| Studies conducted in the last two decades have demonstrated the effectiveness of guided ultrasonic waves (GUWs) for the nondestructive evaluation (NDE), as well as for structural health monitoring (SHM) of waveguides, such as pipes, plates, and rails. Owing to the ability of travelling relatively large distances in dry structures with little attenuation, GUWs allows for the inspection of long waveguides, locating cracks and notches from few monitoring points, while providing full coverage of the cross section. Laser pulses are one of the most effective methods to generate ultrasonic bulk and guided waves in dry structures. In this dissertation we propose a non-contact NDE method based on the generation of broadband ultrasonic signals by means of laser pulses to inspect underwater structures. The waves are then detected by means of an array of immersion transducers and analyzed by means of statistical analysis to search for damages on the wet structure of interest.;In this study we first investigated the effect of water's depth, temperature, and pressure, and the laser energy and wavelength on the amplitude of the laser-induced ultrasonic waves. The results showed that the 0.532 microm wavelength is the most suitable for our applications. A good range of nominal laser energy is comprised between 160 mJ and 190 mJ. Furthermore, the variations of temperature and pressure have minimal effects on the ultrasonic signals. The following phase showed the ability of the technique to detect various types of defects in an immersed plate, which we achieved by building in house A B-scan system, controlled by National Instrument PXI running under LabVIEW. We designed two series of tests in which the number of transducers, their spatial arrangement, as well as the types of features extracted from the time, the frequency and time-frequency domain varied. By developing two unsupervised algorithms based on outlier analysis, we revealed that the method is capable of successfully detecting a crack and a hole-through. Next, the variation of the energy peak in the time-frequency space was shown to decrease with a dependence on the plate thickness. A range of peak energy was experimentally tabulated and the experimental group velocities of the first fundamental symmetric mode were calculated for six plates of different thickness, varying between 1 mm and 10 mm. Finally, the ability of a focused transducer to interrogate the damage state of the original aluminum plate was shown. As predicted, our multivariate algorithm successfully detected all the five defects devised on the plate. This work concluded with a comparison between the two methods. The results showed that both the hybrid laser-immersion transducer technique and the focusing technique can be successfully used for the noncontact monitoring of immersed plates. |
