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DYNAMIC TENSILE FAILURE OF ROCKS SUBJECTED TO PRE-TENSTION STRESS *K. Xia, B. Wu University of Toronto 35 St. George Street Toronto, Canada M5S 1A4 (*Corresponding author: [email protected]) R. Chen National Unversity of Defense Technology Changsha, People’s Republic of China 410073 DYNAMIC TENSILE FAILURE OF ROCKS SUBJECTED TO PRE-TENSTION STRESS ABSTRACT Tensile failure of rocks is the main problem in underground rock engineering projects, in which rocks are subjected to dynamic disturbances while under high in situ stresses. For example, in deep mining activities, blasting induced compressive stress wave would be reflected as tensile wave at a free surface around the openings, which would facilitate the tensile failure of the deep rock since it is much weaker in tension than in compression. It is thus critical to understand the dynamic tensile failure of rocks subjected to static tension for deep rock engineering applications. A modified split Hopkinson pressure bar (SHPB) system is adopted to load Brazillian disc (BD) samples statically, and then exert dynamic load to the sample through stress wave generated by impact, as shown in Figure.1. The pulse shaper technique is used to generate a slowly rising stress wave and thus to facilitate the stress equilibrium of samples during each test. Five groups of Laurentian granite (with tensile strength of 12.8 MPa) BD samples under the pre-tension stress of 0 MPa, 2 MPa, 4 MPa, 8 MPa, and 10 MPa are tested under different loading rates. The result shows that the dynamic tensile strength of the rock decreases with the increase of pre-tension stress, which is easy to understand because pre-tension stress weakens the rock sample. It is also observed that under the same pre-tension stress, the dynamic tensile strength increases with the loading rate, revealing the so-called rate dependency that is common for engineering materials. Furthermore, it is found that the loading rate sensitivity of the rock increases with the increase of pre-tension stress. A qualitative explanation for this phenomenon is that more microcracks are generated and activated under higher pretension stress, resulting in a more viscous material. Based on the results, an empirical equation is proposed to fit the data and explain the dynamic tensile behaviour of rock under pre-tension stress. A physical model is used to simulate the dependency of the dynamic tensile strength of rock on the pre-tension stress and the loading rate, which is shown to be in good agreement with the experimental results. Figure 1 – The schematic of the Brazilian disc test with pre-tension stress in SHPB Figure 2 – The dynamic strength versus loading rates for different pre-tension stresses KEYWORDS Brizilian disc, Dynamic tensile strength, SHPB, Pre-tension stress