# Pull-Out Test and Mechanical Properties Analysis Based on Intelligent Bolt and Internet of Things
## 4 Test Result Analysis  
### Analysis of Pull-Out Effects of Bolts with Varying Anchorage Lengths  
After sorting out the monitoring data, four kinds of bolts with lengths of 12 m, 14 m, 18 m, 22 m and 32 m were selected for force analysis, and the relationship curve between the tension of the rod end and the displacement of the rod end was obtained, as shown in Figure 7.  
According to the diagram, the stress process goes through three stages:  
1. Quasi-elastic stage (OA): the rod end tension and the rod end displacement basically increase linearly, the plastic deformation of the bolt is small, and the rod end tension increases rapidly. The tension value under different anchorage lengths can be found. The load value of point A is the elastic limit. 2. Strengthening stage (AB): Different from the previous studies on a pull-out test of short bolts, it is found that when the bolt length is long, the curve shows obvious  
Figure 6: Field pull-out test process. (**a**) Loading equipment installation; (**b**) Step by step control loading; (**c**) Bolt pull-out. Figure 7: Curve of tension and displacement of rod end. (**a**) The stress history diagram of anchor bolt; (**b**) The measured diagram of bolt stress history. strengthening characteristics. It is because when the length of the bolt is long, with the increase in the tension at the end of the bolt during the drawing process, the friction force, cohesive force and other resistance between the bolt and the rock and soil mass are not evenly exerted. The longer anchorage section enables the bolt to mobilize more rock and soil to participate in the bearing, so that more rock and soil are activated and play a bearing role, which leads to the continuous increase in the tension at the end of the rod and shows the strengthening characteristics. In addition, with the increase in the pull force of the rod end during the drawing process, the relative displacement between the anchor rod and the rock and soil mass increases, and the friction force also increases. The longer anchor rod provides a larger friction area, resulting in the rod end tension continuing to rise, and shows strengthening characteristics. At the same time, when the bolt length is long, the curve does not show the yield characteristics. This is because, compared with the short bolt, the stress distribution of the long bolt body is not uniform. With the gradual increase in the stress in each part during the drawing process, the whole bolt and rock and soil system gradually exert the bearing capacity, and there will be no stress concentration phenomenon, so that a certain part reaches the yield strength first. The synergistic effect between the longer anchor and the surrounding rock and soil is more significant, and the deformation of the anchor will be constrained and limited by the rock and soil, so that the anchor will not show obvious plastic deformation and the yield phenomenon, as will the short anchor under stress, but will show continuous strengthening characteristics. With the increase in the tension at the end of the bolt, the displacement of the end of the bolt increases rapidly, and the plastic deformation of the anchorage body also increases. 3. Failure stage (BC): After reaching the ultimate pull-out bearing capacity, the tension at the end of the rod continues to increase, and the anchorage body disengages and slips or the bolt body is destroyed. The tension value decreases rapidly, and the displacement at the end of the rod remains unchanged or increases. The failure mechanism of the failure stage mainly has the following two aspects: First, the anchorage body disengages and slips. During the pull-out process of the bolt, the load is mainly transmitted by the cohesive force between the bolt and the surrounding rock and soil. With the increase in the tension at the end of the rod, after reaching the ultimate uplift bearing capacity, the bonding force at the interface between the bolt and the rock and soil is gradually overcome.