GLUED STRUCTURES INSPECTION BASED ON LOCK-IN THERMOGRAPHY

Active thermography is a widely employed technique for parametric identification and non-destructive inspection. This attractive method is based on the observation of thermal waves propagation induced by a periodic heating. For non-destructive testing usual approaches are based on a global heating (a large surface of the inspected material is submitted to thermal excitation). In the following a local approach is investigated: the heated area is small (order of magnitude is one square centimeter) and lateral propagation is studied in order to reveal the defect in the sample. In fact, both modulus (heat wave amplitude) and phase lag (delay) of the measured periodic signal are modified by the defect neighborhood and the search for the most effective area leads to the defect localization. Several results are highlighted in this communication in order to investigate an automated procedure. Temperatures are measured by an infrared camera and analyses of modulus cartography are performed in order to estimate the defect location. In such an aim, the downhill simplex method is implemented in order to converge toward defect location. Illustrations are dedicated to glued structures (two plates separated by a thin glue interface) for which unknown defect is a lack of glue which can be considered as a bubble (air trapped between the lower and the upper plane surface). Automated method attractiveness is established in several configurations.