## 3 Test Results and Discussion
### Crack Behavior and Failure Pattern
By observing the final failure patterns of pre-holed specimens and the analysis of fracture process in Section 3.4, especially the cracks characterized by strain localizations in the filled holes, summaries of typical crack patterns for each group of specimens are depicted in Figure 13, where T1 represents tensile cracks that initiate at the rock matrix and propagate straight; T2 represents tensile cracks that  
Figure 12: Failure patterns of sandstone specimens under different filling modes and hole angles: (a) Sq-0-A; (b) local enlarged drawing of the square hole in Figure 12a; (c) Sq-0-B; (d) Sq-0-C; (e) Sq-45-A; (f) Sq-45-B; (g) inclusion fragments and debris from the hole in Figure 12e; (h) Sq-45-C.  
initiate at the inclusion interface and propagate straight; S1 represents shear cracks that initiate at the side corners of the hole in rock matrix and propagate obliquely; S2 represents shear cracks that initiate at the inclusion interface and propagate obliquely; TS represents a mixture of tensile and shear cracks that propagate with curvature; (T) represents tensile debonding at the inclusion boundary or tensile crack inside the inclusion; (S) represents shear slip along the inclusion interface or shear crack inside the inclusion; B represents buckling or slabbing on sidewalls. In this study, as can be seen from Figure 13a,d, four types of cracks can be summarized around a square hole: T1 cracks due to tensile stress concentration at top and bottom parts, TS or S cracks distributed at corners and B cracks induced by compressive stress at sidewalls. Among them, B cracks only occur on the sidewalls of the hole with \(\alpha=0^{\circ}\), and TS cracks only occur at the corners of both sides of the hole with \(\alpha=45^{\circ}\). Compared with the unfilled hole, it can be seen from Figure 13b,c,e,f that the number of cracks around the filled hole significantly increases and the crack pattern is more complex, which can be mainly divided into five types, i.e., T2, S2, TS, (T) and (S). In addition, the crack behavior around the inclusion is also affected by the hole angle, especially the interface angle. For example, it is observed that the crack type in \(0^{\circ}\) inclusions is mainly interface debonding (T), while (S) cracks only occur inside inclusions. On the contrary, no (T) crack is observed in \(45^{\circ}\) inclusion. Interface slipping or internal slipping (S) are dominant in the \(45^{\circ}\) inclusion, and the surrounding cracks are mainly shear ones. It seems that the interface angle is related to the shear behavior. The interface slipping (S) is more prone to initiate at the inclined interface with a certain angle to the loading direction, and then slips along the interface into the rock matrix as S2 cracks. The results are basically consistent with the observations of [PERSON] and [PERSON] [16] and [PERSON] and [PERSON] [22]. They attributed this phenomenon to the shear stress on the inclined interface. Figure 13: Crack patterns around a square hole under different filling modes and hole inclination angles: (**a**) Sq-0-A; (**b**) Sq-0-B; (**c**) Sq-0-C; (**d**) Sq-45-A; (**e**) Sq-45-B; and (**f**) Sq-45-C.