| Two techniques were proposed and evaluated to analyze and control highly dispersive laser-generated Lamb waves propagating in aluminum plates. Both techniques were first validated with nondispersive laser-generated Rayleigh waves propagating on the surface of an aluminum block. For both techniques, the thermoelastic strain was produced by absorption of a Q-switched ruby laser pulse at the surface of the sample and normal displacements were detected with either a pinducer (small piezoelectric transducer) or an argon-ion laser interferometer.; The first technique consists in scanning the surface of the sample, acquiring the laser-generated waveforms, and performing a two-dimensional fast Fourier transform in space and time to obtain the dispersion curves of the medium in wavenumber and frequency. This technique allows one to identify which modes propagate in the specimen as well as their relative amplitudes. This technique, which is common in structural acoustics, has not previously been used in laser ultrasonics.; The second technique consists in forming on the surface of the specimen an array of confocal arc sources by passing the laser beam through a Fresnel lens. The array spacing produces a "forcing wavelength" for which only a few specific frequencies can propagate. Dispersion curves can be obtained by measuring the frequency content of the received signals for a range of wavenumbers. Because of the narrowband nature of the technique, and because of the confocal geometry of the source distribution, this technique offers a relatively high signal-to-noise ratio; it also alleviates the need for scanning the surface of the sample. Good agreement is obtained between theoretical and experimental dispersion curves especially for the lower modes, thus showing that the proposed techniques have potential for some specific applications in laser ultrasonic nondestructive testing. ftn*Work supported by the National Science Foundation. |
