ULTRASONIC PULSED-BEAM INTERACTION WITH A FLUID-LOADED ELASTIC PLATE - THEORY

The interaction of transducer-excited ultrasonic pulsed beams with flu id-loaded elastic plates is treated with a computationally efficient a nalytical model. The model synthesizes the frequency-domain voltage, d ue to a single transducer operated in reflection (pulse-echo) mode and a pair of tranducers in transmission mode, utilizing an approach that is based on (1) expansion of transducer fields in terms of quasi-Gaus sian beams modeled via the complex-source-point technique and its rece nt extension to finite receivers, and on (2) complex wave-number spect ral decomposition and synthesis to solve the beam-structure interactio n problem. First, a reference solution for the frequency-domain reflec ted and transmitted fields is expressed in terms of spectral integrals over two-dimensional infinite spectra of plane waves weighted by the plate reflection or transmission coefficient. Subsequent expansion in terms of a finite sum of integrals representing multiply reflected bea ms propagating within the plate, combined with high-frequency asymptot ics and inverse Fourier transformation of the frequency-domain data, y ields the time-domain voltage as a finite sum of purely compressional (P), purely shear (S), and P-S coupled arrivals. Of particular interes t is the new higher-order asymptotic expansion developed to account fo r shear waves excited in the plate by the finite angular spectrum of b eams at normal incidence. Both reference and asymptotic solutions have been implemented in numerical codes and validated against experimenta lly generated data. This is shown in a follow-up paper. The methodolog y presented here can be applied under more general conditions of trans ducer orientation and focusing, and also for elastic media with more t han one layer. (C) 1996 Acoustical Society of America.