## 5 Acoustic Emission Characteristics  
The uniaxial reload process of the damaged sandstone was monitored using an acoustic emission system. Figure 8 shows the relationship curves of AE energy and ac-cumulated energy with time. From Figure 8a, it can be seen that the intact sandstone does not produce damage in the pore compacting stage or the linear elastic stage, and the acoustic emission energy changes very slightly, fluctuating around the zero value. In the fracture initiation and stable expansion stages, the load exceeds the initiation strength, the internal fractures of the rock start to fracture and expand, and the acoustic emission energy increases by jumps. In the unstable expansion stage of the fractures, the load exceeds the damage strength, and the existing fractures expand rapidly. In the unstable extension stage, the load exceeds the damage strength, and the existing fractures expand rapidly and penetrate, developing into a fracture network and leading to the formation of macroscopic rupture, while the acoustic emission energy increases intensively and stably. From Figure 8b-f, it can be seen that the cumulative energy characteristics of the 40-60% damaged sandstone at the unloading point are similar to those of the intact sandstone and can be roughly divided into three stages (i.e., smooth stage, stable growth stage, and secondary smooth stage), indicating that the 40-60% damaged sandstone at the unloading point has the same uniaxial reload process as the intact sandstone, and the rock undergoes the pore compacting stage, the linear elastic stage, the crack stable extension stage, and the crack unstable extension stage. From Figure 8g,h, it can be observed that the characteristics of acoustic emission energy at 65% and 70% of the unloading point are similar, and there is almost no acoustic emission energy generation before the axial load reaches the crack initiation strength. Combined with Figures 4 and 5, we can see that, compared with 40-60% of the unloading point, the crack initiation strength of the 65% and 70% damaged sandstone at the unloading point has increased, while the damage strength decreases; therefore, the time to produce acoustic emission energy characteristics is significantly shorter and mainly concentrated in the crack extension stage before the macroscopic damage to the rock occurs. From Figure 8i, it can be noted that the acoustic emission energy characteristics of the 75% damaged sandstone at the unloading point show the phenomenon of \"double peaks\"; combined with Figure 4, it can be seen that the crack initiation strength of the 75% damaged sandstone at the unloading point is similar to the damage strength, which means that the first peak point is when the load exceeds the crack initiation strength, and the second peak point is when the load exceeds the crack initiation strength and the fracture inside the rock starts  
Figure 7: Volume strain characteristic curves of damaged sandstone cracks.  
to crack. As mentioned in Section 4.3, the 75% damaged sandstone at the uniaxial reload process, after briefly experiencing the pore compacting stage, the linear elastic stage, and the crack stable expansion stage, enters the crack unstable expansion stage more rapidly, leading to the generation of the \"double peak\" phenomenon of acoustic emission energy characteristics. From Figure 8j-l, it can be seen that at 80% to 90% of the unloading point, the rock sample has macroscopic cracks that are not penetrated, and the crack initiation strength and damage strength coincide completely when the uniaxial reloading is carried out. Once the load is applied, it enters the crack unstable extension stage, and the acoustic emission energy characteristics are very active, with the cumulative energy characteristics showing an obvious \"step-type\" growth.