# Fracture Threshold Analysis and Parameter Matching of Cut-Out Induced Bolts for Subway Couplers
## 4 Fracture Threshold Optimization
### Result Validation
#### 4.3.2 Experiment Validation  
After completing the optimization of the cut-out zone, a verification experiment was conducted. To verify whether the optimized bolts could withstand the extreme collision conditions in real vehicles, the test plan was also adjusted, with the test setup as shown in Figure 27. The test was adjusted to a two-coupler collision energy absorption test. Test coupler one was installed on a rigid wall, and test coupler two was installed on a test bench. The buffer in test coupler one did not have an overload protection bolt installed, while the buffer in test coupler two had cut-out induced bolts installed between the buffer and the coupler yoke. The two couplers were installed with a height difference of 20 mm. In this test, the mass of the test bench was 32 t, the initial speed of the test bench recorded by the speed sensor was 6.90 m/s, and the total impact kinetic energy was 761.76 kJ.  
Figure 26: Bol stress distribution time series diagram.  
The timing changes of the coupler collision were recorded by a high-speed camera as shown in Figure 28. The coupler was set to start contacting at T = 0 ms. When T = 26 ms, the buffers of both couplers were fully consumed, and the expansion tubes of the fixed-end and bench-end couplers began to act. When T = 72 ms, the expansion tube of the bench-end coupler stopped acting, and from this point on, only the expansion tube of the fixed-end coupler was consumed. When T = 95 ms, the expansion tube of the fixed-end coupler was fully consumed, and the expansion tube of the bench-end coupler started acting again. Unlike the previous test, in this test, the re-acting bench-end expansion tube successfully consumed the entire stroke. At T = 138 ms, all energy absorption devices of the two couplers were fully consumed, and the couplers entered the rigid phase. At T = 139 ms, the bench-end breakaway cut-out induced bolts broke, and the remaining kinetic energy was absorbed by the protective devices on both sides, causing the bench to rebound and the collision to end.  
The force-time situation of the test is shown in Figure 29, with Roman numerals corresponding to key moments of the collision: (1) T = 0 ms, where the coupler heads are fully coupled, and the collision begins; (2) T = 26 ms, where the buffers of test coupler one and test coupler two are fully consumed, and the expansion tubes are about to start acting; (3) T = 72 ms, where the expansion tube of test coupler two stops crushing, and at this point, only the expansion tube of test coupler one is absorbing energy; (4) T = 95 ms, where the expansion tube of test coupler one is fully consumed, and the expansion tube of test coupler two is about to start acting again; (5) T = 138 ms, where all energy absorption devices of the couplers are fully consumed, and the couplers are about to enter the rigid phase, at which point the bolts need to be sheared to protect the vehicle body.  
In this coupler coupling collision test, it is clear in the high-speed photography images that the breakaway bolts of the coupler broke shortly after the two expansion tubes had fully consumed their strokes. Table 4 shows that the fracture force was 1828.44 kN, differing from the fracture force obtained from the simulation calculation by 1.48%.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Surrogate Model** & **Simulation** & **Experiment** \\ \hline Fracture Force (kN) & 1894.13 & 1855.59 & 1828.44 \\ Deviation from Experiment & 3.59\% & 1.48\% & / \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of the fracture force of cut-out induced bolts.  
Figure 27: Experimental setup.  
met the requirement for timely fracture in the coupler's energy-absorbing components under overload conditions, thereby enabling the coupler's energy absorption to satisfy the requirements specified in EN 15227 for C-II class vehicles at 25 km/h impact scenarios [36].