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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