Mechanical behavior of different sedimentary rocks in the Brazilian test

Mechanical behavior of the rock in tensile stress environment remains an unresolved problem in the underground mining, where surrounding rocks commonly experience tensile failure. In the present study, tensile failure behavior of three types of sedimentary rocks is investigated experimentally and numerically. The deformation response, fracture propagation, and splitting pattern as well as energy conversion of the rocks are examined in the Brazilian test with a testing machine, high-speed camera, and image scanning system. The tested rocks tend to show elastic-brittle-plastic deformation behavior in the biaxial stress state. Tensile strengths of the coal, mudstone, and sandstone are 1.2, 3.9, and 13.4 MPa, respectively. The coal and mudstone split in a static mode while the sandstone in dynamic mode. The splitting fracture initiates at the disk center in the coal and sandstone, and it emerges at the loading point in the mudstone. The fracture propagates more quickly in the rock with larger strength. It uses 1200.0, 5.8, and 0.4 ms, to break out sample surface of the coal, mudstone, and sandstone, respectively. The joint roughness coefficient (JRC) of the splitting fracture is largest (20.1) in the mudstone, followed by that in the coal (17.7), and it is smallest in the sandstone (15.3). A continuous-discontinuous coupling model, which is capable of analyzing the stress distribution and fracture propagation synergistically, is developed and calibrated against the experimental data. The numerical model accurately reproduces mechanical behavior of the tested rocks observed in the Brazilian test. The splitting fracture propagates along the maximum tensile stress plane in the sandstone. The propagation direction is locally influenced by the grain boundary with small tensile strength in the coal while, in the mudstone, the fracture is mainly formed along the grain boundary. Such differences in the fracture propagation path lead to the increasing trend from the JRC of the splitting surface in the sandstone to that in the coal and finally to that in the mudstone.