# Study on Mechanical Properties and Application of a New Flexible Bolt
## 5 Cyclic Mechanical Property of Flexible Bolts  
The friction between rod body and locking casing has a direct impact on the mechanical property of flexible bolts. Therefore, before the engineering application, it is necessary to study further the mechanical characteristics of flexible bolts under cyclic dynamic loading. An MTS electrohydraulic servo system was used to conduct cyclic loading-unloading tests to two groups of flexible bolts (Group A and Group B). The diameter and length of the locking casing are 30 mm and 120 mm, respectively (same as D1L2). The cyclic loading was divided into two modes: equal-lower-limit-and-equiamplitude loading, and increasing-lower-limit-and-equiamplitude loading. The mechanical responses of the two loading modes are shown in Figures 13 and 14, respectively. Figure 12: Tail structure of the flexible bolt and the situation where the locking casing is pulled out: (**a**) the components of the tail structure of flexible bolts, (**b**) a cross-sectional view of the tail structure after extrusion, (**c**) the failure mode when locking casing is pulled out, and (**d**) spring segments. Figure 13: Tensile resistance–displacement curve of equal-lower-limit-and-equiamplitude cyclic loading. As is shown in Figure 13, a single tensile resistance-displacement curve is added as a reference to the result of the noncyclic static tensile test. The test results indicate that the tensile resistance- displacement curve is divided into three stages: quasi-elastic stage (OD), yield stage (DE), and breaking stage (EF). The yield resistance and peak resistance are 288 and 340 kN, respectively. The equal-lower-limit-and-equiamplitude cyclic loading tests were conducted with flexible bolts (numbered as A1-A4). The loading mode follows: when the tensile force upon the flexible bolts (A1-A4) reaches 280, 300, 320, and 340 kN, respectively, cyclic loading is initiated with an amplitude of 200 kN; the lower limits of tensile force upon the four flexible bolts (A1-A4) are 80, 100, 120, and 140 kN, respectively. Cyclic loading is repeated 50 times; when cyclic loading is finished, tensile force is exerted on the bolts continuously until they break. As can be seen from Figure 13, the upper limit of cyclic loading on the A1 bolt is located in the quasi-elastic stage; most of the deformation can be restored and the relative displacement corresponding to the cyclic loading-unloading stroke reaches 1.4 mm. The residual deformation is mainly ascribed to the fact that the steel-strand rod body is tensioned. The upper limits of cyclic loading on A2-A4 bolts are all located in the yield stage; the deformation can still be partially restored and the relative displacements corresponding to the cyclic loading-unloading strokes finally reach 2.2, 3.7, and 6.0 mm, respectively. As the upper limit of cyclic loading increases constantly, the residual deformation rises correspondingly, whereas the resilience declines gradually. The ultimate peak tensile forces of A1-A4 bolts are 350, 348, 335, and 354 kN, respectively, with a mean peak force of 347 kN. Thus it can be seen that in contrast to the flexible bolts without cyclic loading, the peak resistance of the flexible bolts that undergo cyclic loading does not vary much, although it displays a slight rise. This indicates that cyclic loading does not change fundamentally the stress state within the bolt, which in turn makes it possible for the bolt to maintain its mechanical property. The gradual changes in loading-unloading amplitude, upper limit, and lower limit demonstrate that the mechanical property of flexible bolts is determined by the upper limit of cyclic loading, whereas the unloading amplitude and lower limit do not affect the mechanical property significantly. Furthermore, it proves that the support system of flexible bolts is not affected much by the stress adjustment and small impact disturbance caused by the roadway excavation. Figure 14 displays the tensile resistance-displacement curve of increasing-lower-limit-and-equiamplitude cyclic loading.