Investigating the relationship between the brittleness index and crack initiation stress for the granite under triaxial compression

The brittle failure of rock is closely related to the rock crack initiation, propagation, coalescences and nucleation. Although many brittleness indexes have been proposed to describe the brittle behavior of rocks, the relationship between the brittleness index and crack initiation stress under triaxial compressive states has not been fully understood. This paper introduced the current main brittleness indexes and discussed their applicabilities for triaxial compressive tests. Granite specimens from a gold mine are prepared for experimental tests. The triaxial compressive tests are carried out on stiff testing machine and the confining pressures are 5 to 110 MPa. Experimental results on the stress-strain curves show that the post-peak behaviors of the granite under confining pressures are in Class I and Class II types. The brittleness indexes, crack initiation stress, compressive strength, elastic moduls and Poisson's ratio of the granite are obtained and discussed. It has been found that, the crack initiation stress linearly increases with the increasing of confining pressure. Moreover, a power function can describe the relationship between the brittleness index and crack initiation stress for Class I rock behavior while an exponential function can describe the relationship between the brittleness index and crack initiation stress for Class II rock behavior.