Ultrasonic inspection of sludge accumulated in plastic pipes using meta-slab mode-converting wedge transducers

Noninvasive ultrasonic inspection of sludge accumulated inside fluid-flow pipes is important to avoid a cata-strophic pipeline failure. Although an earlier ultrasonic inspection method is applicable to both metal and plastic pipes, it can be applied only when the height of the accumulated sludge inside a plastic pipe is larger than about 1/5 of the pipe diameter. Considering the wide use of plastic pipes in various industries, lowering the detectable height of the accumulated sludge can be critically important. Here, we propose a non-conventional method to inspect the sludge inside a plastic pipe, the height of which can be as low as zero in theory. While the earlier method used only L (longitudinal) waves for the inspection, the proposed method uses different wave modes in different media along the wave passage from a PZT transducer to the fluid via the pipe wall. Specifically, we generate an L wave from the PZT, fully mode-convert it to an S (shear) wave before transmitting it to the wall of a plastic pipe, and convert it back to an L wave in the fluid. The suggested wave passage should ensure that the L wave in the fluid propagates horizontally or parallel to the fluid surface for accurate height estimation. We show why using the S wave in the pipe wall is critical in lowering the detectable height of the sludge; additionally, for the full-power L-to-S mode-conversion from the PZT to the pipe wall, a meta-slab mode-converting wedge is designed by a PEEK-based metamaterial. The specific wedge configuration will be also presented. Using the proposed novel sludge inspection method, we performed numerical simulations and experiments in PVC pipes in which simulated sludges of different heights were considered. Our numerical and experimental results confirm that the detectable heights of the accumulated sludge were considerably lower than those by the existing approach that does not use the S wave.