## 3 Test Results and Discussion
### AE Behavior and Fracture Process  
In this study, special attentions are paid to the damage and fracture process around the square holes under different filling modes, and the effect of interface angle on the crack behavior. For this purpose, the AE and DIC techniques were jointly applied to analyze the cracking process of rock qualitatively and quantitatively. The AE counts and cumulative acoustic emission counts (CAEC) were obtained to reflect the damage evolution in the whole loading process [33]. The major principal strain fields in the cracking process were also given, and the strain localization characteristics could identify rock fracture as early as possible compared to other strain variables [25; 34]. Through careful observations, it was found that when the major principal strain is about 2.5%, a crack can be considered as macro cracks for the tested sandstone specimen. In order to determine the crack stress thresholds and reduce the influence of subjective factors, the legend range for major principle strain fields is set as 0-2.5% in this paper. Figures 8-10 show the damage and fracture process of the intact specimen, specimens with a \(0^{\circ}\) hole and a \(45^{\circ}\) hole, respectively. The left side of the figures shows the stress-time curves in combination with AE counts and CAEC, and the right side shows the major principal strain fields on specimen surface at the characteristic stress points. Figure 8: Acoustic emission (AE) behavior and strain fields of intact specimen Int-1. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen No.**} & \multicolumn{2}{c}{**10 MPa**} & \multicolumn{2}{c}{**20 MPa**} & \multicolumn{2}{c}{**30 MPa**} & \multicolumn{2}{c}{**40 MPa**} \\ \cline{2-9}  & \(\mathbf{\Delta}\)**D\({}_{\mathbf{x}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** & \(\mathbf{\Delta}\)**D\({}_{\mathbf{y}}\)/mm** \\ \hline Sq-0-A & 0.0082 & \(-\)0.0397 & 0.0262 & \(-\)0.0775 & 0.0468 & \(-\)0.1194 & 0.0762 & \(-\)0.1600 \\ Sq-0-B & 0.0054 & \(-\)0.0204 & 0.0159 & \(-\)0.0412 & 0.0360 & \(-\)0.0687 & 0.0635 & \(-\)0.1051 \\ Sq-0-C & 0.0003 & \(-\)0.0139 & 0.0117 & \(-\)0.0341 & 0.0293 & \(-\)0.0539 & 0.0508 & \(-\)0.0856 \\ Sq-45-A & 0.0248 & \(-\)0.0482 & 0.0556 & \(-\)0.0926 & 0.0944 & \(-\)0.1392 & 0.1533 & \(-\)0.1951 \\ Sq-45-B & 0.0219 & \(-\)0.0350 & 0.0503 & \(-\)0.0756 & 0.0923 & \(-\)0.1213 & 0.1508 & \(-\)0.1723 \\ Sq-45-C & 0.0017 & \(-\)0.0124 & 0.0150 & \(-\)0.0397 & 0.0424 & \(-\)0.0760 & 0.0882 & \(-\)0.1228 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Deformation of \(0^{\circ}\) and \(45^{\circ}\) hole under different filling modes at stresses of 10-40 MPa. Figure 9: AE behavior and strain fields of pre-holed specimens with \(\alpha=0^{\circ}\) under different filling modes: (**a**) Sq-0-A-1; (**b**) Sq-0-B-2; and (**c**) Sq-0-C-1. Figure 10: AE behavior and strain fields of pre-holed specimens with \(\alpha=45^{\circ}\) under different filling modes: (**a**) Sq-45-A-1; (**b**) Sq-45-B-2; and (**c**) Sq-45-C-1. Obviously, the damage and fracture process of rock, including crack initiation, nucleation, propagation and coalescence, is closely related to several characteristic stress thresholds of rock (i.e., \(\sigma_{\rm cc}\), \(\sigma_{\rm ci}\), \(\sigma_{\rm cd}\) and \(\sigma_{\rm c}\)) [35; 36]. Where \(\sigma_{\rm cc}\) and \(\sigma_{\rm ci}\) represent the stresses at the closure of initial defects and the initiation of new cracks, respectively. The crack damage stress \(\sigma_{\rm cd}\) refers to the stress that unstable cracks begin to appear, and the peak stress \(\sigma_{\rm c}\) is equal to the UCS. In regard to the values of \(\sigma_{\rm ci}\) and \(\sigma_{\rm cd}\) for sandstone specimens in this test, they can be qualitatively and quantitatively determined by combining DIC and AE results [37; 38].