## 3 Test Results and Analysis
### Failure Mechanism Analysis  
The test results show that, for the unconstructed specimen, four main roles are considered: chemical bonding force, mechanical occlusion force, friction force, and precompression stress. In the early stage of loading, these factors share interfacial shear. As the interfacial shear increases, slight longitudinal compression of the core concrete occurs, causing the steel tube-concrete interface at the loading end to undergo displacement misalignment and extend toward the measured end, gradually increasing the chemical bonding force. When the tensile wire sensor at the measured end produces a value, the interface misalignment is transferred to the measured end, which also indicates the disappearance of the chemical bonding force. In addition, the core concrete expands laterally under the launching load, producing an extrusion force on the inner wall of the steel tube, increasing the friction force, and slightly increasing the mechanical occlusion force. The steel tube-concrete interface has an uneven abrasive concrete layer, which also leads to the friction coefficient changing constantly, so the friction force increases, and when the friction force is greater than the loss of the bonding force and the mechanical occlusion force, the P-S curve will show a secondary upward section. There will also be a secondary rising section. For specimens constructed with internally welded reinforcement rings, three main roles are considered: (1) The restraining effect of the reinforcement ring. The reinforcement ring is \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen** & \(\mathbf{\tau_{u}}\) & **[PERSON] et al. [13]** & **[PERSON] et al. [31]** & **[PERSON] et al. [19]** & **[PERSON] et al. [32]** \\
**Number** & **MPa** & **MPa** & **MPa** & **MPa** & **MPa** \\ \hline ZU\({}_{0}\)N\({}_{1}\)W & 3.96 & 1.00 & 1.92 & 1.05 & 2.49 \\ ZU\({}_{0.25}\)N\({}_{0.75}\)Y & 6.18 & 1.00 & 1.92 & 1.05 & 2.49 \\ ZU\({}_{0.25}\)N\({}_{0.75}\)E & 8.23 & 1.00 & 1.92 & 1.05 & 2.49 \\ ZU\({}_{0.5}\)N\({}_{0.5}\)Y & 6.21 & 1.00 & 1.92 & 1.05 & 2.49 \\ ZU\({}_{0.5}\)N\({}_{0.5}\)E & 8.36 & 1.00 & 1.92 & 1.05 & 2.49 \\ ZU\({}_{1}\)N\({}_{0}\)W & 4.80 & 1.00 & 1.92 & 1.05 & 2.49 \\ JU\({}_{1}\)N\({}_{0}\)W & 4.57 & 1.00 & 1.92 & 1.05 & 2.41 \\ JU\({}_{0.5}\)N\({}_{0.5}\)E & 7.82 & 1.00 & 1.92 & 1.05 & 2.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of experimental values with calculated values from the existing literature. Figure 5: Longitudinal strain curve of the outer wall of the steel tube for the JU\({}_{0.5}\)N\({}_{0.5}\)E specimen. welded to the inner wall of the steel tube, and because its stiffness in the plane is greater than that of the core concrete, it directly restrains the concrete in the deformation of the steel tube and the concrete under the launching load. This effect converts the resulting squeezing force into mechanical occlusal and frictional forces. (2) The welds play a great role in hindering the slip of the concrete, and even if weld damage occurs, the remaining welds can continue to work until the welds are entirely torn. (3) Regarding the effect of the internal welded reinforcement ring's anti-shear connectors, as the interface chemical bonding force disappears, the welded reinforcement ring on the inner wall of the steel tube will gradually play a role. At the beginning of the application of force, the steel ring in the elastic phase will gradually resist the increasing shear force with increasing concrete deformation, and its force will be similar to that of the cantilever structure under a uniform load. With increasing load, the welded part of the steel ring and the inner wall of the steel tube will gradually increase the load N\({}_{\text{J}}\), and the welded steel parts of the force condition will shift from the original uniform load state into a non-uniform load state. Consequently, the distribution of the force will be transferred to the welded part of the steel ring.