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Elastic wave propagation provides a noninvasive way to probe granularmaterials. The discrete element method using particle configuration as input,allows a micromechanical interpretation on the acoustic response of a givengranular system. This paper compares static and dynamic numerical probingmethods, from which wave velocities are either deduced from elastic moduli orextracted from the time/frequency-domain signals. The dependence of wavevelocities on key characteristics, i.e., perturbation magnitude and directionfor static probing, and maximum travel distance and inserted signals fordynamic probing, is investigated. It is found that processing thefrequency-domain signals obtained from dynamic probing leads to reproduciblewave velocities at all wavenumbers, irrespective of the perturbationcharacteristics, whereas the maximum travel distance and input signals for thetime domain analysis have to be carefully chosen, so as to obtain the samelong-wavelength limits as from the frequency domain. Static and dynamic probesare applied to calibrated representative volumes of glass beads, subjected tocyclic oedometric compression. Although the perturbation magnitudes areselected to reveal only the elastic moduli, the deduced wave velocities areconsistently lower than the long-wavelength limits at various stress states,and thus sensitive to sample size. While the static probes investigate theinfluence of stress history on modulus degradation, dynamic probing offersinsights about how dispersion relations evolve during cyclic compression.Interestingly, immediately after each load reversal the incremental behavior isreversibly elastoplastic, until it becomes truly elastic with furtherunload/reload. With repeating unload/reload, the P- or S-wave dispersionrelations become increasingly scalable with respect to their long-wavelengthlimits.

Elastic wave propagation in dry granular media: effects of probing characteristics and stress history