## 3 Results and Discussion
### Fracture Mechanism
#### 3.4.2 Microscopic-Fracture Characteristics  
The microscopic morphology of the fracture surface of the specimen can be obtained by means of SEM, to help reveal the failure mechanism of rock from a microscopic point of view [48; 49]. To better understand the failure mechanism of transversely isotropic rocks, the specimens with typical fracture characteristics are selected for SEM scanning and their fractures are analyzed in the following  
Figure 20 presents the microscopic morphology of different fractures under monotonic loading. For the tensile fracture along the foliation plane (U1) in sample U-0, it can be seen from the magnified image of 100 times that there are many foliation planes exposed on the fracture surface, forming a wave-like pattern. This indicates that this fracture is peeled from more than one foliation plane due to the thin layered texture possessed by the slate rock. When the fracture surface is magnified up to 5000 times, a lot of thin-section tips can be observed at the fracture edge. This is because the tensile fracture means the tips of sharp lamellae are retained. Moreover, the grain grooves can also be observed, indicating the occurrence of intergranular (IG) fracture. For the shear fracture along the foliation plane (U2) in specimen U-30, at 100-times magnification, the fracture surface is banded and the banded stripes show a morphology of a peak in the middle and a valley on both sides. After further magnification, it can be found that the middle part of the band is filled with rock debris, while the two sides of the band are the foliation surface with a low degree of damage. The reason for the presence of abundant rock debris may be that the foliation planes are broken during shearing, and the foliation planes are staggered and rub against each other. In addition, three typical fracture surfaces, e.g., U3, U4 and U5, in the specimen U-90 are analyzed microscopically. Figure 20c demonstrates the microscopic characteristics of the tensile fracture through the foliation plane (U3). It can be seen in the image, at a magnification of 100 times, that the flaking layer is curved and the fracture surface is relatively rough. These curved flaking layers are not foliations, but consist mainly of an arrangement of grains. Clear rock grains can be observed in the magnified-5000-times image, implying that the fracture mode is dominated by the IG fracture, but the transgranular (TG) fracture can also be found in some localized locations. The microscopic morphology of the tensile fracture along the foliation plane (U4) is presented in Figure 20d. The fracture surface is undulated, as shown in the magnified-100-times image, and the IG fracture is observed at 5000-times magnification, indicating that the fracture mode is dominated by the tensile fracture. Furthermore, Figure 20e demonstrates the morphological characteristics of the shear fracture through the foliation plane (U5) under SEM; a few wavy bumps can be observed at 100-times magnification, and, except for these bumps, the fracture surface is relatively smooth and flat. At 5000-times magnification, rock debris is observed on the fracture surface, and some step-like features that symbolize the TG fracture are found as well. This further validates the fact that the fracture mode of U4 is dominated by the shear failure through the foliation plane. Figure 20: Cont. The microscopic characteristics of different fractures in specimens under cyclic loading are shown in Figure 21. The results manifest the fact that the fracture pattern of fracture C1 in specimen C-0-40 under cyclic loading is very similar to that of fracture U1 in specimen U-0 under monotonic loading, but with more rock debris formed. This phenomenon is more significant in the fracture C2. This may result from the repeated compression and shear on the formed fracture surface during cyclic loading. Thin-section tips and IG fractures are also found in the fracture C1, which symbolize the typical tensile fracture mode.