Mechanical properties of sandstone and prediction model of shear deformation band under true triaxial unloading conditions

The intermediate principal stress and unloading have an important influence on the shear failure characteristics of rock. Based on the multifunctional true triaxial fluid-structure interaction test system, the loading and unloading failure tests of sandstone under different intermediate principal stress conditions were carried out. The deformation characteristics and strength characteristics were analyzed, and the influence of the two on the angle of shear deformation zone was studied by establishing the constitutive relationship and the shear deformation zone angle prediction model. The test results show that with the increase of intermediate principal stress, the deflected shear stress at the peak of the rock sample increases, and the increase under unloading conditions is small. The maximum compression point of the rock sample shows a trend of increasing first and then stabilizing, and after reaching the maximum compression point, the volume strain of the rock sample gradually changes from expansion to compression under loading conditions, while the expansion trend occurs under unloading conditions. With the increase of intermediate principal stress, the deflection shear strain of the rock sample increases sharply, and the steep increase under unloading conditions occurs early. At the peak, the stress and strain Lode angles under the unloading condition are larger than those under the loading condition, and the difference between the two types of Lode angles under the two conditions increases with the increase of the intermediate principal stress. The intermediate principal stress and unloading can reflect the yield characteristics of the rock through the stress Lode angle, and reflect the internal fracture evolution direction of the rock through the strain Lode angle. The equivalent plastic strain was used to characterize the internal friction angle and dilatancy angle, and to establish a hardening intrinsic relationship consisting of three stress invariants. Comparing the experimental values, it was found that the model predicted well. With the increase of the intermediate principal stress, the angle of the deformation zone first decreases and then stabilizes. The unloading effect will reduce the angle of the deformation zone, and the angle decrease level is reduced under the influence of the intermediate principal stress.