## 3 Results and Analysis  
### Indentation Force and AE Events in the Indentation Process  
With respect to the dry specimen, the typical penetration forces and AE events are shown in Figure 5 (red represents the first indentation process, green represents the second indentation process). The variation curve of the penetration force with the penetration depth can be divided into four stages, namely A, B, C, and D, as shown in Figure 5 as the areas divided by the red dotted lines: (1) Micro-crack closure stage (Stage A): at this stage, the rock specimen was in compaction under the action of the cutter. The micro-crack was closed under pressure, and the specimen was gradually compacted, forming an early nonlinear deformation. At this time, no brittle fracture failure occurred, so the number of AE events per unit of time was small, and basically no AE events occurred. (2) Elastic deformation stage (Stage B): once a large number of existing micro-fractures and micro-pores were closed, the specimen entered the elastic deformation stage, and the original cracks in the specimen continued to be compacted. At this stage, the penetration force-penetration depth curve was a concave curve, with the occurrence of a few AE events. (3) Crack propagation stage (Stage C): this stage consisted of stable and unstable crack initiation and propagation periods. At this stage, cracks were  
Figure 4: Schematic diagram of indentation test. continuously generated, rapidly propagated, and coalesced. At the same time, the penetration force increased rapidly at this stage, with the further penetration of the cutter. The penetration force \(F_{cd}\), corresponding to the red dot in Figure 5 reached 55 kN, which was the critical penetration force that turned the rock mass from elastic deformation to brittle fracture. At this stage, the acoustic emission energy increased rapidly, indicating that the AE events were mainly caused by crack propagation. (4) The post-failure to the residual strength (Stage D): when the penetration force reached a certain value (i.e., the peak penetration force \(F_{p}\) in purple-red shown in Figure 4 (340 kN)) the penetration force rapidly dropped to the vertical penetration force \(F^{\prime}_{p}\) (in black in Figure 5 (305 kN)), and then, it dropped even more sharply, accompanied by more obvious acoustic emissions. Unlike in the first indentation, the acoustic emission was maintained at a high level in the early stage of the second indentation, because the first indentation already caused some damage to the specimen, and in the early stage of the second indentation, the cutter was likely to cause further damage, resulting in severe acoustic emissions. Therefore, the variation curve of the penetration force with the penetration depth in the second indentation process can be divided into four stages, namely A\({}^{\prime}\), B\({}^{\prime}\), C\({}^{\prime}\), and D\({}^{\prime}\) with the penetration depth curve shown in Figure 5 as the areas divided by the green dotted lines. In Stage A\({}^{\prime}\), as the indentation increased, the penetration force increased slowly. When the penetration depth increased to 0.18 mm, the penetration force increased with the penetration depth. In Stage B\({}^{\prime}\), when the penetration depth increased to 0.38 mm, the penetration force reached \(F_{cd}\), as shown in Figure 5, which was close to 75 kN. In Stage C\({}^{\prime}\), the penetration force increased greatly with the increase of the penetration depth. When the penetration depth increased to about 0.78 mm, the penetration force reached a peak (i.e., \(F_{p}\) in Figure 5), which was 292 kN. During this process, the number of acoustic emissions per unit of time was consistently maintained at a high level, indicating that the crack development in the specimen was maintained at a high level in this stage. In early Stage D\({}^{\prime}\), the penetration force slowly decreased until the penetration depth reached 0.87 mm.