Crack detection of immersed metallic structure in water with surface oscillation using scanning laser pulse

Abstract

There are some difficulties of non-destructive test and evaluation for immersed or submerged structures such as nuclear reactor pipe line, submarine or huge ship. This paper proposes the method of damage detection of immersed metallic structure which has crack on weld zone and placed in the water with slight and random surface oscillation using ultrasonic wave propagation imaging (UWPI) system with piezoelectric transducer. A T-shape metallic structure with artificial surface crack on weld zone, which with size 2 mm by 0.3 mm and the depth 2 mm, used as the specimen. A 532 nm Q-switched continuous wave laser is used for scanning an area of 20 mm by 40 mm. A piezoelectric sensor with magnetic sensor head, which is attachable to metallic structure is used as a contact ultrasonic sensor. The tests are performed in three cases: a specimen without water, a specimen immersed in water and a specimen immersed in water with random surface oscillation. Ultrasonic wave propagation image algorithm and adjacent wave subtraction (AWS) algorithm are used for visualizing wave propagation and detecting the crack. For the case where the specimen is immersed in the water, the signal amplitude is increased compared with the specimen without water case. In AWS algorithm results of the immersed case, scattering waves which are generated by cracks were observed. In random surface oscillation case, the excitation laser beam is refracted randomly by Snell's law resulting in the diluted wave propagation images. However, improvements in signal-to-noise ratio using repeat scan technology enable to the detection of the crack and estimation of the crack size. This paper examines the damage detection using ultrasonic wave propagation caused by laser excitation from outside of the water even when the water surface oscillation exists and implies the possibility of application of ultrasonic propagation imaging system to submerged structures.