ABSTRACT:

To simulate the in situ stress state of underground rocks, a modified Split Hopkinson Pressure Bar (SHPB) system is utilized to load Brazilian Disc (BD) samples hydrostatically, and then exert dynamic load to the sample by impacting the striker on the incident bar. Five groups of Laurentian granite BD samples (with static BD tensile strength of 12.8 MPa) under the hydrostatic confinement of 0 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa were tested under different loading rates. The result shows that the dynamic tensile strength increases with the hydrostatic confinement stress. It is also observed that under the same hydrostatic pre-stress, the dynamic tensile strength increases with the loading rate, revealing the so called material rate dependency. Furthermore, the increment of the tensile strength decreases with the hydrostatic confinement stress, which resembles the behavior of rock static tensile strength under confining pressure as reported in the literature.

1 INTRODUCTION

Tensile failure is a main failure mode of rocks in underground rock engineering projects, in which rocks are subjected to dynamic disturbances while under in situ stresses. As well known, pores and microcracks are potential sources of failure for rock materials because of stress concentration (Aadnoy & Angellolsen 1995, Bordia 1971, Zhang et al. 2014). When disturbed by dynamic loads from blasting, seismicity, or rockbursts, the underground rocks would be vulnerable to tensile failure. The discontinuities in the rock result in tensile stress locally around the opening, so the dynamic tensile failure of rocks under pre-tension has been investigated by researchers (Wu et al. 2015, Zhou et al. 2014). However, the far field rocks under hydrostatic pressure can also fail in tension under dynamic loads. Therefore, it is the purpose of this paper to investigate the dynamic tensile failure of rock materials under hydrostatic pressure state.

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