An Approach to Determine the Experimental Transmitter-Receiver Geometry for the Reception of Leaky Lamb Waves

摘要

Often, either the swept frequency technique or a combination of swept frequency and geometric analysis is used to produce the experimental Lamb wave dispersion data. This paper describes an approach for constructing dispersion curves in solid plates using Fourier analysis of received leaky Lamb wave signals. The Lamb waves are produced by pulsed ultrasound generated using two broad band transducers positioned in a pitch-catch orientation. The relative distances among the plate and the two transducers are set to specific values as per geometric calculations based on beam diffraction. The transducer defocus is used in conjunction with geometric calculation to determine the phase velocity of the Lamb wave mode being monitored. Subsequent to appropriate positioning of the transducers, the plate wave signals are Fourier transformed to obtain a magnitude versus frequency spectrum. Peaks in the spectrum indicate the presence of a Lamb wave root. The feasibility of this method, tested by successfully constructing dispersion curves for a steel plate, is compared with the "null zone" monitoring method of generation of the dispersion curves. The geometric positioning method is further applied to a metal matrix composite sample wherein the sensitivity of various experimentally generated Lamb wave modes is assessed to detect many types of preprogrammed defects in different layers of the composite plate.