Detection of cracks in cylindrical pipes and plates using piezo-actuated Lamb waves

In this paper, an extension of an earlier study (Tua et at 2004 Smart Mater Struct. 13 643-60) for health monitoring of plate structures using time of flight of propagating Lamb waves is presented. First, the detection of micro-width cracks of size 330 and 220 mu m on an aluminum plate is considered. Second, the ability of the technique to detect and locate cracks in-filled with impurities such as grease, araldite epoxy, metallic epoxy and spray paint is investigated. Third, the technique is applied to the case of welds where a notch is induced on an aluminum weld applied on a slot in an aluminum plate. Comparison of the results obtained for the weld with and without the notch is made. For all three cases, results showed that the technique of Tua et at (2004 Smart Mater Struct. 13 643-60) using a piezoelectric (PZT) actuator and sensor pair can be successfully applied to detect and locate such cracks. The methodology to detect and locate cracks in homogenous cylinders and pipes based on the time-of-flight and strength analysis of propagating Lamb waves is proposed. By observing the attenuation in the strength of the direct wave incidence at the sensor, the presence of a crack can be determined. At least four actuation positions with two on each end of the pipe segment of interest are needed to exhaustively interrogate for the presence of cracks. The detailed procedure for locating and tracing the geometry of the crack(s) is described. It is shown experimentally that the detection using a circular PZT actuator and sensor, with dimensions of 5 mm diameter and 0.5 mm thickness, is possible for an aluminum pipe segment of up to at least 4 m in length. The proposed methodology is also explored for an aluminum pipe under more practical situations, such as burying it in sand with only the actuator and sensor positions exposed. Experimental results obtained showed the feasibility of detecting the 'concealed' crack on pipes buried in sand.