Structural Health Monitoring of Immersed Structures by Means of Guided Ultrasonic Waves

Guided ultrasonic waves (GUWs) are increasingly considered in the non-destructive evaluation and structural health monitoring of engineering systems that benefit from built-in transduction, moderately large inspection ranges, and high sensitivity to small flaws. Sometimes, owing to the kind of system being inspected, a non-contact approach for the generation and detection of GUWs is desired. This article presents an initial study of the feasibility of using a hybrid laser/immersion transducer system for the detection of damage in submerged structures. A pulsed laser was used for the generation of stress waves in an aluminum plate immersed in water, which were detected by a pair of conventional immersion transducers. The detected time waveforms were processed using the joint time-frequency analysis of the Gabor wavelet transform to extract information about the velocity and the attenuation of the propagating modes. Damage was simulated by devising a rectangular notch and a small circle on the face of the plate exposed to the probing system. The study shows promising results and may pave the road toward an innovative approach to the non-contact inspection/monitoring of underwater structures.