# Study on Mechanical Properties and Application of a New Flexible Bolt
## 4 Static Tensile Test Results and Mechanical Analysis of Flexible Bolts
### Analysis and Discussion of Test Results
#### 4.4.2 Mechanism of Action between Locking Casing and Rod Body  
Figure 12 illustrates schematically the tail structure of flexible bolt and the situation where the locking casing is pulled out. As shown in Figure 12a, the tail structure of the flexible bolt is composed of locking casing, spring, and rod body. The locking casing and spring are extruded together onto the rod body by the extrusion machine (as shown in Figure 2a). Figure 12b presents a cross-sectional view of the tail structure after extrusion. Under the action of extrusion force, the locking casing would deform plastically, thus producing great holding force on the rod body. Meanwhile, since the cross-section of the spring is triangular, it will embed into the inner side of locking casing and the surface of rod body, increasing the friction between locking casing and rod body, and thus guaranteeing that the locking casing will not slip. Since the rigidity of locking casing is much lower than that of flexible bolt, the spring will not do serious damage to the rod body. The results of static tensile test also illustrate that the damage done by the spring does not influence the bearing capacity of flexible bolts. [PERSON] et al. (2020) conducted static tensile tests through which the peak resistance of cable was obtained to be approximately 341 kN. By contrast, the average peak resistance of D1L1 flexible bolts is 340 kN. Thus, it can be concluded that the damage done by the locking casing to the bolt is very slight.  
Figure 12c shows the failure mode when the locking casing is pulled out. As can be known from Section 4.3, when the diameter of the locking casing is too large or the length is too short, the mechanical property of flexible bolts becomes unstable. When the peak resistance is reached, the locking casing can be pulled out easily. In the course of locking casing being pulled out, the spring breaks into some small segments (as shown in Figure 12d), indicating that the spring is indeed embedded into the inner side of the locking casing after the extrusion process and produces greater friction. However, when the diameter is 42 mm or the length is 100 mm, the friction between locking casing and rod body is insufficient to resist greater tensile force. With proper specifications (length: 120 mm; diameter: 30 mm), the steel wires break in turn and the locking casing is not pulled out. This  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Maximum Resistant Force**} & \multicolumn{2}{c}{**Average**} & \multicolumn{2}{c}{**Displacement**} & \multicolumn{2}{c}{**Average**} & \multicolumn{2}{c}{**Elongation of**} \\  & & **(kN)** & & **Resistant Force** & \multicolumn{2}{c}{**Corresponding to**} & \multicolumn{2}{c}{**Displacement**} & \multicolumn{2}{c}{**Flexible Bolt**} \\  & & & **(kN)** & & **Maximum Resistance (mm)** & & **(mm)** & **(\%)** \\ \hline D1L1 & 170 & 96 & 236 & 167 & 12.63 & 8.44 & 14.88 & 11.98 & 2.0 \\ D1L2 & 332 & 338 & 351 & 340 & 25.15 & 32.12 & 31.24 & 29.50 & 4.9 \\  
failure mode reflects the most stable mechanical property of flexible bolts, thus enabling the flexible bolts to resist the great deformation in the roadway surrounding rock.