Comparison of normal and oblique incidence ultrasonic measurements for the detection of environmental degradation of adhesive joints

Abstract

Normal and oblique incidence ultrasonic scans have been conducted on two and three layer adhesive joint specimens exposed to water at 50 °C for periods of up to 18 months. The joints consisted of aluminium alloy adherends, which were subjected to one of four different surface pre-treatments prior to being coated (for the two layer specimens) or bonded (for the three layer specimens) with an epoxy polymer. Overall, the oblique incidence technique detected the same types of defect as the normal incidence scans, except when the resolution was too poor to detect the smallest defects. No defect was detected using oblique incidence scans that could not be detected using normal incidence scans. In the two layer specimens, two main types of defects were detected: edge disbonds and micro defects. Micro defects were detected in regions remote from the edges and these small scale isolated defects took several forms. The detection of micro defects was a strong indicator that the interphase of the specimen had been attacked and weakened by the ingressing water molecules and, indeed, the number of such defects correlated to the loss of interfacial toughness. In the case of the three layer specimens, the extent of edge disbonding was much lower than in the corresponding two layer specimens. Also, no micro defects were detected ultrasonically in any of the three layer specimens. However, examination of some of the failure surfaces from the three layer specimens suggested that micro defects might have been present but that they were too small to be detected at the spatial resolution of the ultrasonic scans. Therefore, for the three layer joint specimens, there were no indicators from the ultrasonic scans, which could reveal whether the joint had suffered attack and weakening of the interphase regions by the ingressing moisture. There was no evidence in any of the tests of a gradual change in either the normal or oblique incidence reflection coefficient moving into an apparently well bonded region from either the edge disbonds or the micro defects.