The ultrasound response of disordered granular materials under compression was experimentally and numerically investigated. Experiments employed piezoelectric transducers to generate and detect short-duration ultrasound waves of various frequencies traveling through a random packing of sapphire spheres subjected to compression. The experiments featured in-situ measurements of contact fabric, crystal orientation, average per-particle stress, and inter-particle forces. These measurements were made by combining synchrotron X-ray computed tomography (XRCT) and 3D X-ray diffraction (3DXRD). The packing configuration and inter-particle forces in the granular material at a given level of sample compression were used to construct a linear spring network with stiffness described by a Hessian matrix. The ultrasound response of this network was computed to understand how packing structure and force heterogeneity influence wave velocity, dispersion and attenuation. Comparisons were also made between these calculations, experimental measurements, and the theoretical behavior of ideal granular crystals. Results reveal that packing heterogeneity and force heterogeneity play different roles of varying relative importance in determining characteristics of wave velocity scaling, dispersion and attenuation.

Ultrasound wave propagation in granular materials

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