## 3 Test Results and Discussion
### AE Behavior and Fracture Process
Firstly, find out the time when AE counts increase significantly or CAEC deviate from the linear segment; then, observe the feature of major principal strain fields at these moments. If the characteristic AE behavior corresponds to crack initiation, propagation or coalescence in the major principal strain fields, the corresponding stress is the crack initiation or damage stress. However, the crack closure stress \(\sigma_{\rm cc}\) is difficult to be directly identified by this method. It is generally considered that the axial stress at the critical point of the stress-strain curve from nonlinear to linear is the crack closure stress. Based on this, an objective approximate estimation method of crack closure stress was proposed, which is helpful to reduce human subjective error. Taking the specimen Sq-A-0-1 as an example, the calculation steps are as follows: first, select two points on the stress-strain curve where the stresses are 0.2 and 0.3 times of the peak stress (i.e., 10.33 MPa and 16.05 MPa), and mark them as P1 and P2 respectively; second, the stress-strain curve is linearly fitted between the two points (P1 and P2) to obtain the slope of 10.97 GPa, and then a line (\(\sigma=10.97\epsilon\)) is drawn from the origin Point 0 (see Figure 11a); third, calculate the axial strain difference (\(\Delta\varepsilon=\varepsilon_{\rm x2}-\varepsilon_{\rm x1}\)) between the straight line and the stress-strain curve for the same axial stress \(\sigma_{\rm x}\); finally, plot the curve of axial strain difference versus axial stress, and find out the Point A with the largest axial strain difference. The stress at Point A is the crack closure stress \(\sigma_{\rm cc}\), as shown in Figure 11b. Therefore, the stresses at Points A, B, C and D shown in Figures 8-10 can be defined as crack closure stress (\(\sigma_{\rm cc}\)), crack initiation stress (\(\sigma_{\rm ci}\)), crack damage stress (\(\sigma_{\rm cd}\)) and peak stress (\(\sigma_{\rm c}\)), respectively. The stress Point E is at the post-peak. It should be noted that the crack initiation stress \(\sigma_{\rm ci}\) mentioned in this paper is the stress corresponding to the cracking of rock matrix. However, for the filled specimens, since the interface between the filling material and the hole is the weakest plane, it can be clearly seen from Figure 9b,c and Figure 10b,c that, comparing the principal strain fields at Points A and B', strain localizations first concentrate at the boundary or inside of the filled holes and initiate from it, which indicates that the corresponding stress at Point B' can be regarded as the crack initiation stress \(\sigma^{\prime}_{\rm ci}\) of the inclusions in the filled specimens. Figure 11: Determination of crack closure stress of specimen Sq-0-A-1: (**a**) schematic diagram of axial strain difference and (**b**) location of Point A with the maximum axial strain difference. For the intact specimen, it can be seen from Figure 8 that some AE events can be detected during the pore compaction stage (OA), which is caused by the closure of the micro cracks and pores in the rock specimen. When the stress increases to Point A, the specimen turns into the elastic deformation stage (AB), and the axial stress-time curve increases linearly, with a large amount of strain energy being stored. In this stage, the AE counts enters the quiet period, and the CAEC also show as the platform interval. From Point B, the AE counts become active again, and the CAEC begin to increase slowly and linearly with the increasing stress, which indicates that the micro-cracks in the specimen begin to initiate and propagate steadily, i.e., the stable crack growth stage (BC). Some strain localization dots are randomly distributed on the specimen surface, but the range and density of the micro cracks are not enough to form macro cracks. As the stress increases to Point C, the AE counts are more active than the previous stages, and the CAEC increase nonlinearly, which indicates that the specimen enters into the unstable crack growth stage (CD).