Dynamic weakening is commonly observed when stone is subjected to a single or repetitive impact loading. In a series of impact loading experiments conducted using the split Hopkinson pressure bar system with external confinement pressure,
we observe evident nonlinear dynamic response for each impact loading, an accompanying dynamic weakening effect, and significant plastic deformation of the specimen. The dynamic response can be predicted accurately by the nonlinear granular model (Johnson and Jia in Nature 437:871–874, 2005). In the framework of statistics, an analytical model for dynamic weakening is proposed and it suggests that the weakening effect is induced by the increasing number of broken inter-particle bonds after impact. This is related to a decrease in the dynamic modulus that can be described by introducing a confinement pressure-dependent energy portion parameter 휇. A further nonlinear analysis of the experimental data provides detailed insights into the nonlinear dynamic response. The proposed weakening mechanism is based on inter-particle statistics and it comprises a wealth of dynamic regimes, including modulus softening and damage evolution, which can be extended to other granular materials but not limited to rocks.
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