## 3 Test Results and Discussion
### AE Behavior and Fracture Process
At this time, there are more strain localization points on the specimen surface, and some of them gradually gather and coalesce on the right side of the specimen. When the stress rises to the peak Point D, a large range of strain localization region appears on the specimen surface, and those concentrated on the right side of the specimen evolve into narrow high strain bands. After the peak stress, the high strain bands rapidly propagate and penetrate the specimen, resulting in final brittle splitting failure. Meanwhile, the AE counts and CAEC soar to the highest level, and the axial stress rapidly drops to 0 MPa. For the specimens with an open hole (Sq-0-A and Sq-45-A), as shown in Figures 9a and 10a, the AE behavior of the pre-holed specimens under uniaxial compression are similar to that of the intact specimen, but the stress thresholds are significantly decreased and the corresponding crack behavior is obviously different. At the crack closure stress Point A, there is no obvious feature of major principal strain field on the specimen surface, which indicates that the crack closure stress threshold is hard to be directly determined by only strain field. At the crack initiation stress Point B, due to tensile stress concentration, strain localizations concentrate at the center of the roof and floor of the 0\({}^{\circ}\) hole and the upper and lower corners of the 45\({}^{\circ}\) hole, indicating the initiation of tensile cracks from these areas. With the increase of the stress, the specimen enters the stable crack growth stage (BC), and the tensile cracks propagate stably parallel to the loading direction. At the crack damage stress Point C, two pairs of symmetrical localized bands appear at the corners of both sides of the hole, and gradually gather and nuclear as the stress increases to peak Point D, forming macro secondary cracks. Therefore, the initiation and unstable growth of secondary cracks are the signs that the pre-holed specimens enter the stage of unstable crack growth. Different from the 45\({}^{\circ}\) hole, there are high strain concentrations on both sides of the 0\({}^{\circ}\) square hole at the peak stress, which is an indication of slabbing on the sidewalls of the square hole due to local high compressive stress concentration [39]. With the further propagation of the secondary cracks and the coalescence with the far-field cracks at the corners of the specimen, the primary tensile cracks are forced to be closed [5], and the stress drops to Point E. During the dropping, the AE counts and CAEC both soar to the highest level. The damage and fracture process of the specimens with a square inclusion (Sq-0-B, Sq-0-C, Sq-45-B and Sq-45-C) are shown in Figure 9b,c and Figure 10b,c. Consistent with the unfilled specimen, the ultimate failure of the filled specimen is mainly caused by the propagation and coalescence of secondary cracks. However, striking differences can be observed in the crack initiation position and the corresponding stress value. At the stress Point B\({}^{\prime}\), strain localizations first concentrate at the boundary or inside of the inclusion, causing interface debonding or interface slipping [26]. Then the cracks coalesce with each other, penetrate through the inclusion boundary and finally propagate into the rock matrix to form tensile cracks as the stress increases to Point B. This indicates that under the low axial load, the applied energy is mainly concentrated in the filling material, resulting in inclusion deformation and fracture, which has a certain crack arrest effect on rock matrix. The results also show that the stress thresholds of filled specimens are significantly improved than unfilled specimens, and the mode and distribution of cracks around the inclusion are more complex, which will be discussed in detail in following sections. Notably, after filling, the high strains concentrated on the sidewallsof the 0\({}^{\circ}\) square hole disappear, which indicates that the filling treatment can effectively inhibit the occurrence of the sidewall slabbing.