## 4 Experiment and Discussion  
### Experimental Platform  
Owing to the complicated structure of the thread, the stress distribution in the engaging surface of the thread cannot be measured by a strain gauge. However, the thread will deform after being loaded.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Serial Number** & **Taper** & \begin{tabular}{c} **Thread Height** \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Thread Angle** \\ **(degrees)** \\ \end{tabular} &
\begin{tabular}{c} **Maximum** \\ **Stress (MPa)** \\ \end{tabular} & **K** \\ \hline
1 & 1:4 & 2.24 & 30 & 454.74 & 3.68 \\
2 & 1:6 & 2.24 & 30 & 447.41 & 3.52 \\
3 & 1:16 & 2.24 & 30 & 449.13 & 3.46 \\
4 & 1:32 & 2.24 & 30 & 445.98 & 3.42 \\
5 & 1:32 & 2.04 & 30 & 523.64 & 3.69 \\
6 & 1:32 & 2.24 & 30 & 445.98 & 3.42 \\
7 & 1:32 & 2.44 & 30 & 396.21 & 3.39 \\
8 & 1:32 & 2.24 & 15 & 419.55 & 2.89 \\
9 & 1:32 & 2.24 & 30 & 445.98 & 3.42 \\
10 & 1:32 & 2.24 & 45 & 485.59 & 3.59 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Stress states under different structural parameters.  
Figure 16: Three optimized thread structures: (**a**) optimized thread taper, (**b**) optimized thread height and (**c**) optimized thread angle.  
When the deformation reaches a certain level, the sealing of the thread will fail. If a pressurized gas is filled in the cavity of the joint, the gas will leak through the spiral passage of the thread. By measuring the pressure in the chamber, the small thread deformation can be converted into a change in the pressure reading. Accordingly, the sealing performance of the thread can be evaluated by the change in the pressure reading. Therefore, the relationship between the axial load and the seal of the thread can be obtained.  
Figure 17 shows a schematic diagram of the experimental platform used to perform the tensile tests. The tests are performed on a WAW-2000 DL electrohydraulic servo-controlled universal testing machine, which can provide a maximum tensile force of 2000 kN. The air compressor (model: W-1/8) can provide a maximum air pressure of 1.5 MPa. To ensure the safety of the experiment, the pressure of the gas in the joint cavity during the tensile test is 0.6 MPa. Thus, the air compressor can meet the requirements of the experiment. The tensile load and crosshead displacement of the universal testing machine are collected and processed by the computer. The test piece subjected to the tensile test is shown in Figure 18. The test piece consists of two parts: a pin joint and a box joint. When the two parts are connected, a closed cavity is formed inside. An air injection port is provided in the wall of the box joint, through which a certain air pressure can be injected into the cavity. A clamping surface, which is designed to be gripped by the universal testing machine, is provided at both ends of the joint. In this study, four different threaded joints were tested: _optimization scheme 1_, _optimization scheme 2_, _optimization scheme 3_ and the _original design_. For better sealing, grease is evenly applied to the threads before joining. The prepared test samples are shown in Figure 19. The main parameters of each group of threads are shown in Table 2.  
Figure 17: Tensile test platform.  
Figure 18: Structure of the joint sample.