Abstract
We present an experimental study aiming to understand the local origin of the acoustic bursts recorded globally during a two-dimensional shear experiment of a granular medium. In order to explain the power-law distribution of energy of these events (also called avalanches), the analysis focuses on their acoustic frequencies. Experiments with single grains have also been performed, trying to separate the contributions related to collision between neighbours from shear movements between grains. Preliminary results indicate that both frictional sliding and collision-like mechanisms are involved in the origin of the acoustic bursts. The evolution of the structure of the medium around a large acoustic burst is analysed both with high resolution and high-speed imaging. The energy of the acoustic bursts seems related to single and well-localised events with very large variations in energy. However, these local reorganisation processes may eventually bring the global relaxation of the system, which is also captured by a force drop in the resistance to shear.
Highlights
Shearing compressed granular materials has often been used as a simplified model of a tectonic fault
The Acoustic emissions (AE) are analysed through a time-frequency method: we computed the spectrogram of the acoustic signal of each sensor
The histogram of the frequencies emitted during five different shear experiments is presented in the figure 2c
Summary
Shearing compressed granular materials has often been used as a simplified model of a tectonic fault. As the plates move laterally in relation to each other and at very low speeds, shear stresses build up on the packed grains, and eventually they are liberated through sudden events associated to the reorganisation of the pack. These events –often called avalanches– may be detected from sudden jumps in the global shear force experienced by the plates [2, 8], from acoustics bursts [1, 9] or even from direct visualisation in the case of photoelastic grains [2, 3]. Acoustic emissions (AE) are recorded using 6 piezoelectric pinducers (VP-1.5 from CTS Valpey Corp.) inserted in adjusted holes in the upper and lateral walls Both the force and the acoustics are recorded with a NI-USB6366 card at 100 kHz during the whole shear movement
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