# Fracture Threshold Analysis and Parameter Matching of Cut-Out Induced Bolts for Subway Couplers
## 1 Introduction  
The orderly and controllable energy absorption of the coupler buffer device can effectively reduce casualties in train collision accidents. The role of the overload protection device is to promptly disconnect the coupler from the vehicle body, allowing subsequent anti-climbing energy absorption devices to come into contact and absorb energy. In the collision accidents, Figure 1 indicates that the impact kinetic energy is absorbed in thefollowing order: shock absorbers, crushed tube, anti-climbing energy absorbers, and the train body [1; 2; 3; 4]. The purpose of the overload protection device is to disconnect the coupler (which includes the shock absorber and crushed tube) from the train body after the crushed tube has completed its energy absorption, allowing the coupler to freely retract. Otherwise, the long, slender structure of the coupler can act like a lever, lifting the train and potentially causing climbing between vehicles or derailment [5; 6; 7]. Additionally, if the coupler cannot retract freely, the high-strength coupler can impede the contact of the train's anti-climbing energy absorption device, interfering with the normal operation of the collision energy management system [8; 9]. Currently, the forms of overload protection devices on couplers mainly include drum sleeve type, shear break bolt type, tensile break bolt type, and aluminum alloy plate fracture type [9; 10]. The drum sleeve-type overload protection device requires the installation of guide rails during the installation process and needs special tools to apply pre-tightening force. Improper installation can lead to large errors in pre-tightening force, making it inconvenient for practical use [9]. [PERSON] et al. [9] studied the aluminum alloy plate fracture-type overload protection device by adding a cut-out zone and optimizing the circular cross-section, successfully reducing the breaking force value, and found that the method of optimizing the circular cross-section was more effective than adding a cut-out zone. [PERSON] et al. [10] investigated the effects of impact kinetic energy, bolt preload, and transition end structure on the aluminum alloy plate fracture-type overload protection device. Although the aluminum alloy plate fracture-type overload protection device has the advantages of a simple structure and convenient installation, there is currently no set of general design guidelines, nor a general relationship between the transition section design and the breaking force value, that is not conducive to engineering applications. The shear break bolt type and tensile break bolt type are currently the most commonly used forms of overload protection devices. In this study, the tensile break bolt-type overload protection device was chosen as the research object. The research methods for bolts mainly include experimental methods and numerical simulation methods. [PERSON] et al. [11] studied the tensile properties of bolted end plate connections for structural angle sections using experimental methods. The results indicated that the primary failure mode of the structure was thread failure in the bolts. Additionally, [PERSON] [12], [PERSON] [13], and [PERSON] [14] all studied the mechanical behavior of bolts in various engineering structures under tensile conditions using experimental methods. [PERSON] [15] conducted tensile tests on bolts using four simulation models for modeling, including a  
Figure 1: Collision energy absorption sequence of train components. detailed model, simplified model with end linkages, simplified model with shell head and embedded end, with a hole, and simplified model with shell head and embedded end, without a hole. The results indicated that the simulation data most closely matched the experimental data when the diameter of the bolt shank was equal to the major diameter of the bolt and the diameter of the threaded region was equal to the minor diameter of the thread. [PERSON] et al.