Strain-rate effects on the strength and fragmentation size of rocks

Rock mass possesses a complex structural hierarchy. The size effect on the strength of geomedia is closely related to the structural hierarchy of rock mass. This paper demonstrates that the rock mass strength sensitivity to strain rate may be regarded as the result of competition between the coexisting thermally activated and macro-viscous mechanisms, which dominate at different ranges of strain rates. The dependence of the rock media strength on strain rate may be expressed as the summation of the contributions from these two mechanisms. A new model is proposed using an asymptotic intermediate approximation of viscosity that is based on analyses at different structural levels. Comparison with experimental data shows that the proposed model provides a good description of the rock mass strength over a wide range of strain rates. It also shows that the fragmentation size of rock mass is influenced by the dynamic loading, under which shear deformation energy is increased at the moment of fracture due to the strength enhancement from the hardening effects of confinement stress, plastic deformation and strain rate. The suggested relationship gives a good description of the mean fragmentation size.