## 3 Results
### Plastic Deformation Evolution  
The evolution of plastic strain accompanies the entire process of rock damage and failure. Contrary to previous empirical views, it is now recognized that rocks undergo irreversible deformation even in the compaction and elastic stages. Even at 20% unloading of the axial peak strain, rocks will undergo small irreversible plastic deformation. As the stress level increases, the plastic strain of rocks gradually increases irreversibly. Before reaching the stress peak, the plastic strain of rocks increases almost uniformly. In the plastic stage, the plastic strain begins to accelerate. After entering the softening stage, the plastic strain of rocks increases rapidly. In the residual stress stage, the residual elastic strain of rocks no longer changes significantly, and the increase in strain is almost entirely transformed into plastic strain. To further analyze the variation pattern of plastic strain during rock loading, the plastic strain corresponding to each unloading point is plotted, yielding the axial plastic strain curve depicted in Figure 6.  
From Figure 6, the evolution curve of axial plastic strain in rocks can be divided into four distinct stages. (1) Compaction stage: corresponding to the initial compaction stage of the complete stress-strain curve, the curve of axial plastic strain in rocks exhibits a convex trend during the early loading phase. This indicates that the plastic strain of the rock increases relatively rapidly at the onset of loading. As the rock undergoes compaction, the rate of axial plastic strain gradually decreases. (2) Low-speed increase stage: prior to reaching the peak stress during rock loading, the axial plastic strain shows a linear increasing trend with the axial strain. However, it is evident from Figure 5 that the rate of plastic strain increase is notably slow. This suggests that before reaching the peak stress, the majority of axial deformation in the rock is transformed into elastic deformation, while the plastic strain demonstrates a gradual growth pattern. This is primarily due to the absence of interconnected cracks inside the rock before reaching the peak stress, where the strain is predominantly governed by the overall elastic deformation of the rock. (3) High-speed increase stage: upon entering the stress-softening stage, the rate of plastic strain increase abruptly escalates, and the curve of axial plastic strain in rocks displays an upward-opening blunt V-shape. In the post-peak softening stage, the cracks inside the rock rapidly propagate and form shear slip surfaces. Consequently, the axial deformation of the rock rapidly converts into plastic deformation through shear sliding, retaining only a small residual elastic strain. Hence, the rate of axial plastic strain increase reaches its maximum value during the post-peak stress-softening stage, which is referred to as the high-speed increase stage. (4) Residual stress stage: in the residual stress stage, the elastic strain within the rock remains nearly unchanged, and the axial strain is primarily transformed into shear sliding along the rock's shear slip surface. As a result, the axial plastic strain of the rock exhibits a linear increase with the axial strain. As previously mentioned, the evolution of axial plastic strain in rocks can be classified into four distinct stages: compaction, low-speed increase, high-speed increase, and residual stress. The overall axial plastic strain curve of rocks exhibits an upward-opening blunt V-shape. Furthermore, to investigate the impact of confining pressure on the variation of axial plastic strain in rocks, the axial plastic strain curves under different confining pressures were plotted in Figure 7. Each color in the figure represents the plastic strain curves of different rock samples subjected to the same confining pressure.