# 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 O33 mm Locking Casing  
Figures 8 and 9 show the mechanical response and failure mode of flexible bolts with O33 mm locking casing. As shown in Figure 8a, when the length of the locking casing of the three bolt samples is 100 mm, the bolt resistance shows an overall change trend of sharp rise and sharp fall. Before the bolt displacement reaches 22 mm, all three samples reach their peak resistances. The peak resistance of D2L1-land D2L1-3 is 311 and 303 kN, respectively, but that of D2L1-2 is relatively small, 197 kN. During the late breaking stage, drastic variations appear between D2L1-1 and the other two samples. The resistance of D2L1-1 witnesses a sharp decline from peak resistance to its minimum. By contrast, the resistance of D2L1-2 and D2L1-3 experiences a long and relatively mild decline, the displacement reaching 112 and 78 mm, respectively. As is shown in Figure 9a, the core steel wire of D2L1-1 breaks, causing the resistance to drop suddenly to below 60 kN. The rate of resistance drop reaches up to 80%, triggering the prewarning of the MTS system and in turn terminating the static tensile test. No damage is done to the locking casing of D2L1-2 and D2L1-3. Instead, the locking casing is just pulled out and leaves long scratches on the rod body.  
Figure 8: Mechanical response of flexible bolts with different O33 mm locking casing lengths: (**a**) 100 mm, (**b**) 120 mm, and (**c**) 140 mm.  
Figure 7: Failure mode of flexible bolt with O33 mm locking casing: (**a**) D1L1, (**b**) D1L2, (**c**) D1L3.  
From Figure 8b, it can be known that when the length of the locking casing of the three bolt samples is 120 mm, the resistance change trend of the three samples is approximately the same. During the quasi-elastic stage, the gradient of the three samples varies slightly: that of D2L2-1 takes the lead and is followed by that of D2L2-3 while the gradient of D2L2-2 is the smallest. However, during the yield stage, the resistance change curve of the three samples is virtually close. During the breaking stage, the bolt samples show a characteristic of sequential breakage. The peak resistance of D2L2-1, D2L2-2, and D2L2-3 is 372, 373, and 369 kN, respectively. As shown in Figure 8b, all three sample bolts have a serious deformation, and many steel wires have 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 D2L3-2 and D2L3-3 is quite similar. To be specific, the bolt resistance also experiences three stages: quasi-elastic stage, yield stage, and late breaking stage. However, remarkable variation appears between the two samples and D2L3-1, whose peak resistance is 228 kN, being only 62% of the other two samples. After the peak resistance, as the displacement of D2L3-1 increases, the bolt resistance decreases slowly. This is mainly because the friction between the locking casing and rod body declines gradually when the locking casing is pulled out. During the final breaking stage, the resistance shows a characteristic cyclic oscillation rise. This is mainly because the springs in the locking casing break by stage and new friction is produced between the broken springs and rod body, which in turn increases the resistance. As is shown in Figure 8c, no steel wire breaks in D2L3-1, only leaving scratches on the rod body. Five steel wires break in D2L3-2 and one wire breaks in D2L3-3, which is in line with the change trend shown in Figure 8c.