Noncontact laser ultrasonic crack detection for plates with additional structural complexities

This article presents a new noncontact laser ultrasonic wavefield imaging technique for detecting subsurface cracks in metallic plates with additional structural complexities. The proposed technique offers noncontact, automated, and baseline-free crack diagnosis for complex metal structures with potential to field structural health monitoring applications. First, a complete noncontact laser ultrasonic wavefield imaging system is introduced, and its working principle is presented. Then, a self-referencing frequency-wavenumber (f-k) filter is developed for instantaneous crack detection. The self-referencing f-k filter isolates crack-induced features from the ultrasonic wavefield images obtained only from the current state of the target structure using the noncontact laser ultrasonic wavefield imaging system. Finite element analyses are employed to investigate the characteristics of laser-generated ultrasonic waves and validate the proposed self-referencing f-k filter. Finally, the proposed technique is experimentally validated using asymmetrically tapered and vertically stiffened aluminum plates. The numerical and experimental results confirm that subsurface cracks are well identified and localized. The uniqueness of this study lies in that crack damage in plates even with additional structural features can be autonomously detected without using baseline data from the pristine condition of a target structure and with no sensor placement.