## 3 Experimental Results  
### AE Timing Characteristics of the Grading Loading Process  
During the micro-fracture process of granite loading, it will be accompanied by AE signals. Various changes will occur due to the unlike internal cracks under different stress levels of grading loading and each fracture corresponds to different levels of AE signals. From the test results of several rock samples, due to the small size of the sample, the AE signals received by the AE sensors at different positions show similar regularity in the timing characteristics, so the intact rock sample was analyzed in accordance with the 5-channel AE sensor (the hole rock sample uses the same channel of AE information). Figure 3 is a time-series response characteristic of the 5-channel AE sensor receiving AE signals of the two types of rock samples in the compact and elastic and failure stages under the staged loading. The AE count in the figure mainly reflects the frequency of micro-fracture of the sample under load deformation, and the AE energy represents the energy released during the fracture of the sample. It can be seen from Figure 3a that in the initial stage (0-9 MPa) of the staged loading of the intact sample, the AE count and the energy value are small due to the higher strength of the granite sample and the lower internal crack closure. As the grading loading progresses, the internal compactness of the sample increases, and the original fissures become fewer and fewer. At the same time, the stress level is insufficient to generate new fissures, so the AE count and energy show a slight downward trend. As the stress continues to increase, when the stress is in the rising phase of 9-12 MPa, the AE  
Figure 3: Sequential characteristics of granite AE parameter under grading loading. (**a**) intact rock sample; (**b**) holey rock sample. signal has a high value response and then the sample enters into the elastic phase (12-32 MPa). At this time, micro-cracks begin to be generated inside the sample, and the generation of new fissures is relatively random but the number is relatively stability, while the cumulative energy of AE is also in a relatively stable phase. When the stress is in the rising phase of 30-33 MPa, the AE signal jumps again, for the sample from the elastic phase into the plastic phase (33-45 MPa), and begins to be irreversible deformation. Meanwhile, the micro-crack gradually expands and forms a small regional fracture and nucleation, which leads the AE count and energy all show a significant upward trend and significantly exceeded the first two stages. The cracks also expand and close with the increase of time in the force retention phase, the AE count and the cumulative energy rise faster, and the rheological characteristics of the sample become more and more obvious due to loading damage factors. When the sample reaches the damage fracture limit stage (42-45 MPa), the internal crack of the sample penetrates a lot and forms the main macro-crack, the AE count increases rapidly to the maximum value. When the stress level reaches retention phase (46 MPa), the sample completely cracks and destabilizes, the AE counts and energy increase rapidly and reach a maximum. Compared with the intact granite sample, the AE counts and the energy value of the holey granite sample are lower under the same axial loading, but the general rule still exists. At the initial stage of loading (0-6 MPa), due to the action of the hole, the original crack of the sample is closed less, the regional response of the AE is high, and the energy level is low. The hole in the center of the sample generates to some extent the pressure relief by the axial load and it makes the compaction phase of the sample take a long time. When the stress is in the rising stage of 12-15 MPa, the AE count suddenly increases, the cumulative energy is sensitive, the new crack begins inside the sample, and the sample enters the elastic stage. In the early stage of the sample, due to the action of the hole there are many strains in the compaction stage.