Strength, deformation, and failure characteristics of hollow cylinder sandstone under axial–torsional tests

Due to the excavation disturbance in deep tunnel, the stress magnitude and orientation changed obviously, which affects the mechanical properties of surrounding rock and the stability of tunnel. The axial–torsional test is adopted to study the influence of stress orientation on the strength and deformation behavior of four sandstones, using the self-developed hollow cylinder torsional apparatus for rock. The results show that the peak shear strength of sandstone is nonlinear, positively correlated with axial stress, and negatively correlated with the rotation angle of the principal stress axis. The stress–strain curve obtained under the rotation of principal stress axis can be divided into compaction, elasticity, yield, and softening stages. Furthermore, the sensitivity of the crack damage threshold of various sandstone is analyzed. The results show that the rotation of the principal stress axis can aggravate rock damage, and the greater the initial damage degree, the stronger the impact. Finally, the evolution mechanism of the internal rock cracks under the axial–torsional test is discussed by analyzing the rock failure characteristics, and it can be seen that the initiation, propagation, and coalescence of cracks are not only affected by stress magnitude, but also the stress orientation. Moreover, the crack propagation model considering principal stress axis rotation is verified and improved experimentally. The results of this research are of great significance to studying the influence of stress orientation and provide an important method to investigate the mechanical properties of rock with complex stress states comprehensively.