## 3 Test Results
### Effect of Water on AE Characteristics  
#### 3.2.1 Cumulative and Evolution of Ring Counts  
Ring counts and their cumulative value are the main indexes to characterize the damage evolution within rock materials [50]. Figure 5 demonstrates the variations of axial stress, ring counts and cumulative ring counts versus time. Obviously, the history of ring counts for all specimens showed similar trends regardless of rock type and moisture condition. According to the occurrence of ring counts and the slope of stress history, we can divide the loading process into four stages:  
1. Crack closure stage (stage I), where initial defects are closed, and the stress increases slowly. In theory, very few or even no ring counts occur because of little energy release during this stage [23; 48; 51].
2. Elastic deformation stage (stage II), where the rock specimen deforms elastically. The stress increases linearly accompanied by the slow and stable accumulation of ring counts. This implies that microcracks initiate steadily.
3. Unstable crack propagation stage (stage III), where ring counts exponentially burst with the increasing stress due to the energy storage enough to trigger the eruption of microcracks.
4. Post-peak stage (stage IV), where the stress drops suddenly with the majority of ring counts occurrence.  
Figure 4: Water-induced changes in mechanical properties of three rock types: (**a**) uniaxial compressive strength; (**b**) elastic modulus; and (**c**) critical strain.  
Figure 6 shows the relationship between stress and ring counts, from which the effect of water on the cumulative ring counts under uniaxial compression is observed:  
1. In crack closure stage, the ring counts of the dry sandstone were slightly less than that of the saturated one. On the contrary, for the granite and the marble, the number of ring counts in the saturated state was larger than that in dry state, indicating saturated specimens generate more microcracks during this stage.
2. The stress at the onset of ring counts outburst, i.e., the transition point between stages II and III is defined as the threshold of crack damage [52]. For all rock types, the crack damage stress decreased when the rock became water-saturated. As an example, the crack damage stress of the dry granite specimen was approximately 115.72 MPa (90.1% of UCS), which was 1.20 times  
Figure 5: Variations of axial stress, ring counts and cumulative ring counts versus time for different rock types under dry and saturated conditions.  
greater than that of the saturated one (77.6% of UCS). This implies that the presence of water lowered the threshold of elastic energy release.
3. The cumulative ring counts of dry rocks were dramatically greater than that of saturated ones, meaning that the rocks tested under dry conditions suffered more violent failures. This could be due to the increasing storage of elastic energy before failure, which can be reflected by the larger shaded area under the pre-peak portion of stress-strain curves. Another reason may be due to the higher slope of the stress drop in post-peak stage (i.e., greater unloading rate) causing a catastrophic fracture of the rock [53; 54].  
Figure 6: Variation of cumulative ring counts versus axial stress: **(a)** sandstone; **(b)** granite; and **(c)** marble.