Detection of coatings within liquid-filled tubes and containers by mode conversion of leaky Lamb waves
- 1Coburg University of Applied Sciences, Institute of Sensor and Actuator Technology, Am Hofbrauhaus 1b, 96450 Coburg, Germany
- 2University Paderborn, Faculty of Electrical Engineering, Computer Science and Mathematics, Measurement Engineering Group, Warburger Str. 100, 33098 Paderborn, Germany
- 3University Freiburg, Department of Microsystems Engineering – IMTEK, Laboratory for Electrical Instrumentation, Georges-Koehler-Allee 106, 79110 Freiburg, Germany
Abstract. In this paper, a new acoustic sensor principle for coating detection within liquid-filled tubes and containers based on mode conversion of leaky Lamb waves is introduced. Leaky Lamb waves are excited and detected by single-phase transducers, which are attached on the outer side of a tube or container. By transmission time and amplitude measurements, coating formation within the liquid-filled tube and container is detected non-invasively. This new sensor principle is subdivided into the separate considerations of Lamb wave excitation, mode conversion and inverse mode conversion. The Lamb wave excitation by a single-phase transducer is visualized by scanning laser Doppler vibrometer imaging. The mode conversion process of leaky Lamb waves is measured by membrane hydrophone measurements and Schlieren visualization; afterwards, the measured emission angles are compared with the theoretical one. The inverse mode conversion process of pressure waves back to leaky Lamb waves is visualized by Schlieren images. By merging the results of Lamb wave excitation, mode conversion and inverse mode conversion, the new sensor concept is explained. Theoretical considerations and measurement results of adhesive tape coating inside a liquid-filled plastic tube and a liquid-filled stainless steel container verify the new acoustic sensor principle. Finally the measuring sensitivity and the technical realization are discussed.