## 3 Results and Discussion  
### Analysis of Connector Failure Modes  
Figure 5 shows the different failure modes observed for the connector in the experimental tests. The specimen has no distinct deformation under a relatively small tensile load. As the load gradually increases, the bolt will develop bending deformation. This process is accompanied by a distinct sound, which indicates that the honeycomb core is under pressure from facesheets and becomes partially flattened when the connector bears shear force (Figure 5d). When the load exceeds the bolt's ultimate shear capacity, the bolt is cut off (Figure 5b,e) and a muffled sound is heard. The bolt hole in a honeycomb sandwich structure is also stretched to an oval shape under pressure, and the connector is damaged. As the high-strength bolt has a relatively high shear capacity, local tensile failure of the honeycomb sandwich structure occurs before some connectors reach their ultimate shear capacities. This failure causes the honeycomb sandwich structure to tear at the pad; however, the bolt is not cut off (Figure 5c).  
Figure 4: Material constitutive relation: (**a**) Aluminium alloy honeycomb sandwich structure; (**b**) Connection plate; (**c**) High strength bolt; (**d**) Ordinary bolt and self-tapping bolt.  
Figure 6 shows the deformation and stress from the finite element analysis for the connector failure. As shown in Figure 6a,b, the total stress on the honeycomb sandwich structure is relatively high; the honeycomb core and the honeycomb facesheets, pad, and connection plate have relatively small displacements; the displacement at the opening suddenly increases; the bolt develops bending deformation under load. The total stress on the honeycomb sandwich structure is primarily less than 260 MPa. However, severe stress concentration occurs at the contact points among the opening, bolt, and connection plate.  
Figure 6c,d show displacement and stress distributions of the bolt and connection plate in the entity model for the honeycomb sandwich structure connector. The bolt develops tiny bending deformation and rigid rotation; the displacement in the central part of the bolt is relatively large (Figure 6c). A stress concentration occurs at the contact point with the pad, the maximum stress is 780 MPa, the working stress exceeds 22% of the ultimate strength--in order to intuitively and precisely find the failure position of high strength bolt, we intentionally did not set it in the finite element analysis, and observed the stress concentration degree--and material yield occurs. The bolt develops bending deformation and exerts pressure on the connection plate, and the stress in this area is relatively high. The total connection plate displacement is relatively small, and the bolt hole develops significant deformation. A stress concentration occurs at the honeycomb sandwich structure's contact point with the connection plate, and the maximum stress is 270 MPa. The stress around the bolt hole exceeds the material yield stress, which causes the surrounding yielding bolt hole to become stretched into an oval shape (Figure 6d).  
Figure 5: Specim failure process; (**a–c**) High strength bolt connector; (**d,e**) Ordinary bolt connector; (**f,g**) Self-tapping bolt connector.  
Figure 6: Results of FE simulations: (**a,b**) displacement field and stress distribution for the sandwich structure; (**c**) displacement field and stress distribution for the high-strength bolt; (**d**) displacement field and stress distribution for the connection plate. Displacements are expressed in mm and stress in MPa.