Brittleness evaluation method based on pre-peak crack initiation and post-peak characteristics of rock

The existing researches show that the brittleness of rock is closely related to the initiation and propagation of internal microcracks, but there are few brittleness evaluation indexes considering the characteristics of rock initiation. Based on the theoretical analysis of brittleness and the characteristics of rock initiation, a brittleness evaluation method is proposed based on the pre-peak initiation and post-peak characteristics of rocks. Firstly, based on the descriptions and definitions of brittleness by George, Tarasov & Potvin et al., the feasibility of evaluation method based on the pre-peak crack initiation and post-peak characteristic brittleness is theoretically analyzed. Secondly, component B_i representing pre-peak brittleness of rocks and component B_ii representing the post-peak brittleness of rocks are constructed, and the product of the two is the brittleness index B_I representing the pre-peak fracture initiation and post-peak stress drop. Finally, it is verified based on the test data of granite, sandstone and marble under different loading methods, confining pressures and lithology conditions. The results show that the brittleness of marble decreases with the increase of confining pressure from 5 to 35 MPa. At confining pressure of 5 MPa, the brittleness of granite under the triaxial unloading tests is greater than that under the triaxial compression tests. The values of the corresponding brittleness index B_I of granite, sandstone and marble are 0.684, 0.336 and 0.186 respectively, and the calculated results are consistent with the experimental ones. This study provides a new way to evaluate the rock brittleness from the perspective of crack initiation and stress drop, which has guiding significance to enrich the analysis and evaluation of indoor rock brittleness.