Scattering of the fundamental torsional mode by an axisymmetric layer inside a pipe

The scattering of the fundamental guided torsional mode by a local axisymmetrical layer coated inside a pipe, referred to as a bilayered pipe, is studied. In a prescribed frequency range, the number of torsional modes which can propagate in an empty pipe is increased by the presence of the coating layer, including new cutoff frequencies that depend on the layer thickness and shear acoustic properties. This principle suggests the potential for detecting, and perhaps characterizing layers inside pipes, which may be exploited in two ways using either remote or local measurements. A remote measurement may be employed to measure the reflection from the entry point of the layer inside the pipe. Such a measurement shows that the reflection coefficient spectrum exhibits periodic maxima that occur at the cutoff frequencies of the torsional modes in the bilayered pipe. On the other hand, when the location of the coating layer is accessible, the local guided wave measurement can be made to measure the dispersion curves of the torsional modes in the bilayered pipe by applying a reassigned spectrogram analysis. The idea is investigated through theoretical analysis and finite element modeling and is validated by experimental measurements.