# Study on Mechanical Properties and Application of a New Flexible Bolt
## 4 Static Tensile Test Results and Mechanical Analysis of Flexible Bolts
### Flexible Bolts with O42 mm Locking Casing  
Figures 10 and 11 show the mechanical response and failure mode of flexible bolts with O42 mm locking casing. As shown in Figure 10a, when the length of the locking casing of the three bolt samples is 100 mm, the resistance change trend of the three samples are approximately the same: firstly, a sharp increase to the peak resistance, then a drastic decline, and finally a long constant resistance deformation. Before the bolt displacement reaches 20 mm, all the three samples reach their respective peak resistances: D3L1-1, 333 kN; D3L1-2, 328 kN; and D3L1-3, 339 kN. After the peak resistance, the locking casings of the three samples are all pulled out, causing a sharp decline in bolt resistance, which is finally maintained within 80-120 kN. In the process of the locking casing being pulled out, the displacement of all three sample bolts increases continuously, but the resistance does not change remarkably. The final displacement of the three samples is 100, 63, and 75 mm. As can be seen from Figure 11a, no failure occurs to the rod body of the three samples, and the steel wires do not loosen, either. However, the locking casings of the three samples are pulled out to different degrees, leaving scratches of different length on the rod body.  
Figure 9: Failure mode of flexible bolts with O33 mm locking casing: (**a**) D2L1, (**b**) D2L2, and (**c**) D2L3.  
Figure 10b, it can be known that when the length of the locking casing of the three bolt samples is 120 mm, the resistance change curves of the three samples are approximately the same during the quasi-elastic stage. However, during the yield stage and breaking stage, the resistance change trend varies. The bolts of D3L2-2 and D3L2-3 have a similar change during the yield stage; when reaching the peak resistance, one steel wire breaks, causing the instant drop of resistance. In addition, the locking casing of D3L2-2 shows a slight trace of being pulled out. The resistance of D3L2-1 fluctuates slightly during the yield stage; its locking casing is pulled out during the breaking stage and its final displacement exceeds 90 mm. As can be seen from Figure 11b, the deformation of all three sample bolts is rather slight: the locking casing of D3L2-1 is pulled out, and the core steel wire of D3L2-2 and D3L2-3 is broken.  
From Figure 8c, it can be known that when the length of the locking casing of the three samples further increases to 140 mm, the resistance change trend of the three samples  
Figure 11: Failure mode of flexible bolt with Ø42 mm locking casing: (**a**) D3L1, (**b**) D3L2, and (**c**) D3L3.  
Figure 10: Mechanical response of flexible bolts with different Ø42 mm locking casing lengths: (**a**) 100 mm, (**b**) 120 mm, and (**c**) 140 mm.  
differs. For D3L3-1, its locking casing is pulled out after the quasi-elastic stage and yield stage, causing the instant drop of bolt resistance and the subsequent long constant resistance state. For D3L3-2, its displacement is rather small during the yield stage until its locking casing is pulled out. When its resistance drops to 160 kN, it has a long constant resistance deformation. Its final displacement reaches 83 mm. For D3L3-3, its steel wires are broken after the quasi-elastic stage and yield stage. The peak resistance of D3L3-1, D3L3-2, and D3L3-3 is 362, 345, and 369 kN. As is shown in Figure 11c, the locking casings of D3L3-1 and D3L3-2 are pulled out to different degrees. Three steel wires are broken in the rod body of D3L3-3 and the rod body bends remarkably, with the wires loosened. What is shown in Figure 11c is in line with the resistance change trend shown in Figure 10c.  
The drastic increase in the diameter of locking casing causes the reduction of socketed force between the locking casing and rod body, which in turn leads to the slip of rod body and invalidation of flexible bolts in the end.