Contact-free detection and visualization of guided waves in composites using an optical microphone

Abstract

With increasing adoption of carbon fiber reinforced polymers in modern aircraft, new challenges in the maintenance of these structures arise. These materials are prone to internal damages such as delaminations, necessitating advanced non- destructive testing methods like ultrasonic testing. Guided ultrasound waves are a promising solution for testing large thin- walled structures like aircraft fuselages due to their long propagation range and high sensitivity to internal damage. However, traditional methods like laser-doppler vibrometry, while effective, are limited by their reliance on surface reflective properties, restricting their applicability in real-world maintenance scenarios. In this paper, a laser-based optical microphone is used to detect and visualize guided waves in carbon fiber reinforced polymer materials, featuring a very high bandwidth and independence from surface reflectivity. A robot test setup is developed to sample areas on the plate through repeated experiments. Signal processing enables the analysis of the wavefield and the visualization as two- dimensional wavefield images. By introducing defects into the plate, it is shown that these can be detected by changes in wave propagation patterns. With the flexible test setup using a robotic arm, the scalability of this approach to more complex structures with curvatures is demonstrated, highlighting its relevance for future use in aircraft maintenance.