## 3 Experimental Result Analysis
### Energy Characteristics
#### 3.3.2 Energy Conversion Characteristics  
Figure 3 shows the relationship between the strain energy and strain. At the initial stage of loading, the primary fractures inside the rock sample undergo progressive closure under the influence of axial stress. During this period, the dissipated strain energy gradually increases with the increase in the strain. The total strain energy input from the outside is mainly converted into dissipated strain energy, and the energy dissipation is mainly manifested as frictional sliding at the fracture surface and the fragmentation of mineral particles. After entering the middle stage of loading, there are still residual fractures that have not been completely closed inside the rock sample. Under the action of stress, these fissures continuously generate frictional effects and lead to the dissipation strain energy maintaining an increasing trend. However, with the improvement of the bearing capacity of rock samples, the energy conversion mechanism has changed. The total strain energy input from the outside is mainly converted into elastic strain energy and is mainly used for the elastic deformation of rock samples. The change of this energy conversion characteristic marks that the rock sample begins to have a mechanical response dominated by elastic deformation, and its conversion mechanism can be explained as a significant enhancement in the ability of mineral crystals to store elastic strain energy. As the strain increases to the later stage of loading, the secondary cracks inside the rock sample accelerate their bifurcation and expansion, resulting in a further increase in the dissipated strain energy.  
Figure 2: Failure characteristics: (**a**) 1 mm/min; (**b**) 3 mm/min; (**c**) 5 mm/min; (**d**) 7 mm/min.  
However, the total strain energy input from the outside is still mainly converted into elastic strain energy. When the strain reaches the peak strain, the elastic strain can be released sharply, resulting in macroscopic brittle failure of the rock sample. This indicates that the elastic strain energy stored in the mineral crystals of the rock sample forms a high stress concentration at the tip of the secondary crack. When the secondary crack propagation reaches the critical length, the stored elastic strain energy is dissipated in the forms of accelerated propagation of the secondary crack, clastic kinetic energy, and acoustic emission.  
Figure 4 shows the relationship between the strain energy and the loading rate. With the increase in the strain rate, the total strain energy, elastic strain energy, and dissipated strain energy all show a monotonically increasing trend. Among them, the growth ratio of the dissipated strain energy is the largest, followed by the total strain energy, and the elastic strain energy is the smallest, as shown in Figure 4. The main reasons for this are as follows. The high-rate loading inhibits the stress relaxation process at the tip of the secondary crack, prompts the bifurcated propagation mode of the secondary crack to shift from quasi-static to dynamic instability, accelerates the bifurcated propagation of the secondary crack, and significantly increases the energy required for the propagation of the secondary crack. Furthermore, the high-speed friction effect intensifies the thermodynamic coupling effect of the existing fracture surface, resulting in an increase in the energy dissipation efficiency of frictional slip.  
Figure 3: Relationship between strain energy and strain: (**a**) 1 mm/min; (**b**) 3 mm/min; (**c**) 5 mm/min; (**d**) 7 mm/min.