## 3 Triaxial Cyclic Loading and Unloading Experiment on High-Strength Sandstone
### Patterns of Energy Distribution in Sandstone  
The energy absorbed by a loaded sandstone sample from the exterior is mainly converted into elastic and dissipation energy. The ratios of elastic energy and dissipation energy may affect the damage and destruction characteristics of sandstone samples. According to Figure 13, the overall fluctuation trend of the ratio of elastic energy and the ratio of dissipation energy with the increase in cyclic loading and unloading times are both close to a sinusoidal function. In Figure 13a, the elastic energy percentage increases and peaks as the cycle count increases in the same level of cyclic loading. From the different cyclic loading stages, the elastic energy percentage in the first cyclic loading stage reaches the highest. Afterwards, the peak value of the elastic energy percentage decreases as the number and level of loading cycles increase. The fluctuation of the elastic energy percentage of the samples gradually decreases. This also indicates that a decrease in the modulus of elasticity of the sandstone samples may have occurred with the increase in the cycle count of loading. The accumulation of micro cracks may have occurred within the rock samples.  
Figure 12: Total energy density, elastic energy density, and dissipation energy density curves of cyclic loading and unloading samples under different surrounding pressures. (**a**) Total energy density; (**b**) elastic energy density; (**c**) dissipative energy density.  
The elastic energy percentage will show an overall weakly increasing trend with increasing surrounding pressure. As an example, the average value of the elastic energy percentage at a surrounding pressure of 20 MPa is approximately 1.5% higher than that at a surrounding pressure of 0 MPa. This indicates that higher surrounding pressure can promote the elastic recovery of sandstone samples. It is possible that the effect of surrounding pressure reduces the expansion and slip of micro cracks within the rock. The trend of dissipation energy percentage changes in sandstone samples at different surrounding pressures in Figure 13b is similar to the trend of elastic energy percentage changes. The dissipation energy percentage of sandstone samples showed a fluctuating decrease with the increase in cycle count. The percentage of dissipation energy decreases as the cycle count increases for sandstone samples under the same cyclic loading. This indicates that cracks and defects within the rock stabilise after many cycles of loading, resulting in less energy being dissipated for each cycle thereafter. Regarding the effect of different surrounding pressures on the percentage of dissipation energy of hard sandstone samples, it can be found that at lower surrounding pressures (0 MPa and 5 MPa), the percentage of dissipation energy is relatively high. This occurs due to the greater susceptibility of sandstone samples to plastic deformation and internal damage at low surrounding pressures. As the surrounding pressure increased to 15 MPa or 20 MPa, the proportion of dissipated energy decreased. It may be due to the high surrounding pressure, which enhances the bearing capacity of sandstone samples and inhibits crack extension.