calculated from the maximum load using…”

“…In Part 1 of these two papers the possible correlation of dielectric measurements and changes in the mechanical properties of a set of joints subjected to hygrothermal ageing was presented [1]. A study of the hygrothermal ageing of dicyandiamide-cured DGEBA epoxy resins [2][3][4] indicated that in the initial stages of moisture absorption it was possible to dry the adhesive films and recover their initial mechanical properties.…”

“…those used in aircraft structures, might be anticipated to age owing to moisture absorption in the adhesive as well as on account of changes in the adhesive/ adherent interface. It was therefore appropriate to extend the previous study [1] to include an investigation of the effects of drying on aged joints. A number of systematic investigations of the ageing of bonded joints have been reported [5][6][7][8][9].…”

“…In Part 1 [1], and also previous dielectric investigations of ageing [10][11][12][13][14], the effects of conversion of the oxide to hydroxide has been discussed. A study of the highfrequency dielectric characteristics of aluminium hydroxides indicated that, while there were subtle differences in the way in which the conversion of the oxide to hydroxide occurred with the various forms, there was a general correlation between the magnitude of the dielectric permittivity changes observed and the extent of the conversion that occurs.…”