The influence of a polycrystals' grain structure on elastic wave scattering is studied with analytical and numerical methods in a broad frequency range. A semi-analytical attenuation model, based on an established scattering theory, is presented. This technique accurately accounts for the grain morphology without prior assumptions on grain statistics. This is achieved by incorporating a samples' exact spatial two-point correlation function into the theory. The approach is verified by using a finite element method (FEM) to simulate P-wave propagation in 3D Voronoi crystals with equal mean grain diameter, but different grain shape uniformity. Aluminum and Inconel serve as representatives for weak and strong scattering cubic class materials for simulations and analytical calculations. It was found that the shape of the grains has a strong influence on the attenuation curve progression in the Rayleigh-stochastic transition region, which was attributed to mode conversion scattering. Comparisons between simulations and theory show excellent agreement for both materials. This demonstrates the need for accurately taking the microstructure of heterogeneous materials into account, to get precise analytical predictions for their scattering behaviour. It also demonstrates the impressive accuracy and flexibility of the scattering theory which was used.

Attenuation of Rayleigh-type surface acoustic waves induced by grain-boundary scattering is studied experimentally and theoretically by an effective medium approach. A frequency domain opto-acoustic laboratory setup, capable of measuring a coherent Rayleigh wave response by emulating an ensemble average via spatial averaging, is presented. Measurements are conducted on polycrystalline aluminum at ultrasonic frequencies from 10 MHz to 130 MHz. A constant effective phase velocity of 2893 m s−1 is found below 80 MHz. The effective attenuation coefficient varies in the whole frequency range by nearly two orders of magnitude, and shows classical scattering behavior, comprising stochastic and geometric scattering regimes. A semi-analytical attenuation model is presented, valid below the geometric limit. The model incorporates the material’s spatial two-point correlation function obtained from metallurgical micrographs. Comparisons to experimentally obtained attenuation coefficients show good quantitative agreement, with differences in the frequency power-law dependence. This study attempts to elucidate microstructure induced surface acoustic wave attenuation experimentally by means of a statistical approach. The proposed method and the obtained findings contribute to the understanding of wave propagation in heterogeneous media, and promote the use of surface acoustic waves in non-destructive microstructure characterization.

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A method for the determination of longitudinal and transverse bulk acoustic wave attenuation from measurements of the decay-rate of two independent zero-group-velocity resonances in a couple of matched plates is presented. A linear relation is derived, which links the bulk-wave attenuation coefficients to the decay-rate of plate-resonances. The relation is used to determine the acoustic loss of tungsten at GHz frequencies from noncontact laser-ultrasonic measurements in plates with thicknesses of about 1 µm. The longitudinal and transverse attenuation was found to amount to 1918 m−1 and 7828 m−1 at 2.16 GHz and 3265 m−1 and 12181 m−1 at 2.46 GHz. The presented approach is validated with calculated responses to a thermoelastic source, and the accuracy of the obtained attenuation values is estimated to be in the range of 10%.

We present an evaluation of attenuation of elastic waves in the GHz range, based on the decay of thickness-stretch resonances of plates. We measured the elastic response of micro-scale aluminum plates, using a laser-ultrasound technique. The thermo-elastic laser excitation provides significant coupling into thickness-stretch modes in the frequency range above 1.5 GHz. To suppress interference from other resonant and transient modes, we used an excitation spot size well above the plate thickness and applied signal processing in the time and frequency domain. We adapted existing theory on the decay of thickness-shear modes to apply for thickness-stretch modes, validated the derived theory with simulations, and applied it to experiments. A set of samples with different thicknesses in the range of 1.1–2.15 μm allowed us to obtain damping values in the corresponding frequency range of 1.5–3 GHz.