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