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Enhanced Coating Disbond Detection Capabilities in Pipe Using Circumferential Shear Horizontal Guided Waves

Circumferential shear horizontal guided waves propagate in the circumferential direction of a pipe or pipelike structure and have particle displacements in the axial direction only. In this work, the circumferential shear horizontal wave phase and group velocity dispersion curves and wave structures are calculated for a layered elastic/viscoelastic annular structure using a semianalytical finite element technique. The results are used to select modes and frequencies that are appropriate for the reliable detection of disbonded protective coatings. Time, amplitude and frequency based disbond detection features are identified and utilized in the design of a disbond detection algorithm. Circumferential shear horizontal waves are generated in a coal tar enamel coated pipe specimen using a pair of electromagnetic acoustic transducers (EMATs) arranged in a manner conducive to data normalization. Data are collected in several regions with coating disbonds of varying size. It is found that the technique is able to reliably detect coating disbonds and sort them according to size. Results from a field test are also presented in which data were collected from a moving sensor carriage. Though experimentally validated for a single coating type and thickness, the developed disbond detection philosophy is readily applied to other coating types and thicknesses.

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