Abstract
To investigate the fracture pattern and crack propagation of rock under different stresses, Brazilian split tests, preset angle tests, and uniaxial compression acoustic emission (AE) tests were carried out on sandstone to obtain the mechanical parameters and AE signals of the whole test process. Based on the analysis of the distribution of the characteristic AE indicators rise time/amplitude (RA) and average frequency (AF) combined with the core definition of areal density, the test results showed the following. In Brazilian split tests and preset angle tests, shear fractures and tensile fractures accompanied the whole process of rock damage and failure, and the crack type reflected by the RA-AF distribution was consistent with the theoretical analysis and actual failure results. The fracture pattern of sandstone under uniaxial compression was dominated by shear fracture, and tensile and shear cracks had the same evolutionary trend. In uniaxial compression, the development of cracks was complicated from the stage of steady development of microcracks, and the crack development was different in different specimens, which eventually leads to different failure modes. That is, the failure mode of sandstone could provide positive feedback to the RA-AF distribution. Under different stresses, the characteristics of the cumulative event rate of tensile or shear fractures varied significantly among stages, which corresponded to the various stages of rock damage and deformation.
Highlights
In recent years, with the construction of many underground geotechnical engineering projects, the construction depth continues to increase, and geological disasters such as deformation, collapse, and rock-burst of the surrounding rock in underground chambers often occur, seriously threatening construction safety [1,2,3]. ese geological disasters are the macromechanical manifestations of rock instability and failure
The Brazilian split test method was used in this study. e prepared cylindrical disc specimens each had a diameter of 50 mm and a thickness of 25 mm. e specimens for Brazilian split testing were labelled BT. e commonly used rock shear test methods include direct shear [36] and variable-angle compression-shear [37]. e shape of the direct tensile specimen is to be of the dog bone shape, which is not easy to prepare and is easy to produce stress concentration at the specimen’s ends [36]
The Brazilian split test method was used in this study, and the latter with a preset angle of 60° was used in this study. e specimens for preset angle shear testing were cubes with a side length of 50 mm and were labelled CS. e cylindrical specimens used in the rock uniaxial compression tests each had a diameter of 50 mm and a height of 100 mm and were labelled UC
Summary
With the construction of many underground geotechnical engineering projects, the construction depth continues to increase, and geological disasters such as deformation, collapse, and rock-burst of the surrounding rock in underground chambers often occur, seriously threatening construction safety [1,2,3]. ese geological disasters are the macromechanical manifestations of rock instability and failure. The rock contains various defects (e.g., microcracks, pores, joints, and fissures), and the instability and failure of an engineered rock mass are essentially a progressive process from the development of damage of mesoscopic defects to the macroscopic fracture of rock under a stress field [4,5,6]. AE waveform characteristics have been employed for determining the failure mechanisms of intact and jointed rocks [16], identifying the fracture behavior of the concrete [20], exploring the rock AE rate-dependence at different strain rates [21], and extracting the intrinsic dynamical rock-burst precursors [22]
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