In-situ ultrasonic testing of polymeric adhesive bonds exposed to complex mechanical and environmental loads

For the evaluation of adhesive bonds, a variety of non-destructive testing methods is known. Nevertheless, there is still a demand for improved quantitative characterization of bond quality. Furthermore, in service, adhesively bonded joints are subjected to complex combinations of mechanical and environmental loads, which are not covered by conventional failure and life prediction models. In a co-operative project, wedge test specimens, thick adherent specimens for shear strength determination, and bulk specimens were manufactured in order to investigate three different adhesives, a commercial epoxy adhesive and two model adhesives, a two component and a one component epoxy adhesive, respectively. Before and after combined mechanical and environmental loading, the samples were examined with ultrasonic non-destructive techniques by IZFP. In order to understand the damage mechanisms, the adhesives were analysed via IR spectroscopy by one of the project partners. The wedge test samples were made of 3 mm thick stainless steel substrates with 0.3 mm thick adhesive layers. They were aged at 60°C in water immersion and at 60°C in dry air. In addition, they were loaded mechanically by wedge insertion. Two sets of wedge test samples were imaged in situ during the ageing process by high-frequency ultrasonic microscopy at 25 MHz and at 50 MHz. To this end, a special set-up was developed allowing ultrasonic imaging in immersion in water at controlled temperatures up to 60°C. By evaluation of the ultrasonic data, the crack propagation was imaged during the ageing process. As shown here, the final crack front detected non-destructively by ultrasound agreed to the optical fracture image taken after destruction of the samples. In addition, the ultrasonic data are discussed with respect to the damage mechanisms in comparison to the IR spectroscopic findings.