## 3 Results and Analysis
### Indentation Force and AE Events in the Indentation Process
After that, with the further increase of the penetration depth, the penetration force dropped sharply until the specimen failed. During the decrease of the penetration force, the acoustic emission phenomenon still remained at a high level. Figure 6 summarizes the set of tests in various conditions. This phenomenon was most obvious at the time when the penetration force reached the peak and was consistent with the penetration force-penetration depth curve. Unlike in the first indentation, the acoustic emission phenomenon  
Figure 5: Typical penetration forces and AE events (red represents the first indentation process, green represents the second indentation process). was maintained at a high level even in the early stage of the second indentation, mainly because there was already some damage inside the specimen after the first indentation, and on this basis, the cutter was extremely likely to cause further damage during the second indentation. This conclusion is basically consistent with the results obtained by other researchers [29]. However, there are still many differences between the test results of the granite specimens with different water contents and those of the specimens in the dry condition. At the micro-crack closure and elastic deformation stages in the first indentation process, granite specimens with different water contents showed almost no acoustic emission, and those in the dry state showed only a small number of acoustic emissions and basically no change. In the crack propagation stage, the penetration force increased approximately linearly with the penetration depth. This stage was accompanied by leaps in the number of acoustic emissions, indicating that the internal crack developed significantly at this stage. However, due to the water contained in the specimens with different water contents, the rocks had lower strengths. The number of AE events was concentrated before and after the failure, and the moisture-bearing specimens saw fewer AE events than those in the dry condition. In the second indentation process, the number of AE events in the dry state was maintained at a relatively high level before the elastic deformation stage, but with the increase of the water content, the number of AE events decreased. During the crack propagation stage, the acoustic emission of different water content specimens was maintained at a high level. At the post-failure stage, the average number of AE events in the moisture-bearing specimens was smaller than that in the dry specimens. The main reason was that the moisture-bearing specimens were softened after being exposed to water, resulting in smaller deformation and failure. This is consistent with the conclusion that water can reduce rock strength. During the rock saturation process, the internal, mineral particles expand in volume, making the cementation looser, which softens the rock and reduces its strength. The energy release rate reflects the energy release of AE per unit time. However, it is difficult to reflect the cumulative effect of cutter intrusion on rock. In order to study the cumulative effect of cutter penetration on rock fracture, the AE parameter of cumulative energy was analyzed. Due to space limitations, only dry specimens were selected for AE energy analysis. The typical relationship between the AE cumulative energy and the penetration depth is shown in Figure 7. Cumulative energy is the accumulation of energy in the process of cutter penetration, which indicates the total strength of AE energy. As shown in Figure 7, the cumulative energy increases little at the initial stage, this shows that the specimen has a small amount of damage, and the damage to energy is very low at the beginning of the load. With the increase of load increasing, AE cumulative energy increases sharply after a large number of cracks emanated and propagated, this phenomenon can be used as the precursor information of rock specimen damage and imminent failure. It also can be observed that, a lot of energy was generated at the second indentation process.