# 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 O30 mm Locking Casing  
Figures 6 and 7 show the mechanical response and failure mode of flexible bolts with O30 mm locking casing. As Figure 6a shows, when the length of 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 then a slight increase for a long time, and finally oscillation fall. Before the bolt displacement reaches 15 mm, all three samples reach their respective peak resistances, but their peak resistances vary remarkably; the peak resistance of D1L1-1, D1L1-2, and D1L1-3 is 170 kN, 96 kN, and 236 kN, respectively. The higher the peak resistance, the greater the resistance decline. Meanwhile, the mechanical response of the three samples during the final failure is different from each other. The resistance change curves also demonstrate that the bolt performance is unstable when the length of locking casing is 100 mm. When the displacement of D1L1-2 sample reaches 55 mm, its resistance remains unchanged; in other words, the bolt is in a constant resistance state. This indicates that the locking casing is being pulled out at a constant speed and the friction between the locking casing and rod body sets off the resistance. However, in the final test phase of D1L1-1 and D1L1-3 samples, the resistance experiences an oscillation fall, indicating that the spring in the locking casing is breaking gradually, which in turn causes the constant change of friction between the locking casing and rod body. From Figure 7a, it can be seen that the steel-wire rod body does not fail; the locking casings of the three samples are pulled out, leaving long and fresh scratches on the rod body. The displacement in the test is mostly the relative displacement between the locking casing and rod body. The length of scratch on the rod body is naturally the length of relative displacement. From Figure 6b, it can be known that when the length of the locking casing of the three bolt samples is 120 mm, the trend of the resistance change of the three samples is approximately the same and the peak resistances are quite close to each other. Before the bolt displacement reaches 20 mm, the bolt resistance increases linearly with the displacement and the bolt is in quasi-elastic stage. When the displacement exceeds 20 mm, the bolt is in a yield stage and the increase of bolt resistance slows down remarkably compared with that of the quasi-elastic stage. Finally, the resistance reaches its peak resistance. The peak resistance of D1L1-1, D1L1-2, and D1L1-3 is 332, 338, and 351 kN, and the corresponding displacement is 24.65, 32.12, and 31.24 mm, respectively. After reaching the peak resistance,  
Figure 5: Test equipment: (A) denotes the rod body, (B) refers to the locking casing, (C) indicates the steel lagging, and (D) shows the nut. breakage occurs in the bolts of all three samples. New characteristics appear in the failure mode and mechanical response. To be specific, sequential breakage of steel wire is the main characteristic. Each breakage of the steel wire is followed by a slight rise of bolt resistance, but the resistance is smaller than the peak resistance of the preceding breakage. In other words, after each breakage, the peak resistance reduces by stage. Meanwhile, the number of stage peak resistances also denotes the number of broken steel wires. Take D1L2-3 as an example, there are four stage peak resistances in bolt resistance, indicating that there are four steel wires breaking during the static tensile test. From Figure 7b, it can be known that the characteristics of bolt failure change when the length of locking casing increases. The failure mode turns from locking casing being pulled out to steel wire breaking by stage.