NDE of adhesive joints and riveted structures with lock-in thermography methods

Elastic waves sent into the component propagate inside the sample until they are converted into heat. A defect causes locally enhanced losses and consequently selective heating up. Therefore amplitude modulation of the injected elastic wave turns a defect into a thermal wave transmitter whose signal is detected at the surface by lock-in thermography synchronized to the frequency of amplitude modulation. This way ultrasound lock-in thermography allows for selective defect detection which enhances the probability of defect detection in the presence of complicated intact structures. Measurements were performed on various kinds of mechanical joints. The results of our feasibility study indicate that both optical (OLT) and ultrasound exited lock-in thermography (ULT) are reliable tools for the rapid identification of damaged riveted structures. We demonstrate that one can locate a screw joint or a riveting which provide only a reduced stress. Investigations on airplane components were performed which confirmed the applicability of lock-in thermography for remote maintenance inspection within a few minutes.