Elastic waves in particulate glass-rubber mixtures

Abstract

We investigate the propagation of waves in dense static granular packings made of soft and stiff particles subjected to hydrostatic stress. Physical experiments in a triaxial cell equipped with broadband piezoelectric wave transducers have been performed at ultrasound frequencies. The time of flight is measured in order to study the combined effect of applied stress and rubber content on the elastic properties of the mixtures. The bulk stiffness deduced from the wave speed is nonlinear and non-monotonic with the increasing percentage of rubber with a more prominent effect at higher pressures. Moreover, in the frequency domain, a spectral analysis gives insights on the transition from a glass- to a rubber-dominated regime and the influence of rubber particles on the energy dissipation. Mixtures with rubber content below 30% show enhanced damping properties, associated with slightly higher stiffness and lighter weight.

Multi‐scale characterization of granular media by in situ laboratory X‐ray computed tomography

“…Here, we first report the results on the bulk stiffness of granular mixtures with diverse rubber content obtained by the experimental tests. Particular attention is devoted to the dependence of sound velocity on the applied load and soft-stiff composition of samples, since this is an important controllable experimental parameter [21,26,55]. After that, X-ray images taken during wave propagation are analysed to obtain micro-insights of packings.…”

“…This allows to probe the stiffness of the soil or particulate sample along a given stress path. The transmitting transducer transforms the input F I G U R E 2 Schematic view of wave propagation measurement setup [55] electric signal to an acoustic wave and sends it through the medium. The receiving transducer receives the propagated mechanical sound wave and transforms it back to an electronical signal which can be acquired by the digitizer, compare [27,43] for technical details on the transducers.…”

“…All investigated samples are made of monodisperse glass and rubber beads with 4 mm diameter. Further information on the material characteristics for both, glass and rubber beads, can be found in [55]. Particles are poured carefully into the oedometer cell described in Section 3. shows the steps taken from a reconstructed raw image (first row) to segmented (second row), and labeled (third row).…”

“…During the image acquisition, it is switched from force‐control to displacement‐control to avoid slight movements of the particles due to relaxation processes. The selection is based on previous knowledge from comparable experiments in a triaxial cell without XRCT imaging [55] as well as the before performed ex situ measurements in the oedometer cell. A summary of the applied in situ workflow is given in Figure 6.…”

“…To study this systematically, the simplest synthetic granular medium is a biphasic mixture of monodisperse soft and stiff particles. In [55], such mixtures under different hydrostatic stresses as well as for different volume fractions were investigated. Measured P-wave moduli showed a significant decline while more soft particles are added, that is, higher rubber volume fractions, due to a change in the microstructure of the granular medium.…”