## 3 Test Results and Analysis
### Triaxial Cyclic Test Results of Sandstone  
During the process of CLU on rock samples, energy is dissipated through the expansion and progression of existing fractures, as well as through the formation and merging of new fissures within the specimens. The main aim of this research is to explore the variations in energy density and the laws governing energy distribution during the deformation and failure stages of sandstone under triaxial CLU conditions. Through examining the evolution of energy throughout this process, the study delves into the underlying mechanisms of deformation and failure in sandstone. For each level of confining pressure, three sandstone samples were chosen to undergo triaxial cyclic loading and unloading experiments. Due to space constraints, only a single set of experimental data is presented for each confining pressure. Figure 5 depicts the stress-strain curves of sandstone during triaxial CLU experiments conducted under different confining pressures. The analysis of Figure 5 indicates the following insights: (1) in the triaxial CLU experiments conducted on sandstone, the observed stress-strain curves generate closed loops, also known as hysteresis loops. These loops, which result from an unloading curve in each cycle paired with a reloading curve in the subsequent cycle, manifest due to the inherent defects, like pre-existing fractures, within the natural rock, exhibiting non-linear elastic behavior. This effect is seen when the rock is loaded to a specific point and then unloaded, where the unloading path does not align with the initial loading path and is situated lower, thereby creating a closed loop with the next reloading path; (2) observations on the quantity of loading and unloading cycles alongside the area encompassed by the hysteresis loops reveal that with an increase in cycle numbers, the loops move rightward and their magnitude expands, leading to denser loops and an enlarged loop area. The maximum area enclosed by these loops corresponds to the peak stress point, indicating a direct relationship between stress and the loop area; (3) the external shapes of the stress-strain curves during triaxial cyclic loading and unloading bear resemblance to those seen in conventional triaxial compression tests. However, with increasing load during CLU, the curves start to diverge from the conventional test profiles. The presence of confining pressure accentuates the weakening stage before the peak stress, suggesting an enhancement in the specimens' toughness under such pressure; (4) analogous to conventional triaxial tests, the triaxial CLU tests show that under elevated hydrostatic pressure, the rock specimens' pores begin to close before the application of axial deviatoric stress, especially as confining pressure increases. Consequently, the compaction phase is less pronounced, and the axial peak strength sees an uplift. After failure, the internal residual stress levels within the rock samples increase, implying an increase in the specimens' load-bearing capacity with heightened confining pressure.  
Figure 5: Stress versus strain profiles for sandstone during triaxial CLU experiments.