Mechanisms for acoustic emissions generation during granular shearing

Abstract

Shear deformation of granular media leads to continual restructuring of particle contact network and mechanical interactions. These changes to the mechanical state include jamming of grains, collisions, and frictional slip of particles—all of which present abrupt perturbations of internal forces and release of strain energy. Such energy release events typically result in the generation of elastic waves in the kHz frequency range, termed acoustic emissions (AE). The close association between grain-scale mechanics and AE generation motivated the use of AE as surrogate observations to assess the mechanical state of complex materials and granular flows. The study characterizes AE generation mechanisms stemming from grain-scale mechanical interactions. Basic mechanisms are considered, including frictional slip between particles, and mechanical excitation of particle configurations during force network restructuring events. The intrinsic frequencies and energy content of generated AEs bear the signature of source mechanisms and of structural features of the grain network. Acoustic measurements in simple shear experiments of glass beads reveal distinct characteristics of AE associated with different source mechanisms. These findings offer new capabilities for non-invasive interrogation of micromechancial interactions and linkage to a stochastic model of shear zone mechanics. Certain statistical features of restructuring events and associated energy release during shearing were predicted with a conceptual fiber-bundle model (FBM). In the FBM the collective behavior of a large number of basic mechanical elements (representing e.g. grain contacts), termed fibers, reproduces the reaction of disordered materials to progressive loading. The failure of fibers at an individual threshold force corresponds to slipping of a particle contact or a single rearrangement event of the granular network. The energy release from model fiber breakage is the equivalent to elastic energy from abrupt grain rearrangement events and provides an estimate of the energy available for elastic wave generation. The coupled FBM–AE model was in reasonable agreement with direct shear experiments that were performed on large granular assemblies. The results underline the potential of using AE as a diagnostic tool to study micro-mechanical interactions, shear failure and mobilization in granular material.