High-Temperature Pipeline Wall Thickness Monitoring Using a Waveguide Sensor Structure Based on Wave Mode Conversion

Ultrasonic waveguide sensor is suitable for monitoring pipeline wall thickness at high temperature ( $\gt 500~^{\circ }$ C) due to the heat dissipation and heat insulation effect of guide plate. The shear wave is produced by shear-wave piezoelectric ceramic. The plate wave used to measure pipeline wall thickness is formed by the shear wave. However, conventional high-temperature sensors using shear-wave piezoelectric ceramics were limited in cost, frequency, and size. Especially, it is difficult to obtain high-frequency shear wave with small-sized sensors. In this article, a waveguide sensor structure based on wave mode conversion is developed to overcome the problems above. Shear horizontal (SH0) mode plate waves can be converted by longitudinal waves generated from standard, low-frequency, and small-sized piezoelectric ceramics. A suitable frequency-thickness product is chosen to avoid dispersion and signal distortion. The feasibility of the designed sensor is verified by experiments. The mature ultrasonic longitudinal wave probe is used as the transducer to improve the frequency and reduce the cost and size of the sensor, which provides a basis for achieving large-area matrix measurement.