Mechanical and Energy Evolution Characteristics of Sandstone under True Triaxial Cyclic Loading

To study the mechanical and energy evolution characteristics of sandstone under true triaxial cyclic loading, a sandstone mechanical test with different intermediate principal stress under true triaxial loading was conducted using the rock true triaxial disturbance unloading test system. The influence of axial load on the deformation, energy evolution, and macroscopic failure characteristics of sandstone under different intermediate principal stress in a true triaxial test was systematically analyzed, and the damage evolution law of sandstone under true triaxial cyclic load was revealed. Results showed that the failure mode of sandstone under true triaxial compression changed from tension–shear composite failure to tension failure. Grading cyclic load ${\sigma }_1$ greatly influenced maximum principal strain ${\epsilon }_1$ and minimum principal strain ${\epsilon }_3$ but had little influence on intermediate principal strain ${\epsilon }_2$. Under the same ${\sigma }_2$ condition, the input energy and elastic energy in ${\sigma }_1$, ${\sigma }_2$, and ${\sigma }_3$ directions increased nonlinearly. Under different ${\sigma }_2$ conditions, the dissipated energy in ${\sigma }_1$, ${\sigma }_2$, and ${\sigma }_3$ directions decreased with the increase in ${\sigma }_2$. With the increase in ${\sigma }_2$, graded cycles ${\sigma }_1$, ${\epsilon }_2$, and ${\epsilon }_3$ decreased considerably, and the failure mode changed from tensile failure to shear failure. When the cyclic loading rate increased, the ${\sigma }_1$, ${\epsilon }_1$, ${\epsilon }_2$, ${\epsilon }_3$, and volume strain ${\epsilon }_v$ of sandstone failure decreased, but the expansion point increased. Under true triaxial grading cyclic loading and unloading, the total dissipated energy of sandstone increased exponentially. The larger ${\sigma }_2$ was, the smaller the damage variable was.