Using Passive Acoustics to Monitor Impending Failure in a Sheared Granular Material

Abstract:

Failure of granular materials plays an important role in numerous geophysical processes, from mobilization of sediments to fracture of sedimentary rock to the intermittent failure of gouge which regulates earthquake dynamics. However, measurements and analytic models which forecast failure in this class of materials, while of both fundamental and practical interest, remain elusive. Materials including numerical packings of spheres, colloidal glasses, and granular materials have been known to develop an excess of low-frequency vibrational modes as the confining pressure is reduced. We share results from experiments on sheared granular materials in which we monitor the evolving density of excited modes via passive monitoring of acoustic emissions. We observe a broadening of the distribution of excited modes coincident with both bulk and local plasticity, and evolution in the shape of the distribution before and after bulk failure. These results provide a new interpretation of the changing state of the material on its approach to stick-slip failure, and may suggest a path towards forecasting failure in geophysical systems. Details of this work are also reported in our publication “Sounds of Failure: Passive Acoustic Measurements of Excited Vibrational Modes.” Phys. Rev. Lett. 120, 218003 (2018).