The detection of cracks in beams and plates using piezo-actuated Lamb waves has been presented in the last SPIE Symposium. This paper is an extension of the technique to pipes. It has been shown that for a thin-walled pipe, the assumption of Lamb wave propagation is valid. Such waves can be efficiently excited using piezoceramic transducers (PZT) with good control on the pulse characteristics to assess the health of structural components, such as the presence of cracks. In this paper, a systematic methodology to detect and locate cracks in homogenous cylinder/pipe based on the time-of-flight and strength analysis of propagating Lamb wave is proposed. By observing the attenuation in strength of the direct wave incidence at the sensor, the presence of a crack along the propagation path can be determined. At least four actuation positions, 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 circular PZT actuator and sensor, with dimensions of 5.0 mm diameter and 0.5 mm thick, is possible for an aluminum pipe segment of up to at least 4.0 m in length. The proposed methodology is also explored for the 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 the pipe buried in sand.
Lamb waves have attracted great attention in non-destructive evaluation (NDE) due to its efficiency in interrogating a reasonably extensive distance along the plate. Such waves can be efficiently excited using piezoceramic transducers with good control on the pulse charactertistics to assess the health of structural components, such as the presence of cracks. Through selective generation of Lamb waves within a frequency range, linear cracks can be detected via time-of-flight analysis of the wave, by using plain piezoceramics transducers with strategic positioning. Alternatively, using a well-designed inter-digital transducer (IDT), a single Lamb mode can be generated. It is shown that using IDT enhances detection accuracy and robustness in view of its controllability on the duration and direction of the generated wave. It is thus able to locate curved crack accurately as well as trace its geometry. The performance of both actuators are compared experimentally using both plain piezoceramics and IDT to detect different cracks, namely, linear crack, curved crack and multiple cracks, on aluminum plates. Plain piezoceramics provide accurate detection for linear and multiple cracks, and are able to estimate the geometry of a curved crack reasonably well. However, IDT is more efficient and provides accurate results for these three cases.