Lamb Wave Tomography and Its Application in Pipe Erosion/corrosion Monitoring

number of NDE applications. To excite Lamb waves, Ultrasonic Lamb wave techniques are widely used in a mode conversion of bulk waves or photo acoustic excitation are often used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach good signal to noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through demonstrate the power of this approach in ultrasonic dry, elastic contacts to monitor velocity changes. We pipe erosionlcorrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness, and erosionlcorrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate. MOTIVATION FOR ULTRASONIC LAMB WAVE THICKNESS MEASUREMENT Ultrasonic measurement of the thickness of plates is most often done by measuring the time of flight of bulk waves in the material'. This technique usually requires a liquid couplant between the transducer and the plate, and is especially difficult when the plate has surface curvature or roughness. Traditional techniques are of limited use in hostile nvironments because of transducer material. Plate thickness can also be limitations on the fluid couplant and the piezoelectric measured with photo acoustic excitation of Lamb waves2.3. However, ablation of the test material is often needed to reach good signal to noise ratio in order to perform accurate measurements4. In this paper, we present a new technique that relies on using dry, point contact transducerhuffer pin sets to excite and detect the zeroth order antisymmetric (Ao) Lamb wave mode in plates. Due to the dispersive nature of the Ao mode Lamb wave, its velocity depends on the thickness of the plate. Hence, a measure of the velocity of the Lamb wave yields a measure of the average plate thickness between the transmitter and the receiver. This technique does not require any couplant, and can be applied in hostile environments to monitor erosion or corrosion 0-7803-2940-6/95/$4.00