# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 3 Results
### Observation of Crack Characteristics
#### 3.2.2 Internal Cracks Observation Based on X-ray Computed Tomography (CT)  
Many related studies based on crack geometries and propagation mechanisms were obtained by surface crack observation. However, cracks are much more complex under three-dimensional conditions than plane conditions, so it is difficult for nontransparent material to obtain the crack characteristics only by surface crack observation. In order to determine the morphology of internal cracks, X-ray computed tomography (CT) was employed.  
As shown in Figure 10a, specimens were scanned which generated a spatial model, and then the model was cut into slices every pixel (34.5 \(\upmu\)m) along the thickness direction (i.e., x-axis) and the vertical direction (i.e., z-axis). Figures 10d and 11e show the cross-sections of 30-4-1 specimen after failure; the corresponding spatial position of each slice is shown in Figures 10b and 11c, respectively. The scales on the right of the pictures also indicate the spatial position relationship between the front  
Figure 9: Surface crack photographs of 45\({}^{\circ}\) specimens after failure.  
vertical cross-sections and the horizontal cross-sections. Note that the original CT scanning images had some randomly distributed bubbles, which were removed without affecting the spatial cracks.  
As shown in Figure 10, seven internal cracks can be observed in this specimen. Cracks, C1 and C2, were throughout the specimen and caused the failure. They coalesce  
Figure 10: Part of cross-sections of 30-4-1 specimen after failure. (**a**) Schematic diagram of space coordinate system; (**b**,**c**) schematic diagram of section cutting position; (**d**) front vertical cross-sections with different depths; (**e**) horizontal cross-sections with different heights.  
preexisting flaw, while in most areas (x-axis from \(-12\) mm to \(25\) mm); they were connected by C7, as shown in Figure 10d4. Judging by geometrical morphology, C3, C4, and C5 were tensile cracks that were not fully propagated. C3 and C4 formed macro-cracks, while they did not directly lead to specimen failure. It is possible that the specimen was fractured under the competition between C1-C2 and C3-C4, but the latter was preferred under the current test condition. The C5 was only a small fragment which ranged near the lower crack tip. C6 was a far-field crack on both sides of the water injection channel, which was similar to hydraulic fracture cracks in morphology. In 90\({}^{\circ}\) specimens under 5 MPa water pressure (90-6-5), cracks like C6 caused the failure. C7 was a horizontal coupled crack (HCC) that was not fully formed. As shown in Figure 10e4, the shaded area was C7.  
CT observations show that not every specimen's internal cracks were as complex as shown above. Generally, the internal cracks of pure tensile or pure shear failure specimens are relatively simple. Compared with specimens in triaxial and uniaxial compression tests, the internal cracks of specimens in a hydraulic coupled triaxial test are more complex due to hydro-mechanical coupling.