# Fracture Threshold Analysis and Parameter Matching of Cut-Out Induced Bolts for Subway Couplers
## 2 Experiment and Simulation Analysis
### Impact Experiment
#### 2.1.3 Results Analysis  
According to the data transmitted back by the speed sensor, the speed of the test bench when it impacted the rigid wall was 7.504 m/s. The high-speed photography camera recorded the displacement-time image of the coupler collision process, as shown in Figure 5. Combining the high-speed photography images with the force-time curve, T = 0 ms when the couplers were in contact according to the high-speed photography, at which point the contact force between the couplers was zero. When T = 8.2 ms, the expansion tubes began to crush, and the contact force between the couplers fluctuated around 1200 kN. When T = 21.2 ms, the contact force between the couplers was about to rapidly drop to zero, so it was judged that the bolts break at this time. When T = 24 ms, the broken bolts can be clearly observed flying out from the high-speed photography images. When T = 40 ms, the couplers completely retracted and separated from the rigid wall, and the overload protection devices on both sides were in contact with the rigid wall.  
The force sensor on the rigid wall recorded the corresponding force-time data, which were filtered to obtain the data shown in Figure 6. From the force-time data, it can be observed that the data from the force sensor are zero between T = 2.9 ms and T = 6.9 ms. This may be due to the presence of a certain gap between the internal components of the coupler buffer and the buffer housing. During the loading process, this gap needs to be consumed first, leading to pressure unloading.  
The appearance of the bolts after the test is shown in Figure 7. It can be observed from the figure that the bolts experienced axial tensile necking at the cut-out zone, followed by a process of breaking at the necking surface, with a relatively flat fracture surface.  
Figure 4: Experimental setup.  
Figure 5: Time sequence diagram of experiment.  
Figure 6: Time–force curve of experiment.  
Figure 8 shows the force-displacement curve from the experiment, where the red segment represents the force-displacement characteristics of the expansion tube. It can be observed that the displacement of the expansion tube in the experiment is approximately 92.03 mm, which is much less than the designed effective displacement of 245 mm. At this stage, the cut-out induced bolts exhibit a fracture force of 1147.5 kN and a fracture duration of 20.9 ms. This is due to the insufficient strength of the cut-out induced bolts, causing the bolts to break prematurely during the collision, resulting in dangerous conditions in the experiment.  
However, it is also important to note that the strength of the cut-out induced bolt should not be too high. A too-strong bolt may fail to promptly disconnect the coupler from the vehicle body, causing the vehicle body to experience excessive impact force, which can lead to undesirable deformation of the vehicle body. Therefore, it is necessary to optimize the cut-out zone of the cut-out induced bolt to ensure that the bolt does not break during the normal energy absorption of the coupler but can quickly break after the coupler completes its energy absorption work.