## 3 Results
### Plastic Deformation Evolution
As depicted in Figure 7, the influence of confining pressure on the axial plastic strain curve of rocks can be summarized as follows: (1) Prior to reaching the peak stress, the axial plastic strain decreases slightly with increasing confining pressure, although the variation between different confining pressures is relatively small. (2) The inflection point between the low-speed increase stage and the high-speed linear increase stage of the plastic strain curve shifts to higher axial strains as the confining pressure increases. (3) Notably, a significant difference in plastic strain is observed among rocks subjected to different confining pressures within the axial strain range of 1% to 2%. Specifically, the axial plastic strain under low confining pressure is noticeably larger than that under high confining pressure. This disparity may arise from the fact that the axial plastic strain curve under low confining pressure enters the high-speed increase stage earlier than its counterpart under high confining pressure. (4) During the high-speed increase stage, the growth rate of the axial plastic strain curve under high confining pressure surpasses that under low confining pressure. This observation suggests that different confining pressure conditions can yield  
Figure 6: Plastic strain curves under different confining pressures ((**a**): 5 MPa, (**b**): 10 MPa, (**c**): 20 MPa, (**d**): 30 MPa). into plastic strain at a fixed transformation ratio, which aligns closely with the slope of the fitting curve. This transformation ratio serves as an indicator of the extent of plastic deformation in the rock, whereby a higher transformation ratio signifies a relatively weaker ability of the rock material to store elastic energy. Remarkably, as the confining pressure increases, the transformation ratio gradually diminishes, implying an augmented ability of the rock material to store elastic energy with rising confining pressures. Combining Figures 8 and 9, it can be observed that prior to the stress peak, dissipated energy and axial plastic strain have very similar evolution trends from both the energy and strain perspectives. Dissipated energy and axial plastic strain both increase linearly with input energy and axial strain, respectively. Prior to the stress peak, the proportion of input energy converted to dissipated energy is also relatively close to the proportion of axial strain converted to plastic strain. Under different confining pressures, the range of input energy converted to dissipated energy is approximately 0.24-0.32, while the proportion of axial strain converted to plastic strain is about 0.23-0.29. Furthermore, the evolution trends of the two quantities under the influence of confining pressure are also exactly the same. Figure 8: Correspondence between dissipated energy and input energy (**(a)**: 5 MPa, **(b)**: 10 MPa, **(c)**: 20 MPa, **(d)**: 30 MPa). According to the experimental data under a 30 MPa confining pressure, a curve is plotted with axial plastic strain as the horizontal axis and dissipated energy as the vertical axis, as shown in Figure 10. From Figure 10, the evolution process of dissipated energy and plastic strain can be divided into four stages. (1) During the compaction stage, plastic strain is generated due to the closure of primary pores and cracks in the rock, accompanied by a small amount of dissipated energy. The dissipated energy-plastic strain curve increases slowly, the slope gradually increases, and the curve is concave upward. (2) From the elastic stage to the stress peak, dissipated energy increases gradually with plastic strain, and the dissipated energy-plastic strain curve increases rapidly, the slope gradually increases, and the curve is concave upward. (3) In the post-peak softening stage, due to the difficulty in unloading the stress curve, there are fewer data points. The dissipated energy-plastic strain curve shows a generally increasing trend, with a reduced growth rate.