Based on three‐dimensional digital image correlation (3D‐DIC) technique, the deformation behavior, damage and fracture characteristics of the granite specimens have been tested and observed by the short core in compression (SCC) method. The test results show that (1) the damage degree of rock in the region of interest (ROI) increases with axial stress. Before peak stress, damage factor based on apparent principal strain increases slowly, then rises rapidly; (2) relative displacements in both horizontal and vertical directions are observed, indicating that the fracture mode for SCC specimens subjected to uniaxial compression is the tensile‐shear mixed mode fracture; (3) new fracture initiates from the middle part of the expected shear fracture band (ESFB) then propagates along the loading direction. A dominant macro‐fracture, linking up the tips of two notches, eventually forms. Meanwhile, a calculation method based on the energy conservation law for the fracture energy of SCC specimens is proposed. The fracture energy of granite is estimated as 1760.4 J/m2.
Rock masses around fluid injection projects are usually subject to complex stress states, including hydraulic pressure, in situ stresses, and fatigue loads.Thus, a series of hydraulic fracturing experiments were performed on granite to simulate such stress states using the true triaxial dynamic testing system.Computed tomographic (CT) imaging was performed to identify the fatigue effects on hydraulic fractures. The results indicate that the increasing amplitudes of cyclic load applied on the minimum principal stress direction will change fracture initiation pressure and generate nonplanar and narrow fractures. When the disturbance direction was changed to the intermediate principal stress, the higher amplitude corresponds to the lower breakdown pressure and the shorter pressurize duration and leads to wider fractures. With the increasing disturbance frequency, complex fatigue cracks were generated, which might weaken the rock strength. The present experiments can enhance the understanding of the hazard of fatigue loads on hydraulic fracturing.
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