Non-destructive testing for health monitoring of adaptive structures

Due to the increasing demand for adaptive materials and structures in various fields, new sensors and actuators are being developed that require advanced processing of materials. In this field both damage identification and integrity characterisation are important for a reliable operation of the system. Our contribution first compares the performance of various modern non destructive testing methods for the quality assurance of smart structures. Also the potentials of the methods are discussed regarding different types of defects. We tested adaptive structure models (consisting of glass fibre reinforced composites and piezoelectric actuators) using modal analysis, non-linear vibrometry, lockin-thermography, and air-coupled ultrasound. The results are correlated (data fusion) in order to combine the efficiency of the methods. In the second part we report about a variable coupling between actuator and matrix to simulate both curing and aging of the matrix material (epoxy resin) and also about such a variable coupling between components within the adaptive (or "smart") structure. Subsequently we monitored the transfer function and especially its changes caused by modifying the properties of the coupling in a controlled way. The results were correlated with those of destructive measurements. Some measurement techniques use the actuator as a probe for monitoring structural changes while others use it as a transmitter. Load induced defects (e.g. local disbonds of the materials) are verified non-destructively. As we used thermoplastic material, these defects can be removed by heating and subsequent solidification of the coupling medium. The application-relevant result is that the model structures can be reproducibly modified in order to simulate various conditions and also to monitor the sensitivity of NDE methods responding to these controlled modifications.