Fully Noncontact Wave Propagation Imaging in an Immersed Metallic Plate with a Crack

Abstract

This study presents a noncontact sensing technique with ultrasonic wave propagation imaging algorithm, for damage visualization of liquid-immersed structures. An aluminum plate specimen (400 mm × 400 mm × 3 mm) with a 12 mm slit was immersed in water and in glycerin. A 532 nm Q-switched continuous wave laser is used at an energy level of 1.2 mJ to scan an area of 100 mm × 100 mm. A laser Doppler vibrometer is used as a noncontact ultrasonic sensor, which measures guided wave displacement at a fixed point. The tests are performed with two different cases of specimen: without water and filled with water and with glycerin. Lamb wave dispersion curves for the respective cases are calculated, to investigate the velocity-frequency relationship of each wave mode. Experimental propagation velocities of Lamb waves for different cases are compared with the theoretical dispersion curves. This study shows that the dispersion and attenuation of the Lamb wave is affected by the surrounding liquid, and the comparative experimental results are presented to verify it. In addition, it is demonstrated that the developed fully noncontact ultrasonic propagation imaging system is capable of damage sizing in submerged structures.

Defects Detection and Localization in Underwater Plates Using Laser Laterally Generated Pure Non-Dispersive S0 Mode

“…The weak dispersion region of the S 0 mode exists in the low-fd range around 1MHz·mm, as shown in the dotted frame in Figure 1. When the plate is fully immersed in water, the dispersion curves of S 0 and A 0 mode are almost invariant [27,31]. The energy of A 0 mode partly converts into a Scholte wave, which decreases exponentially away from the surface into water.…”

“…In the study, Lamb waves were generated by a pulsed laser focusing on the surface of plate and were detected by an array of immersed transducers. Lee et al [27] developed a noncontact laser ultrasonic propagation imaging (UPI) system for damage visualization of liquid-immersed structures using the A 0 mode. When the generating laser irradiates the plate surface, both symmetric modes and antisymmetric modes can be generated, while the symmetric modes normally have small out-of-plane displacement because of the asymmetric excitation (the source is asymmetric with respect to the middle plane of the plate).…”

“…In addition, using an optical method, e.g., laser deflectometry or interferometry, it is very difficult to detect the S 0 mode generated by pulsed laser irradiation on the surface of plate. This is not only because of the interference introduced by the antisymmetric modes, but also because most of the interferometry and the deflectometry is only sensitive to out-of-plane displacement, while the S 0 mode has low out-of-plane displacement, especially when it is generated by asymmetric excitation [13,27].…”

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

“…Lee et al [10] visualized ultrasonic wave propagation and detected the crack damage of liquid-immerse structures using ultrasonic propagation imaging system which are consist of laser pulse excitation to generate Lamb wave on aluminum plate and laser Doppler Vibrometer sensor to signal. According to Lee et al [10], the depth of water doesn't have affect to wave propagation signal and the system measure the crack size successfully.…”

“…According to Lee et al [10], the depth of water doesn't have affect to wave propagation signal and the system measure the crack size successfully. The size of crack, however, is affected by the density of the liquid.…”