The propagation characteristic of ultrasonic longitudinal guided waves in viscous liquid-filled elastic pipes is investigated. The complex dispersion curves were computed on the assumption that the frequency is real number and wave number is complex number. Then an analysis is carried out of the Axial Power Flow (APF) amplitude and attenuation distributions. Finally, the optimal location, optimal mode and its frequency-thickness products (fd) of testing for pipes filled with viscous liquid are chosen by the APF and attenuation distributions. The results showed that the APF amplitude distribution is an effective parameter to choose modes and parameters of testing for pipes. On the other hand, at the ranges of fd of larger group velocity of each mode, this mode has feeble dispersion, while the other modes have smaller group velocity and stronger dispersion. And in that range, the mode has a larger APF at the wall of pipes. Therefore, at the range of fd of larger group velocity of each mode, this mode is used to test elastic pipes filled with viscous liquid should be more effective, the results agree with each other.

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