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Notches (often used to stimulate crack-like defects) in pipes are characterized using Helical-Guided Ultrasonic Waves (HGUW). In thin-walled curved structures with a radius-to-thickness ratio of more than 10/1, Lamb-type guided waves, called the HGUW, propagate. HGUW are plane-strain guided waves propagating circumferentially in helical paths in large-diameter cylindrical structures, with the properties of Lamb waves. They travel in multiple trajectories between two points, and these paths are indexed as orders of the helical path. When the HGUW encounters a notch in its path, it scatters, and the information in the scattering is used to characterize the notch (i.e., determine the notch size). In this work, an approach called the stepped wavelength method is presented to determine the notch size. In this approach, the directivity plots, quantifying from the scattering of the HGUW in all directions around the notch, are evaluated for a set of frequencies (each corresponding to a specific wavelength) from the numerical model of the pristine and damaged pipes. As the wavelength-to-notch size ratio approaches one and increases beyond that, a change in the directivity plot’s profile is witnessed, suggesting a change in the nature of the interaction between the notch and the incident wave. A criterion based on the change in the nature of interaction is developed to estimate the notch size.
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