Oil storage tanks are crucial components in the petrochemical industry for transportation and storage. Corrosion of the tank floor and vertical walls is one of the most important safety concerns. In-situ inspection of the tank floor and walls with ultrasonic guided wave technique by edge- and wall-mounted transducers is still an active research area nowadays. A great deal of research has been conducted in detecting the defect of plate-like structures using Lamb waves. This application has been motivated by the fact that the Lamb wave velocity is the function of frequency and structural geometry (thickness), also the Lamb waves can propagate with low attenuation for long distance in plate structure. However, the complexity due to the existence of multiple Lamb wave modes weakens its applicability. Lamb waves are typically excited by transducers mounted on the surface of the inspected plates. Higher order modes are generated if the frequency-thickness product exceeds a certain threshold value that is dependent on the material of the plates. Low frequency guided waves can reduce this complication as only a small number of wave modes are generated. So operation with low frequency guided waves is usually recommended, but it encounters a problem that the resolution gets degraded hence small defects (compared to the wavelength) are difficult to detect. Our investigation considers the generation of guided waves in tank floor and vertical wall by edge excitation and wall-mounted excitation, respectively. Only when there is no access to the edge of the vertical wall, the wall-mounted excitation (surface excitation) is used. When the frequency-thickness product is small Lamb waves are generated, otherwise, longitudinal waves are generated. We investigate the intermediate frequency-thickness products where a pulse train of semi-longitudinal waves is excited. The pulse train is comprised of a leading pulse and some railing pulses, which have been predicted by the acoustic theory. The transition from Lamb wave to semi-longitudinal wave is investigated. In addition, energy transport from leading pulse to trailing pulses and the interaction of these trailing pulses with defects are studied. The results revealed that the semi-longitudinal wave is a viable option for tank floor and wall defect inspection.

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