# Fracture Threshold Analysis and Parameter Matching of Cut-Out Induced Bolts for Subway Couplers
## 1 Introduction
[16] proposed a novel triangular headed one-sided bolt and studied the performance of T-stubs using triangular headed one-sided bolts through numerical and experimental methods. They also provided an analysis of the factors influencing the strength of T-stubs using triangular headed one-sided bolts. [PERSON] et al. [17] studied the load-bearing capacity and fracture behavior of superalloy bolts under various loading conditions. By comparing the ultimate loads of bolts under combined tensile-bending loads in experiments and simulations, they validated the effectiveness of the finite element model. [PERSON] et al. [18] investigated the influence of different arrangements of Q690 high-strength bolts on the failure modes and ultimate load-bearing capacity of connection structures through static tensile tests and numerical simulations. [PERSON] [19], [PERSON] [20], and [PERSON] [21] all used a combination of experimental and simulation methods to study specific bolted connection structures. There are many factors that affect the axial impact resistance performance of bolts, in addition to basic material and diameter; studies have shown that the impact resistance performance of bolts is also related to the bolt preload and the length of the connecting threads [22; 23; 24]. The tensile failure of bolts mainly includes two forms: thread failure and bolt shank failure [16; 25]. When the bolt experiences thread failure, its breaking force value will be much lower than the force required to break the smooth shank of the bolt, which is contradictory to the need for a controllable and stable breaking force value in overload protection devices. In the study of the dynamic mechanical properties of thin-walled structures, controlling deformation through inducing structures has been proven to be reliable [26; 27; 28]. Based on this design concept, cut-out induced bolts have emerged. These bolts weaken the strength of the cut-out zone by setting a cut-out zone on the smooth shank of the bolt, allowing tensile failure to occur at this location and providing better control over the breaking force value. [PERSON] et al. [29] studied the effect of preload on the cut-out induced bolt. The results showed that excessive preload force could cause deformation of the cut-out zone, thereby affecting the breaking force value of the bolt. It can be seen that reasonable cut-out zone structural parameters are an effective means to improve the stability of the fracture mode of the cut-out induced bolts and the reliability of the overload protection function. From the analysis of the current research status on the overload protection device, it can be found that the matching relationship between the fracture threshold of the overload protection device and the working force of the coupler energy-absorbing device is still unclear. When designing the fracture threshold of the overload protection device, designers lack clear specifications and usually set the fracture threshold of the overload protection system slightly higher than the working force of the coupler energy-absorbing device. The overload protection device designed in this way tends to exhibit premature failure during impact tests. Therefore, this paper adopts a method combining simulation and experimental verification to investigate the influence of structural parameters of the cut-out induced bolt's cut-out zone on the fracture threshold, reveal the mechanism behind the premature fracture phenomenon of the overload protection device, and clarify the matching relationship between the fracture threshold of the tensile break bolt-type overload protection device and the working force of the coupler energy-absorbing device. Based on the aforementioned research background, this paper begins with a cut-out induced bolt impact test and investigates the reasons for premature fracture of the cut-out induced bolts during the test. A finite element simulation model aligned with the test was established.