Pipe Defect Visualization and Quantification Using Longitudinal Ultrasonic Modes

In this study, a new ultrasonic wave based imaging techniques developed using longitudinal ultrasonic waves for detecting defects in pipeline structures. Ultrasonic waves are gaining popularity for pipeline monitoring because of its sensitivity to small defects and a long sensing range. Based on the merits of the ultrasonic waves, several research groups have developed ultrasonic wave based imaging techniques for pipeline monitoring. Conventionally, a pure torsional mode is often generated using shear-mode piezoelectric transducers or electromagnetic acoustic transducers (EMAT) and used for pipe damage detection. In this study, a new ultrasonic wave based imaging technique is developed using a longitudinal wave mode instead of the pure torsional mode. The longitudinal mode is generated using inexpensive macro fiber composite (MFC) transducers attached at one end of the pipe, eliminating the need for the shear-mode transducers or EMATs. Then, the reflections generated by the interaction of the incident longitudinal mode with a defect are measured in a pulse echo manner. Using a normal mode expansion technique, flexural modes are extracted from the reflected signals. When a defect-induced reflected wave mode is propagated back along the longitudinal direction of the pipe, its dispersive nature is minimized and best-compensated at the defection location. Therefore, by virtually propagating each defect-induced flexural mode back in the wave propagation direction, an image which visualizes the focusing of the back-propagated flexural modes can be obtained and the defect location can be identified. Numerical simulations and experimental tests are conducted to demonstrate that a wall-thinning in a steel pipe can be detected and quantified using the proposed imaging technique.