Steps for Characterizing Crack-Like Defects in Real-World Hollow Cylindrical Structures from Scattering of Helically Guided Ultrasonic Waves

Abstract

Recently, a method called the stepped wavelength scattering analysis was proposed for tracking the growth of circumferential crack-like defects in hollow cylindrical structures. As the name implies, this method is based on the scattering of helical guided ultrasonic waves (HGUW). To identify the defect length, the method quantifies the perceivable change in the nature of interactions as the wavelength-to-crack length ratio goes from being less than one to equal to or greater than one. Specifically, when the wavelength is smaller than the defect size, the scattering directivity plots are well-explainable by ray-like interactions. However, when the wavelength becomes comparable or greater than the defect size, the interaction is diffraction-dominated. This change in scattering pattern with respect to wavelength-to-crack length was quantified to establish a criterion for estimating the defect length. Building on this foundation, this paper discusses the steps for extending the method for practical implementation in real-world structures. As such, a three-step strategy with circumferential arrays of transducers is proposed. The steps include (1) locating the defect through HGUW tomography, (2) estimating its orientation from the angular distribution of scattered lobes, and (3) determining its length using the stepped-wavelength scattering analysis. The discussion also highlights a key requirement for implementation: reconstructing full circular-array responses from circumferential measurements. Once a DNN-based model is developed to accomplish this, the proposed three-step framework can enable practical in-situ monitoring and tracking of crack-like defects in hollow cylindrical structures.