The propagation of ultrasonic guided waves in an elastic hollow cylinder coated with viscoelastic materials of varying thickness is studied. The principal motivation is to improve the range of practical guided wave testing for the non-destructive inspection of coated steel pipes. The dispersion curve characteristics of such a system are examined, with particular focus on the differences to the free case due to the presence of bi-layer modes. Particularly, for every propagating mode, several cut-of frequencies of high attenuation are identified; The location of these attenuation peaks are shown to be directly related to the acoustic material properties and the thickness of the coating. It is demonstrated that in the case of coated pipes careful selection of both the excitation mode family (torsional or longitudinal) and the testing frequency is necessary in order to improve the range of practical guided wave testing. It is also shown that when for a given pipe the coating is not of uniform thickness, along the propagating direction, high attenuation is observed for all frequencies above some cut-off. The theoretical predictions are compared to experimental results with a good match being achieved.
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