# Experimental Study on Rock-Like Specimens with Single Flaw under Hydro-Mechanical Coupling
## 3 Results
### Mechanical Properties
#### 3.1.1 Stress-Strain Curve Behavior  
The stress-strain curve reflects the mechanical response of the specimens under loading. In the uniaxial compression test, all specimens showed the characteristics of brittle failure. In the triaxial compression test and hydraulic coupled triaxial test, the stress-strain curve varied with confining pressure and water pressure. With the increase of confining pressure, the failure mode of specimens transferred to ductile failure, and with the increase of water pressure, the failure mode changed to brittle failure. Figure 4 shows part of the stress-strain curves in these tests as an example. According to the test results in the triaxial compression test and hydraulic coupled triaxial test, the types of stress-strain curve are summarized, as shown in Figure 5.  
In the triaxial compression test, all specimens showed a good linear relationship at the elastic compression stage, as shown in Figure 4a,b. The peak deviator stress increased with the confining pressure, and interestingly specimens with different inclination angles showed a different response to the confining pressure. The post-peak stage of 30\({}^{\circ}\), 45\({}^{\circ}\), and 60\({}^{\circ}\) specimens presented a smooth strain softening, and the stress-strain curve dropped gradually with post-peak loading (Figure 4a as an example). While 0\({}^{\circ}\), 15\({}^{\circ}\), 75\({}^{\circ}\), and 90\({}^{\circ}\) specimens showed different trends that the drop range of the post-peak stress-strain curve decreased significantly with the increase of confining pressure. Under 6 MPa confining pressure, these specimens retained their peak deviator stress for a long time or decreased slightly (Figure 4b as an example). With the increase of confining pressure, the stress-strain curve of 0\({}^{\circ}\), 15\({}^{\circ}\), 75\({}^{\circ}\), and 90\({}^{\circ}\) specimens seemed more easily transformed from type 1 to type 2 in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experimental Type** & **Specimen** & **Specimen** & **Confining** & **Water** \\  & **Number** & & **Pressure** & **Pressure** \\ \hline Uniaxial compression test & \(\alpha-\text{UCT}\) & & - & - \\ Triaxial compression test & \(\alpha-\text{CP}\) & & 15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), & 2, 4, 6 MPa & - \\ Hydraulic coupled & & 60\({}^{\circ}\), 75\({}^{\circ}\), 90\({}^{\circ}\) & 2 MPa & 1 MPa \\ triaxial test & \(\alpha-\text{CP}-\text{WP}\) & & specimens & 4 MPa & 1, 3 MPa \\  & & & 6 MPa & 1, 3, 5 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental scheme. \(\alpha\): inclination angle; UCT: uniaxial compression test; CP: confining pressure; WP: water pressure. Figure 5a than the other specimens. Confining pressure increased the slope at the elastic compression stage and the peak deviator stress, while the effect on yield strain was not significant. Under low confining pressure, the stress gradually dropped in the post-peak stage (type 1 in Figure 5a); under high confining pressure, the stress almost remained unchanged or just slightly dropped to residual strength (type 2 in Figure 5a). The stress-strain curves of shear failure specimens showed the former type, and the stress-strain curves of mixed failure specimens showed the latter. In the hydraulic coupled triaxial test (Figure 4c,d) the elastic compression stage was relatively short and bent, except for the condition of 1 MPa water pressure and 6 MPa confining pressure. The  
Figure 4: Part of stress–strain curves as an example. (**a**) 15\({}^{\circ}\) specimens in uniaxial and triaxial compression tests; (**b**) 15\({}^{\circ}\) specimens in hydraulic coupled triaxial tests; (**c**) 60\({}^{\circ}\) specimens in uniaxial and triaxial compression tests; (**d**) 60\({}^{\circ}\) specimens in hydraulic coupled triaxial tests; (**e**) 75\({}^{\circ}\) specimens in uniaxial and triaxial compression tests; (**f**) 75\({}^{\circ}\) specimens in hydraulic coupled triaxial tests.