A granular system composed of frictional glass beads is simulated using theDiscrete Element Method. The inter-grain forces are based on the Hertz contactlaw in the normal direction with frictional tangential force. The damping dueto collision is also accounted for. Systems are loaded at various stresses andtheir quasi-static elastic moduli are characterized. Each system is subjectedto an extensive dynamic testing protocol by measuring the resonant response toa broad range of AC drive amplitudes and frequencies via a set of diagnosticstrains. The system, linear at small AC drive amplitudes has resonancefrequencies that shift downward (i.e., modulus softening) with increased ACdrive amplitude. Detailed testing shows that the slipping contact ratio doesnot contribute significantly to this dynamic modulus softening, but thecoordination number is strongly correlated to this reduction. This suggeststhat the softening arises from the extended structural change via break andremake of contacts during the rearrangement of bead positions driven by the ACamplitude.
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