## 3 Results and Discussion
### Failure Mode and Mechanical Response  
The compression shear test at 800 \({}^{\circ}\)C was conducted on the C/C composite bonded-bolted hybrid single-lap joints. The failure morphology of the test piece after cooling was observed using a digital  
Figure 6: Calculation process of the bolt progressive damage. \begin{table}
\begin{tabular}{c c} \hline \hline
**Failure Mode** & **Stiffness Degradation** \\ \hline Fiber tension & \(E_{11}=0.07E_{11}\) \\ \hline Fiber compression & \(E_{11}=0.07E_{11}\) \\ \hline Fiber tension & \(E_{22}=0.2E_{22}\), \(G_{22}=0.2G_{22}\), \(G_{23}=0.2G_{23}\), \\ \hline Fiber compression & \(E_{22}=0.4E_{22}\), \(G_{12}=0.4G_{12}\), \(G_{23}=0.4G_{23}\), \\ \hline Matrix/fiber shear & \(G_{12}=0\), \(\
u_{12}=0\) \\ \hline Delamination & \(E_{22}=0\), \(G_{13}=0\), \(G_{23}=0\), \(\
u_{13}=0\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Element stiffness degradation mode. microscopic system and a scanning electron microscope (Helios Nanolab 600i, FEI, Hillsboro, Oregon, USA), as shown in Figure 7. It can be observed that shear failure occurred in both the adhesive layer and the bolt, and the shear plane of the bolt is flat and coincides with the middle plane of the adhesive layer. The bolt rod had no obvious deformation, and the bolt hole had no obvious extrusion failure, indicating that the bolt rod had a brittle fracture. In contrast to [30], for hybrid joints, the whole adhesive layer exhibited cohesive failure (adhesives were observed on both overlapped plates after the failure), and no interface failure occurs (adhesives were only found on one overlapped plate after the failure). This is due to the secondary bending caused by the eccentricity of the load, which causes the bolt to compress the adhesive layer perpendicular to the lap surface, reducing the peeling behavior of the adhesive layer as shown in Figure 8.  
Figure 7: (**a**) Failure morphology observed by the digital microscopic system; (**b**) failure morphology observed by scanning electron microscopy. The test and simulation load-displacement curves of C/C composite bonded joints [30] and bonded-bolted hybrid joints at 800 \({}^{\circ}\)C are shown in Figure 9, respectively. It can be seen that the load on the hybrid joints had two peaks with the increase of displacement. The load shared by the adhesive layer and the bolt resulted from the hole clearance filled with the cured adhesive. After reaching the first peak, a brittle fracture occurred in the adhesive layer, the load-bearing capacity dropped rapidly, the curve presented as a falling straight line, and the load was then entirely taken by the bolt. Subsequently, the load rose again, the hole wall was compressed, and the bolt was sheared. The eccentricity of the load caused the bolt to bend slightly, resulting in stress concentration on the middle plane of the bolt. Finally, the bolt failed in the shear, and the shear plane was the middle plane of the bolt rod. Similar to bonded joints, the load on the hybrid joints increased nonlinearly until the first peak. In contrast to the bonded joint, the hybrid joint can be seen as replacing the original adhesive layer with a higher modulus adhesive at the center of the adhesive layer. The stiffness of the bonded joints was less than that of the hybrid joints before reaching the first peak, which is because the existence of the bolt increased the overall stiffness of the hybrid joint. The displacement corresponding to the failure point of the adhesive layer in the hybrid joint is greater than the bonded joint, because the existence of the bolt slowed down the crack propagation in the adhesive layer and improved the bearing performance of the joint. For hybrid joints, the failure load of the adhesive layer is defined as \(P_{\text{max}}\), the load at the point where the curve descended after the failure of the adhesive layer as \(P_{\text{a}}\), and the difference between \(P_{\text{max}}\) and \(P_{\text{a}}\) as \(\Delta P\). Therefore, \(\Delta P\) was the load shared by the adhesive layer in the hybrid joints.