Experimental investigation on crack propagation law of sandstone containing a single fissure under tensile⁃shear stress

The rock mass of underground engineering such as the roof of underground mined⁃out area is often faced with tensile⁃shear failure disaster. However，the failure mechanism of fractured rock mass under tensile⁃shear stress state is currently not well understood. To investigate the tensile⁃shear failure law of fractured rock mass，a self⁃devel⁃ oped tension⁃shear auxiliary device was used to carry out the direct shear tests of sandstone specimens containing a single fissure under normal tensile stress conditions. The results show that the angle between the fissure and the horizontal shear direction (anti⁃clockwise direction，“fissure angle” for short) has a great influence on the tensile⁃shear strength and crack propagation form. As the fissure angle varies from 0° to 180°， the strength increases first and then decreases，and the maximum appears in the range of fissure angle from 90° to 120°. When the fissure angle is an acute angle，it is easy to develop secondary cracks from the edge of the specimen to form a nucleation failure with the primary crack developed at the end of the fissure. When the angle is obtuse，cracks only develop from the end of the fissure，and the crack path is relatively horizontal. As the fissure angle varies from 0° to 180°，the cracking di⁃ rection angle first decreases and then increases and the minimum cracking direction angle is reached when the fissure angle is 30°. With the increase of the normal tensile stress，the cracks are more horizontal，and the cracking direction angle generally increases. The specimen failure is mainly the tensile failure by the wing cracks. When the fissure is steep，the crack can start and expand from the inside of the fissure. The difference between the tension⁃shear and com⁃ pression⁃shear failure is obvious. In the case of compression⁃shear，the specimen failure is mainly shear failure or ten⁃ sion⁃shear mixed failure. Anti⁃wing cracks can appear when the fissure angle is obtuse. Through DIC (digital im⁃ age correlation) observation，the strain evolution and displacement field characteristics were analyzed. With the appli⁃ cation of shear stress， the strain⁃concentrated area at the prefabricated fissure gradually extends from the middle to the two ends of the fissure，and the crack starts to expand along the inner edge of the stress⁃concentrated are⁃ a. The stress state of the prefabricated fissure has a great relationship with the fissure angle. When the fissure angle is below 90°，as it increases，the stress state of prefabricated fissure changes from tension to tension⁃shear and then to shear. When the fissure angle is above 90°，the stress state is tension⁃shear. However，the mechanical properties of propagating cracks under different fissure angles are mainly tensile. The study reveals the unique law of crack propaga⁃ tion of fractured rock mass under tensile⁃shear stress state，which can provide a theoretical reference for the stability evaluation of underground rock mass excavation.