Bert Verstraeten

Medium characterization from interface-wave impedance and ellipticity using simultaneous displacement and pressure measurements.

The interface-wave impedance and ellipticity are wave attributes that interrelate the full waveforms as observed in different components. For each of the fluid/elastic-solid interface waves, i.e., the pseudo-Rayleigh (pR) and Stoneley (St) waves, the impedance and ellipticity are found to have different functional dependencies on Youngs modulus and Poissons ratio. By combining the attributes in a cost function, unique and stable estimates of these parameters can be obtained, particularly when using the St wave. In a validation experiment, the impedance of the laser-excited pR wave is successfully extracted from simultaneous measurements of the normal particle displacement and the fluid pressure at a water/aluminum interface. The displacement is measured using a laser Doppler vibrometer (LDV) and the pressure with a needle hydrophone. Any LDV measurement is perturbed by refractive-index changes along the LDV beam once acoustic waves interfere with the beam. Using a model that accounts for these perturbations, an impedance decrease of 28% with respect to the plane wave impedance of the pR wave is predicted for the water/aluminum configuration. Although this deviation is different for the experimentally extracted impedance, there is excellent agreement between the observed and predicted pR waveforms in both the particle displacement and fluid pressure.

Temperature Dependent Elasticity of a Ni2MnGa Film as Measured by Impulsive Stimulated Scattering

We report the results of an impulsive stimulated scattering measurement conducted on a 550 nm film of Ni2MnGa on a MgO substrate, aiming at the direct experimental observation of the highly temperature dependent elasticity. The phase velocity of the surface acoustic waves excited on the surface of the coating-substrate shows an anomaly around the phase transition temperature of Ni2MnGa, showing that it is possible to use impulsive stimulated scattering as a characterization tool to determine temperature dependent elasticity of a thin film.

Combined Particle Motion And Fluid Pressure Measurements of Surface Waves

In this paper we investigate the use of combined particle motion and fluid pressure measurements of surface waves. In particular, we show that the surface-wave impedance of the pseudo-Rayleigh (pR) wave, excited at a water/aluminum interface, can be successfully extracted from such a combined experiment at ultrasonic frequencies. The displacement is measured using a laser Doppler vibrometer (LDV) and the pressure with a needle hydrophone. The impedance of the pR-wave can be clearly distinguished in the wavenumberfrequency domain. Our results suggest that the surface-wave impedance can also be extracted from measurements on seismic or borehole logging scales. Further, the excellent agreement between the observed and predicted pR-waveforms in both the particle displacement and fluid pressure shows that laboratory scale experiments using laser ultrasonics and small pressure detectors can offer a useful link between theoretical models and real field experiments.

In-situ spectroscopic investigation of ultrasonic assisted unfolding and aggregation of insulin

It is well-known that fibrillogenesis of proteins can be influenced by diverse external parameters, such as temperature, pressure, agitation or chemical agents. The present preliminary study suggests that ultrasonic excitation at moderate intensities has a significant influence on the unfolding and aggregation behaviour of insulin. Irradiation with an average sound intensity of even as low as 70mW/cm(2) leads to a lowering of the unfolding and aggregation temperature up to 7K. The effect could be explained by an increase of the aggregation kinetics due to ultrasonically induced acoustic micro-streaming in the insulin solution that most probably enhances the aggregation rate. The clear and remarkable effect at relatively low sound intensities offers interesting options for further applications of ultrasound in biophysics and biochemistry. On the other hand, a process that causes a change of kinetics equivalent to 7K also gives a warning signal concerning the safety of those medical ultrasonic devices that work in this intensity range.

Full elastic characterization of absorptive rubber using laser excited guided ultrasonic waves

Because of the highly damping nature of rubber, it is difficult to characterize its dynamic elastic properties using classical methods. In this paper, an experimental approach employing laser excited guided acoustic waves is proposed to accurately determine the real and imaginary part of the longitudinal and shear elastic modulus of a rubber layer. From the spatiotemporal evolution of a propagating laser excited Lamb wave measured by a laser Doppler vibrometer, which is scanning along a line perpendicular to a line of excitation, the phase velocity dispersion curves in the wave number - frequency domain are obtained. The results are interpreted in the framework of a detailed semianalytical study, analyzing the influence of elastic damping on the Lamb dispersion curves. This analysis is exploited to adequately fit the experimental dispersion curves and thus extract information about the elastic moduli and absorption coefficients of the rubber plate. The results are validated by a pulse-echo measurement, an...

Thermomechanical properties of single crystals evaluated by impulsive stimulated thermal scattering technique

This paper describes the application of Impulse Stimulated Thermal Scattering (ISTS) for measurement of surface acoustic wave (SAW) velocity and thermal diffusion along a free surface of a strongly anisotropic material. The motivation for this work stems from the study of thermoelastic properties of individual phases of ferroelastics; experimental results were obtained on a single crystal in the austenitic phase a Cu-Al-Ni alloy (bcc single crystal having elastic anisotropy factor of about 12). The measured SAW velocities in specific directions are in a good agreement with the values calculated for the elastic constants obtained by other ultrasonic methods. Similarly, the evaluated thermal diffusivity coefficient (22 ± 2).10−6 m2/s of the austenite is consistent with the data in the literature. The proposed approach has also a potential for characterization of thin films grown on anisotropic substrates.