## 5 Conclusions  
In this research, conventional triaxial compression tests and triaxial cyclic loading and unloading tests were carried out using a triaxial rock multi-field coupled test platform for high-strength hard sandstones. In the research programme, multiple stress cycles were set up and repeated loading and unloading was carried out in each stress cycle. The mechanical properties and energy evolution characteristics of high-strength hard sandstone under triaxial cyclic loading and unloading conditions were investigated from the stress-strain response characteristics, strength characteristics, energy characteristics, and macroscopic damage characteristics of high-strength hard sandstone under different surrounding pressures. The main conclusions are as follows:  
1. The stress-strain curve of high-strength hard sandstone under cyclic loading can be divided into the stages of pore compaction, elasticity, yielding, and destruction. As the cycle count increases and the surrounding pressure increases, the sandstone shows significant stiffness degradation. The hysteresis loops in the same level of stress cycling show a significant sparse distribution with the increase in cycle count. The first three to four cycles produce a cumulative cyclic damage displacement that is approximately half of the cumulative damage displacement at that stage. The effect of cyclic loading and unloading under low surrounding pressure conditions on the peak strength of sandstone is more pronounced. The peak volumetric strain of hard sandstone samples under cyclic loading and unloading conditions increases with increasing surrounding pressure. 2. The total energy density, elastic energy density, and dissipated energy density of high-strength sandstone at the same stage of cyclic loading show an overall decrease with the increase in cyclic times. The cyclic load dissipation energy decreases as the cyclic loading times increase. The dissipation energy is larger at the beginning of the cycle (first to fourth) of each cycle stage. This part of the cyclic loading plays a major role in damaging rock. There is a rapid increase in the dissipation energy of hard sandstone samples under cyclic loading with high surrounding pressure and high stress levels. 3. The elastic energy density ratio and dissipation energy density ratio of hard sandstone under cyclic loading tend to fluctuate as a sinusoidal function. As the cycle count increases, the elastic energy density ratio gradually rises to reach the peak value of this cycle stage. The fluctuations of the ratio of elastic energy and dissipation energy decreased gradually with the increase in cyclic stress level and cycle count. The accumulation of micro cracks within the sandstone also reduces the elastic energy percentage. Higher surrounding pressure promotes the recovery of elasticity in hard sandstone samples. It can also lead to a faster stabilisation of the percentage of elastic energy and dissipation energy. Cracks and defects within the rock stabilise after many cycles of loading. This causes the percentage of dissipation energy within the same cyclic phase to decrease with increasing cycle count. 4. Hard sandstones subjected to cyclic loading have more pronounced brittle damage characteristics. The form of damage at low surrounding pressure is mainly shear damage with a small amount of tension cracking. The generation of through cracks when surrounding pressure is high is accompanied by secondary cracks perpendicular to the direction of the maximum principal stress. The sandstone samples are partially damaged badly. Its damage form is also mainly shear damage, tensile damage, and partial compression damage. **Author Contributions:** [PERSON], [PERSON], and [PERSON] conceived and designed the experimental programme; [PERSON], [PERSON], and [PERSON] derived theoretical equations; [PERSON], [PERSON], [PERSON], and [PERSON] conducted laboratory experiments; [PERSON] and [PERSON] collated and analysed experimental data; [PERSON] wrote the paper; [PERSON] helped [PERSON] revise the manuscript.