Fracture Threshold Analysis and Parameter Matching of Cut-Out Induced Bolts for Subway Couplers

The overload protection device is crucial in ensuring the orderly absorption of kinetic energy by the coupler buffer device. This paper studies an overload protection bolt with a cut-out zone. In the bolt impact experiment, a premature fracture of 10.9-grade M24 bolts was observed. Based on the analysis of the results, it was concluded that this phenomenon was caused by the mismatch between the mechanical properties of the bolts and the dynamic performance of the coupler. Building on this test, a numerical simulation model was established and subsequently validated. The width and depth of the inducing structure were selected as the research objects. Using the Latin Hypercube method, 78 sets of cut-out zone structure parameters were generated, and numerical simulations were performed on the cut-out induced bolts. The simulation results indicate that the peak force generated by the coupler collision leads to necking in the cut-out induced bolts, which consequently weakens their mechanical properties to some extent. Therefore, it is necessary to consider a strength margin when designing cut-out induced bolts. Based on the simulation results, a surrogate model was constructed, and the optimal bolt cut-out zone was obtained through optimization: a width of 17.74 mm and a depth of 1.37 mm. The surrogate model predicted a fracture force of 1894.13 kN for the bolts. An impact test was conducted to verify the performance of the optimized cut-out induced bolts. The experimental results showed that the cut-out induced bolts broke after the crush tube completed its kinetic energy absorption, with a fracture force of 1828.44 kN, which was a 3.59% difference from the predicted value of the surrogate model. After optimization, the fracture force of the cut-out induced bolts increased from 1147.5 kN to 1828.44 kN (a 59.34% improvement), while the fracture time extended from 20.9 ms to 69 ms, fully meeting the design requirements of the overload protection device.