Abstract
It is critical to understand the dynamic tensile failure of rocks subjected to pre-tension for rock engineering applications such as underground mining and tunneling. A modified split Hopkinson pressure bar (SHPB) system is applied to load Brazilian disc (BD) samples statically, and then exert dynamic load to the sample through a stress wave generated by impact. The pulse shaper technique is used to generate a slowly rising stress wave to facilitate the stress equilibrium of samples. Five groups of Laurentian granite BD samples (with tensile strength of 12.8 MPa) under the pre-tension of 0 MPa, 2 MPa, 4 MPa, 8 MPa, and 10 MPa were tested under different loading rates. The results show that the rock dynamic tensile strength decreases with the increase of the pre-tension. It is also observed that under the same pre-tension, the dynamic tensile strength increases with the loading rate. However, the total tensile strength of the rock is roughly independent of the pre-tension. The failure patterns of the samples also reveal the rate dependence of the dynamic strength of rocks.
1. INTRODUCTION
Tensile failure is a main failure mode of rocks in underground projects, in which rocks are subjected to dynamic disturbances while under in situ stresses. As is well known, pores and microcracks are potential sources of failure for rock materials because of stress concentration [1-3]. When disturbed by dynamic loads from blasting, seismicity, or rockbursts, the underground rocks would be vulnerable to tensile failure. Even though the far-field load is compressive, the local stresses may be tensile as shown in Fig. 1. From a macroscopic point of view, the bending of the roof induces tensile stress at the roof of the opening; from a microscopic view, the discontinuities in the rock result in tensile stress locally. On top of these pre-tension, dynamic disturbance may be tensile in nature and lead to the ultimate failure of the rock material. Therefore, it is necessary to investigate the dynamic tensile failure of rock materials under pre-tension.
