AN APPROACH TO DETERMINE THE EXPERIMENTAL TRANSMITTER-RECEIVER GEOMETRY FOR THE RECEPTION OF LEAKY LAMB WAVES

Often, either the swept frequency technique or a combination of swept frequency and geometric analysis is used to produce the experimental L amb wave dispersion data. This paper describes an approach for constru cting dispersion curves in solid plates using Fourier analysis of rece ived leaky Lamb wave signals. The Lamb waves are produced by pulsed ul trasound generated using two broad band transducers positioned in a pi tch-catch orientation. The relative distances among the plate and the two transducers are set to specific values as per geometric calculatio ns based on beam diffraction. The transducer defocus is used in conjun ction with geometric calculation to determine the phase velocity of th e Lamb wave mode being monitored. Subsequent to appropriate positionin g of the transducers, the plate wave signals are Fourier transformed t o obtain a magnitude versus frequency spectrum. Peaks in the spectrum indicate the presence of a Lamb wave root. The feasibility of this met hod, tested by successfully constructing dispersion curves for a steel plate, is compared with the ''null zone'' monitoring method of genera tion of the dispersion curves. The geometric positioning method is fur ther applied to a metal matrix composite sample wherein the sensitivit y of various experimentally generated Lamb wave modes is assessed to d etect many types of preprogrammed defects in different layers of the c omposite plate.