Dynamic tensile behavior of rocks under static pre-tension using the flattened Brazilian disc method

Rocks in underground engineering projects, are likely to be subjected to static pre-stress and dynamic loading simultaneously. To measure the tensile strength of brittle rocks under coupled static-dynamic loads, a coupled static-dynamic flattened Brazilian disc (FBD) testing method is proposed in this study via a modified split Hopkinson pressure bar (SHPB) system. Our method has unique advantages on coupled static-dynamic tests using SHPB, because under different static pre-loads, the contact area between the loading platens and the specimen remain nearly constant, contributing to maintaining a consistent contact condition. Using the method, we experimentally and numerically investigate the tensile response of sandstone subjected to different static pretensions and different dynamic loading rates. By careful pulse shaping, the dynamic force equilibrium on both loading ends of FBD specimens can be satisfied. Detailed progressive fracture processes of tested specimens are captured via a high-speed camera, and the central crack initiation is observed in all our coupled static-dynamic tests. The dynamic tensile strength of sandstone decreases with the increase of the pre-tension, while the total tensile strength features an increase, and both the dynamic tensile strength and total tensile strength show the obvious rate dependence. The numerical simulation reveals that under a higher pre-tension ratio, more microcracks are observed at the static pre-tension stage, suggesting that more damage is induced in the rock. Our method is feasible to indirectly measure the tensile strength of rocks under coupled static-dynamic loads.