Experimental Study on Mesoscopic Failure and Acoustic Emission Characteristics of Sandstone under Various Joint Angles

ObjectiveThis study investigates the shear mechanical behavior and acoustic emission characteristics of jointed sandstone in detail, with a particular focus on analyzing the mechanism of shear fracture from both macroscopic and microscopic perspectives. Shear failure is the primary mode of failure in underground engineering, such as landslides and pillar instability. Therefore, this study begins by examining the shear fracture of jointed sandstone specimens, obtaining their mechanical behaviors, deformation properties, and acoustic emission signals, and proposes a method for evaluating the fracture mode of rock. An in-depth analysis of rock shear fracture modes not only plays a crucial role in the stability of surrounding rock in engineering but also contributes to the advancement of deep rock mechanics. The ultimate goal is to provide practical support and effective theoretical guidance for addressing the deformation and fracture of surrounding rock encountered during the design and construction of underground engineering projects.MethodsTo conduct the meso-shear test of jointed rock, a self-developed rock meso-shear test device was constructed. This device primarily consists of a loading system and a crack meso-observation system. Additionally, an acoustic emission monitoring system is used to capture the characteristics of acoustic emission events during the shear fracture of sandstone specimens. The sandstone cube specimens used in this study are all processed from the same intact laminated sandstone. Successive specimens were selected for testing to minimize differences between the specimens. Joint angles (0°, 30°, 60°, and 90°) and specimen dimensions (40 mm × 40 mm × 40 mm) were chosen according to the requirements of the test. Meanwhile, the tests select two conditions: no normal stress and 3 MPa normal stress, to explore the influence of normal stress on rock fracture behavior. During the specimen shear process, microscopic observation was used to monitor the development of surface cracks. Acoustic emission monitoring technology also recorded the propagation of internal cracks and the real-time number of acoustic emission events. These valuable data were used to investigate the effects of different conditions on rock fracture patterns during shear failure from multiple perspectives.Results and discussions The shear stress-displacement curves of sandstone specimens with different joint angles can be divided into five stages based on their mechanical properties and acoustic emission characteristics. High-amplitude acoustic emission events primarily occur near failure, with frequencies concentrated in the range of 50-250 kHz. The morphology of the main crack and the presence of wing cracks are the primary differences in the failure modes of specimens with various joint angles. When there is a certain inclination between the shear force and the joint angle, a stress drop occurs in the shear stress before the peak. This phenomenon is caused by the joint angle, which leads to the premature release of energy accumulated prior to peak stress. The application of normal stress causes the main crack morphology to become vertical, increasing the occurrence of tensile failure. It also causes the main fracture on the surface of sandstone to align more closely with the direction of the applied shear force during shear fracture. The joint angle leads to the formation of wing cracks and secondary cracks near the main crack, expanding the damage zone. The overall fracture mode of sandstone is significantly influenced by joint angle conditions, primarily characterized by wing and secondary cracks resulting from the expansion of tensile cracks. A method was used to categorize the distribution modes of tensile and shear failure. The number of tensile failures observed in the shear test was significantly higher than that of shear failures. As the joint angle increases, the RA value range for tensile failure and the AF value for shear failure gradually expand. When the joint angle is inclined in the direction of shear force, the formation of tensile cracks is promoted, while a vertical joint angle inhibits their formation. Under the influence of normal stress, regardless of the joint angle, although the number of tensile cracks exceeds the number of shear cracks, most of the energy stored in the rock is used for shear fracture generation, making shear failure the main failure mode of sandstone. However, under the application of normal stress, the energy associated with tensile failure gradually decreases as the joint angle increases. When the joint angle is 0° or 30°, tensile failure is the dominant failure mode. When the joint angle exceeds 60°, the energy for shear failure becomes greater than that for tensile failure, and shear failure again becomes the primary failure mode. The presence of normal stress causes the specimen to shift from shear failure to tensile failure. This inhibits the crack development in the direction of the joint angle and introduces friction from the sliding of mineral particles on the fracture surface, ultimately leading to tensile failure.ConclusionsIn summary, the characteristics of sandstone meso-shear failure and acoustic emission are thoroughly studied through rock meso-shear tests and acoustic emission monitoring equipment. The results show that the energy distribution between tensile and shear failures varies with the joint angle and normal stress conditions. Specifically, when the joint angle aligns with the shear direction, the number of tensile fractures becomes more pronounced, but the rock still undergoes shear fractures. These findings offer valuable insights into the fracture mechanisms of jointed rocks, which are relevant to a wide range of underground engineering projects where understanding rock behavior under stress is crucial. By analyzing the shear fracture behavior of jointed rock under different joint angles and normal stresses, the significant influence of these factors on the fracture mode of the rock is revealed. As the joint angle changes, the shear fracture mode transitions from shear failure to tensile failure. This provides a theoretical basis for rock stability analysis in underground engineering design. In the future, we will continue to deepen our research into the mechanism of rock shear degradation, explore additional factors that affect rock fracture mechanics and micro-failure mechanisms, and provide further theoretical support for the safe construction of underground engineering and the efficient extraction of deep resources.