C Desmet

Theory of Scholte, leaky Rayleigh, and lateral wave excitation via the laser-induced thermoelastic effect

The analysis of the laser‐induced thermoelastic excitation of acoustic waves propagating along a plane interface between two elastic media is presented. The general solution for the interface motion is derived. The detailed description of the liquid–solid interface motion caused by the photoexcited leaky Rayleigh, Scholte and lateral wave in the liquid is given both in frequency and time domain. The presented theory predicts that laser‐induced thermoelastic stresses in the liquid and the solid can contribute in phase to the excitation of a Scholte wave and that the lateral wave excitation is suppressed when the light penetration depth and Scholte wave penetration depth in the liquid are equal. The obtained analytical solutions provide necessary theoretical background for the optimization of the laser‐induced generation of interface waves in experiments.

Laser‐induced thermoelastic excitation of Scholte waves

The results of experiments on an all‐optical generation and detection of Scholte waves are reported. The Scholte wave was excited thermoelastically by pulsed laser action on mercury/plexiglas or mercury/fused silica interfaces and was subsequently detected by the technique of light beam deflection. Both the shape of the Scholte pulse in the time domain and its measured velocity of propagation are in line with theoretical predictions.

All-optical excitation and detection of leaky Rayleigh waves

The results of experiments on all-optical monitoring of leaky Rayleigh waves are reported. Leaky Rayleigh waves were excited by pulsed laser action on a liquid-solid interface and were detected by the light-beam-deflection technique. Both the measured velocity of their propagation and the attenuation are in good agreement with theoretical predictions. Possible applications include acoustic spectroscopy of materials, depth profiling of layered structures, and tabletop modeling of seismic phenomena.

THE BEHAVIOR OF LAMB WAVES IN STRESSED POLYMER FOILS

The influence of externally applied unidirectional stress on the lowest‐order Lamb modes of a 12‐μm poly(ethylene terephthalate) (PET) film has been investigated both theoretically and experimentally. The Lamb wave dispersion was determined in a fully contactless way by using optical generation and detection of ultrasound. The elastic parameters of the film material were obtained by fitting the angle dependence of the low‐frequency limit of the velocity of the fundamental Lamb modes. Whereas the dispersion of the symmetric mode S0 is only slightly influenced by an external stress, the antisymmetric mode A0 shows a transition from dispersion behavior determined by the elastic constants at small stresses to a behavior dominated by the applied stress at large stresses.

Investigation of the dispersion relations of surface acoustic waves propagating on a layered cylinder

The propagation of pulsed surface acoustic waves (SAW) on a layered cylinder has been investigated both theoretically and experimentally. The pulses were generated by a ‘‘line‐source’’ pulsed laser and detected by an optical beam deflection technique providing information on pulse dispersion in a fully contactless way. The sample investigated was a hollow steel tube fitted tightly to a massive cylinder of a tin alloy. The theoretical dispersion relation for surface waves propagating in this system, under the assumption that the wave fields are uniform along the cylinder axis, was derived and good agreement with the experimental results was found. These results confirm the applicability of an optical, contactless technique for the characterization of layers on curved substrates.

ALL-OPTICAL INVESTIGATION OF THE LOWEST-ORDER ANTISYMMETRICAL ACOUSTIC MODES IN LIQUID-LOADED MEMBRANES

The antisymmetrical Scholte–Stoneley wave in liquid-loaded metal and polymer membranes were excited via the laser-induced thermoelastic effect and were detected with a laser beam deflection technique. The experimentally determined dispersion relation for this wave is in accordance with theory in a wide frequency range. However, no evidence of the propagating subsonic lowest-order anitsymmetrical pseudo-Lamb wave was found.

The Green’s function for surface acoustic waves: Comparison between theory and experiment

The transient displacement of the surface acoustic wave caused by a transient impulse line source on a sample consisting of a substrate covered by a thin layer is calculated. Starting from the Green’s tensor for an anisotropic semi‐infinite half‐space, the layer, which will introduce dispersion, is considered as a perturbation of this half‐space. Numerical results and experimental data, obtained with an all‐optical technique, for an isotropic layer on an isotropic half‐space are compared and show good agreement.

Splitting of the first generalized Rayleigh mode in thin layers deposited on anisotropic media

The dispersion curve for the first generalized Rayleigh mode on the surface on an anisotropic substrate covered with an anisotropic layer is calculated. We show that, for directions in which both the Rayleigh and leaky Rayleigh waves can propagate on the uncovered substrate, the dispersion curve of this generalized mode splits into two branches at a certain value of the product of the wave number and the thickness. This splitting has to be taken into consideration when deriving layer properties from dispersion curves. For the derivation, we considered the anisotropic layer as a perturbation of the anisotropic substrate. An efficient computer program based on the extended Tiersten approach was written to numerically calculate the phase velocity as a function of frequency.

On the polarity of laser-generated Rayleigh waves

We propose an exact formulation for the near, intermediate and far field of a finite length, thermoelastic, pulsed laser generated acoustic line source. Separating the analytic terms corresponding to the edge effects, we analyse the polarity of the Rayleigh wave. The generalization of this formulation to plane sources presents an alternative to the classical Rayleigh integral method.