New Ultrasonic Guided Wave Testing using Remote Excitation of Trapped Energy Mode

Ultrasonic guided waves have been used in testing of such an elongated object as pipe. Its sensitivity and distance resolution, however, have been limited by velocity dispersion and interference of numerous propagation modes of guided waves. Trapped energy mode of vibration of thickness shear waves in a quartz plate has been widely used in frequency control and selection due to its high Q and localization. One of the authors (MO) found similar trapped modes of horizontal shear or torsional waves in a cylinder, which are suitable for inspection of pipes. The resonant frequency and Q of a trapped energy mode can be remotely measured by reflection of a propagating guided wave at its boundary. The lowest mode is preferred, because it propagates without waveform distortion due to dispersion. A part of energy in a propagating guided wave excites the trapped energy mode through mode conversion and then leaks out to the original propagating wave. This leak appears as a long-tailed ringing due to a high Q of the trapped mode. The resonant frequency and Q of a trapped energy is sensitive to changes in its vibrating region and hence can be used to sense changes in plate thickness, surface condition (roughness and corrosion) and liquid loading (density and viscosity), etc. An analysis based on a multiple transmission line model and an experimental verification of remote excitation of a trapped mode are presented.