# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 3 Results
### Crack Types Observed in This Experiment  
Based on the macro and micro observation method, the author summarized the crack types observed in this experiment. Figure 13 shows the schematic diagram of different types of cracks that led the jointed specimen failure, including three kinds of tensile cracks, two kinds of shear cracks, three kinds of mixed cracks, and a horizontal coupled crack (HCC).  
The type 1 tensile crack (T1) was a three-dimensional wing crack, and the type 2 tensile crack (T2) was the anti-wing crack. Both of them were usually observed in the uniaxial test and led to tensile failure. The type 3 tensile crack (T3) appeared when confining pressure was small or water pressure was large. The vertical splitting failure in 0\({}^{\circ}\) and 90\({}^{\circ}\) specimens were classified as type 3 tensile cracks (T3) in this study.  
Due to the complexity of 3D cracking behavior in mixed failure specimens, different types of mixed cracks may be difficult to distinguish by geometry. In this paper, the authors divided them into three types by comparing the initiation mechanism, direction, and propagation properties (shear/tensile). The comparison between the crack types obtained by CT and SEM observation in this paper and those obtained by high-speed cameras in previous studies will be discussed in Section 4.  
The morphology of type 1 mixed crack (M1) was similar to that of type 3 tensile crack (T3). Both of them initiated almost along the pre-existing flaw direction and bent inward in propagation. The difference was that M1 has an obvious shearing phenomenon near the flaw tips, while the shear sliding traces were rarely observed in T3. Additionally, the bending amplitude of M1 was smaller than that of T3. The initiation direction of type 2 mixed crack (M2) had a small angle with the pre-existing flaw and propagated along the initiation direction without bending. The initiation was a mixed failure dominated by tensile, while the crack surfaces far away from the tips were mainly formed by shear. The geometry of type 3 mixed crack (M3) was similar to that of M2. The difference was that the crack initiation position of M3 was not in two flaw tips, and the shear characteristics at the flaw tips were more obvious than M2.  
Shear cracks appeared in specimens under high confining pressure, and the crack surfaces showed remarkable shear characteristics. The crack surfaces of type 1 shear crack (S1) passed through the pre-existing flaw, while the direction of type 2 shear crack (S2) was not consistent with the pre-existing flaw. S1 appeared in specimens under 6 MPa confining pressure in triaxial compression tests and hydraulic coupled triaxial test with 1 MPa water pressure. S2 usually occurred in 0\({}^{\circ}\) and 90\({}^{\circ}\) specimens both in conventional and hydraulic coupled triaxial tests.  
The initiation of HCCs were dominated by tensile failure, and tensile-shear mixed failure occurred at the end of the propagation. The HCCs occurred when the crack inclination angle ranged from 15\({}^{\circ}\) to 45\({}^{\circ}\), and under high water pressure. The other types of cracks, such as far-field cracks, secondary cracks, and slight end damage, were not the main cracks which caused specimen failure; therefore, these cracks were not concerned in this paper.