Structural health monitoring using geophysical migration technique with built-in piezoelectric sensor-actuator arrays

The objective of this study was to validate the feasibility of adopting the geophysical migration method to interpret the ultrasonic Lamb wave signals in an active structural health monitoring system with built-in piezoelectric (PZT) sensor/actuator arrays. The PZT disks act as actuators to excite Lamb waves and also as sensors to receive the waves reflected from the structural anomaly in a plate. The migration technique, which is an advanced technique in geophysics to reverse the reflection wave field and to image the Earth interior, is then used to back-propagate the recorded wave signals and to visually image the damage in the plate. A finite difference algorithm is used both to synthesize the reflection waves and to implement the migration process, and the accuracy of the algorithm is verified through an analytical solution derived from Mindlin plate theory. A one-way version of flexural wave equation is derived for the poststack migration. For prestack migration, an excitation-time imaging condition is introduced based on ray-tracing concepts and the migration is proceeded through the full-way wave equation. An experimental apparatus is then set up and an analytical model of the waves in a plate incorporated with PZT sensors/actuators is developed to prove that A ₀ mode Lamb waves are accurately generated and collected. Finally, the migration results from the reflection waves of an artificial damage in an arc shape recorded in the experiment are presented. The results of both numerical simulation and experiment studies validate that the migration method possesses the capability of identifying multiple discrete damages without a priori assumption on the distribution pattern of the damages and could be a promising method of interpreting wave signals for damage identification applications.