## 3 Triaxial Cyclic Loading and Unloading Experiment on High-Strength Sandstone
### Damage Characteristics of Hard Sandstone under Triaxial Cyclic Loading and Unloading  
The macroscopic damage characteristics of rock depend on the distribution of micro cracks and the expression of detailed mechanical properties [26]. The main forms of macroscopic damage in loaded rock are shear damage, tensile damage, compression damage, and laminar spalling. The macroscopic crack morphology and crack distribution of hard sandstone samples after conventional triaxial compression and triaxial cyclic loading experiments can be observed to obtain their macroscopic damage characteristics and patterns. The damage characteristics of the loaded sandstone samples are shown in Figure 14. The red part indicates the main macroscopic cracks that lead to the strength loss and destabilisation of the sample, while the green part indicates the micro cracks or tension cracks due to the loading of the rock. Figure 13: Energy percentage of samples under triaxial cyclic loading. (a) Percentage of elastic energy; (b) percentage of dissipated energy. As shown in the figure, conventional triaxial compression conditions, sandstone samples in the surrounding pressure of 0 MPa (without the constraints of the surrounding pressure), and the load damage after the rock of the obvious macro cracks are more developed. The form of damage is mainly dominated by shear damage and the samples are more broken locally. As the surrounding pressure increases, the main cracks that cause the sample to lose strength become more and more obvious. The form of damage is dominated by shear damage. Shear misalignment of a sample along one or more planes inclined in the direction of the maximum principal stress is observed. These planes are usually located at the interface of the weaker mineral grains in sandstone samples. Smooth friction surfaces and shear marks can be observed on the damaged surface. Under triaxial cyclic loading and unloading conditions, the damage characteristics of the samples are more obviously different from those of conventional triaxial compression due to repeated loading and unloading. It is also closer to the damage form under actual natural conditions. Under cyclic loading, the sandstone samples were always subjected to alternating compression-expansion cycles. This also promotes the alteration of primary cracks within the sample and the generation of new cracks, which in turn leads to an increase in the level of rock damage. As the loading and unloading frequency increases, the internal particles of the sample may be damaged and collapse. Deformation damage of rock is the whole process of energy dissipation and energy release. Destabilisation damage of rock is the result of an instantaneous release of energy in the rock [27]. When there is no constraint of surrounding pressure (the surrounding pressure is 0 MPa), the macroscopic cracks distributed throughout the sandstone samples are more developed. The form of damage is mainly shear damage and damage through the sample, accompanied by tensile damage secondary cracks approximately perpendicular to the direction of the maximum principal stress. At lower surrounding pressures (5 MPa and 10 MPa) at which the samples were exposed, shear damage penetrated the samples. Some of the samples showed crack extension and connection. Micro cracks within the sandstone samples also show stress concentration and produce fewer tension cracks as the cracks slip and expand. When the surrounding pressure is higher (15 MPa and 20 MPa), the sandstone  
Figure 14: Damage characteristics of loaded sandstone samples. samples will also develop more secondary cracks perpendicular to the penetrating cracks. The sample is more broken locally. The form of damage mainly consists of shear damage and tensile damage inclined to the direction of the principal stress. There is also localised compression damage manifested under high surrounding pressure due to the contraction of the internal pore structure of the sandstone sample under high stress. The damage form of the sample is more complex.