## 4 Experimental Study and Results Analysis of the Device
### Pressure Testing and Results Analysis of the Test Cabin
#### 4.1.1 Pressure Testing of the Rock Cabin  
The inner diameter of the rock cabin was 500 mm, and the length was 2400 mm. During the experiment, hydraulic oil is injected into the interior of the cabin body through pressure testing pump to achieve pressure application, and the axial deformation of the rock cabin was detected. Due to the symmetrical structure of the rock cabin, deformation data for only one end face should be recorded when axial deformation is detected, and it should then be multiplied by 2 to obtain the total axial deformation. Therefore, in pressure tests of the rock cabin, an axial displacement detection meter was placed on the end face to measure axial deformation, as shown in Figure 10. The original reading of the displacement detector during the test and the data after modification are provided in Table 5.  
As shown in Table 5, the second column is the original reading of the axial displacement detector during the test. Because the initial reading of the axial displacement detector is 1 mm when the pressure is 0 MPa, the data recorded by the displacement detector should be subtracted from the initial readings to obtain the real axial displacement of one side, as shown in the second column of Table 5. Meanwhile, the axial displacement of one side should be multiplied by 2 to obtain the total axial deformation of the cabin, as shown in the third column of Table 5. The relation curves between the axial displacement and test pressure are shown in Figure 11.  
According to Figure 11, with the gradual increase of the test pressure, the axial deformation of the rock cabin also increases gradually and presents a linear growth trend with the change of the pressure. When the highest pressure is 190 MPa, the total axial deformation reaches the maximum value of 6.980 mm, and the relative deformation is 0.291%. Therefore, in the process of the internal pressure of the rock cabin rising from 0 MPa to 190 MPa, the axial deformation of the cabin body is always in the stage of elastic deformation, and the deformation is small. At the same time, no leakage of pressure or flow was detected on the outer surface of the cabin during the test. Therefore, the design of the cabin is reasonable. Figure 10: Pressure system for testing the rock cabin. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Pressure,** & **Axial Displacement** & **Unilateral Axial** & **Total Axial** \\
**MPa** & **Test Data, mm** & **Deformation, mm** & **Deformation, mm** \\ \hline
0 & 1.000 & 0 & 0 \\
50 & 1.865 & 0.865 & 1.730 \\
100 & 2.769 & 1.769 & 3.538 \\
150 & 3.742 & 2.742 & 5.484 \\
190 & 4.490 & 3.490 & 6.980 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Axial deformation of the rock cabin. According to Figure 13, when the internal pressure of the drill pipe chamber rises, the axial deformation detected at both ends of the drill pipe cabin presents a linear increase trend with the increase of pressure, indicating that the drill pipe cabin was always in the elastic deformation stage under the pressure condition of 0 MPa to 190 MPa, and can operate safely and stably. Furthermore, it is found that the deformation data of the two detectors are close to each other, which indicates that the deformation of one end is half of the total axial deformation of the drill pipe cabin, as long as their structure is consistent, even if the length of the two drill pipe cabins is not consistent. Meanwhile, under the condition of maximum pressure of 190 MPa, the maximum deformation of the drill pipe cabin is 2.309 mm, and the relative deformation is 0.046%, which is greatly reduced compared with that of rock sample chamber, which is 0.291%. It is further explained that, in the calculation of the wall thickness of the drill pipe cabin in Section 3.1.3 of this paper, considering that the drill pipe cabins are slender tubes, it is reasonable to adjust the safety factor upward to 3 to ensure its small deformation under ultrahigh-pressure. Therefore, the drill pipe cabin can operate stably in the extreme environment of ultrahigh-pressure.