Quantitative ultrasonic characterization of environmental degradation of adhesive bonds

Environmental degradation of adhesive joints is characterized using angle beam ultrasonic spectroscopy. The method is based on reconstruction of bulk and interfacial properties of an adhesive bond from ultrasonic experimental data using the inverse model, which uses normal and transverse effective springs to simulate poor contact between substrates and adhesive layer. Using standard manufacturing processes, phosphoric acid surface pretreated and unpretreated aluminum alloy 2024-T3 bonded samples were prepared and exposed to accelerated environmental degradation: immersion in saturated NaCl solution at 68°C and 4.45 kN (1000 lb) load. It is shown that property-reconstruction-based angle beam ultrasonic spectroscopy provides an efficient measurement of adhesive bond degradation in the environment. Pixel-by-pixel normal and angle beam ultrasonic measurements were made to quantify the degradation of bondline properties by reconstructing absolute values of adhesive-bond parameters, such as thickness, effective longitudinal and shear moduli, density and ultrasonic longitudinal- and shear-wave attenuations. The properties were monitored as a function of time in the environment and by measuring these parameters over the joint area, environmentally degraded areas in the bonded structure were determined. In particular, it was concluded from measurements on a large set of samples that parameters reconstructed from ultrasonic measurements began to vary significantly after about 70% of the lap shear joint lifetime. To isolate the interfacial degradation from changes in bulk properties of the adhesive, aluminum adherends were prepared without anodization and priming assuring fast interface degradation without degradation of the adhesive layer. Reconstruction of interfacial and bulk properties of the bondline was performed over the bonded area from normal and oblique reflection spectra. The reconstruction was validated by independent measurements of the bulk parameters on the adhesive film extracted from the joint after degradation.