Application of the ultrasonic propagation imaging system to an immersed metallic structure with a crack under a randomly oscillating water surface

Ultrasound is widely used and studied to satisfy the increased demands of the Non-destructive evaluation (NDE) and testing of underwater structures. However, because of the large size and mass of underwater structures, such as submarines, ship hulls, or nuclear reactor pipe lines, it is difficult to inspect the structures during operation. This underwater NDE technology is challenging but could be highly beneficial because the time and cost of maintenance will be effectively reduced. We propose an NDE method for immersed structures using an ultrasonic propagation imaging system with a piezoelectric sensor. The underwater sensing capability of a piezoelectric sensor is experimentally demonstrated using an aluminum plate specimen. A piezoelectric sensor can compensate for the decreased signal amplitudes due to leaky waves that are generated on interfaces between structures and water, since water transmits signals better than air. Additionally, a piezoelectric sensor can be applied even if the water surface is oscillating. Using these properties, the laser induced guided Ultrasonic propagation imager (UPI) inspected a T-shaped steel structure with a 2-mm crack on the weld zone. The inspection was implemented in three cases: a specimen without water, a specimen immersed in water and a specimen immersed in water with a randomly oscillating surface. The crack was visualized and measured using the ultrasonic wave propagation imaging algorithm, the adjacent wave subtraction algorithm, and the variable time window amplitude mapping algorithm. In the case with a randomly oscillating water surface, the laser pulse was refracted randomly based on Snell’s law. This phenomenon may cause degradation of the inspecting results. However, a repeated scanning process and outlier elimination led to an improved signal-to-noise ratio such that it was able to detect the crack. These results demonstrate the possibility to apply the laser UPI to submerged structures even if the water surface is randomly oscillating.