## 3 Triaxial Cyclic Loading and Unloading Experiment on High-Strength Sandstone  
Characteristics of Stress-Strain Response of Triaxial Cyclic Loading and Unloading under Different Surrounding Pressures  
Sandstone is a common brittle rock. Its stress-strain relationship has certain response characteristics from the deformation to the destruction of the sample under triaxial cyclic loading and unloading conditions. [PERSON] [23] previously suggested that the volumetric strain of rocks could be used as a research method to analyse the mechanical properties of rocks. The volumetric strain \(\varepsilon_{v}\) of a rock sample can be expressed in terms of axial strain \(\varepsilon_{1}\) and lateral strain \(\varepsilon_{3}\) as shown in Equation (1). \[\varepsilon_{v}=\varepsilon_{1}+2\varepsilon_{3} \tag{1}\]  
According to the experimental results, it can be seen that the difference between the peak values of the stress-strain curves obtained from the three samples under the same stress condition is small, which is less than 6%. High-strength sandstone samples are more homogeneous and less discrete. In order to simplify the analysis while ensuring scientific and reasonable experimental conclusions, one sample was selected from each group of three samples to be analysed. The conventional triaxial compressive stress-strain curves of the samples under different surrounding pressures are shown in Figure 5.  
When the sample is in a certain constant surrounding pressure condition, the stress-strain curve exists in the pore-compacting stage, elastic stage, yield stage, and destructive stage. In the pore-compacting stage, the internal pore of the rock is continuously reduced by external stress, which is manifested by the rapid increase in axial strain and radial strain under low-stress conditions within a short period of time. With the increase in the surrounding pressure, the axial strain in the pore-compacting stage increases. The increase in surrounding pressure in this process has a hindering effect on the deformation of the rock produced by axial pressure. However, hard sandstone is internally homogeneous and has a small pore volume, so the pore-compacting stage of the sample is shorter. The stress-strain curve will rapidly enter the elastic stage. In the elastic stage, the stress and strain of sandstone samples are linearly proportional to each other. As the axial stress on the sample increases during the yield stage when the sample is approaching its peak strength, small cracks grow internally and the strain in the sample increases rapidly. The sample will enter the destructive stage after reaching the peak strength. The sample will produce a large number of cracks and the strain will increase rapidly. Figure 4: Triaxial cyclic loading and unloading stress loading paths. The stress-strain curves of triaxial cyclic loading and unloading under different surrounding pressures are shown in Figure 6. A significant hysteresis effect was observed in the stress-strain curve of the rock samples during cyclic loading experiments, forming a closed stress-strain hysteresis loop between the unloading curve and the loading curve of the next cycle [24; 25]. Defects such as microfractures, pores, and joints contained within natural rock cause non-ideal elasticity in rock deformation. When unloading after loading to a certain stress level, the unloading curve does not match the original loading curve and is lower than the original loading curve. The unloading curve forms a closed annular region with the loading curve of the next cycle. The presence of hysteresis loops in the stress-strain curve also represents energy dissipation. The area of the hysteresis loop can represent the amount of energy dissipated in the loaded rock sample as a result of crack closure, expansion, connection penetration, and destruction (dissipation energy). The larger the area of the hysteresis loop, the more energy is dissipated and the greater the damage to the rock sample.