# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 3 Results
### Observation of Crack Characteristics
#### 3.2.4 Microscope Observation Based on Scanning Electron Microscope (SEM)  
Cracking behavior of hydro-mechanical coupling under three-dimensional conditions has not been fully studied and understood. Therefore, it needs convincing evidence more than macroscopic morphological observation to judge the tensile or shear properties. Microscale observation based on scanning electron microscope (SEM) provides another approach.  
As we know, shear cracks develop friction sliding traces in the crack surfaces during their propagation. Before SEM observation, the crack surface is sprayed with gold foil, and the sliding traces will cause a different response of the gold foil to the electron beam. Compared to the direct observation of crack surface, SEM observation can find microscopic traces, especially when no difference can be observed directly. Figure 12 shows the SEM scanning images of the crack surface of two specimens and Figure 12d shows the positions of the three scanning samples. The scanning sample S1 shown in Figure 12a was taken from the crack surface of 30-6-1 specimen near the crack tip. Figure 12b,c were taken from the HCC surface of 30-6-5 specimen near the upper tip of the pre-existing flaw (scanning samples S2) and the edge of the specimen (scanning samples S3), respectively. By their microscopic characteristics, the crack mechanism can be determined.  
Figure 12: Scanning electron microscope (SEM) scanning images of shear, tensile, and mixed cracks.  
As shown in Figure 12a, at 50\(\times\) magnification, sliding traces can be seen on the crack surface in most of the field; the stripes indicate that the crack has undergone intense shearing. In Figure 12b, no such shear traces were observed at either 50 or 300\(\times\) magnification. Even the authors searched higher magnification, but only a jumble of gypsum crystals was seen. Then the authors observed several scanning samples taken from the initial segment of the HCC. By moving the position of scanning samples under SEM to cover the whole scanning surface, no evidence of shear cracks was observed. This suggests that HCCs were formed by tensile stress. However, the characteristics of mixed cracks were observed in scanning samples taken from the ends of HCCs. Scanning sample S3 in Figure 12c is an example; no significant features can be seen at lower multiples, while mixed shear traces can be observed with the increase of magnification multiples. As shown in the red box, obvious tensile-shear mixed characteristics can be observed at 500\(\times\) magnification. The upper left part of the white dotted line was dominated by shear failure, while the lower right part was dominated by tensile failure, and this feature can also be observed elsewhere in S3. This indicates that the crack in S3 was subjected to tensile-shear mixed effect. In fact, by further microscopic observation in other scanning samples, we found that many of them were not simply tensile or shear. However, even in the case of mixed failure, the main properties can be distinguished by the magnitude of shear characteristics under SEM. The microscope observation showed HCCs were tensile cracks when they just initiated from the outer tips of the pre-existing flaw, and tensile failure was dominant during the whole fracturing processes, while the HCCs became tensile and shear mixed cracks when the specimens were about to fail.