# Damage Analysis of Rolling Stock Automatic Coupler under Cyclic Loads
## 1 Introduction  
Railways are one of the modes of transport for critical freight and passengers. Wagons and locomotives are connected using specially designed traction devices that maintain a fixed distance between the former and transmit and mitigate the effects of longitudinal (tensile and compressive) forces. Until 1934, screw couplings were used to connect wagons and locomotives. However, screw couplings were associated with multiple risks, including insufficient strength and a hazard to the railway worker who had to crawl between the wagons when coupling and uncoupling them. The coupling process was also slowed down by maneuvering due to the need for precise alignment of the axes of devices. Automatic couplers have eliminated the disadvantages of screw couplings. Since 1935, the Russian-type automatic coupler was adopted across Eastern Europe. Automatic couplers have provided less time-intensive coupling and uncoupling operations and increased freight and passenger train safety (Figure 1) [1,2]. However, with increasing rolling stock power and speed, loads may reach levels where stress values may exceed the proportionality limit of the material in the event of improper operation. This is acceptable if it happens only a few times during the service life. However, in the case of elastic-plastic strain, frequent overloading may cause low cycle fatigue [3, 4, 5]. With increasing equipment power and speed, the actual stress values occasionally exceed the proportionality limit of the material. Under the action of random intensive loads, equipment elements and structures fail due to fatigue damage of various types. The accumulated damage develops as a result of high-cycle and low-cycle fatigue associated with cyclic overload, which causes plastic strain [6; 7]. Usually, the stress and strain fields are distributed unevenly in the parts. In the concentration zones, the stresses exceed the proportionality limit of the material. Only an approximate prediction of stress distribution of the unique structures that make up a spatial body is possible using analytical methods. However, inaccuracies of this kind related to responsible parts may lead to accidents, while excessive safety factors increase the weight of the structure and consumption of the material. It is therefore reasonable to use more accurate numerical methods to determine the stress and strain distribution in complex structures [8]. An automatic coupler of a rolling stock is a typical example of such a loading case. The automatic coupler of a rolling stock is a crucial part of determining passenger, freight, and environmental safety. Material is an important factor in the calculation of the durability of the coupler. Prior to the investigation of the remaining service life of the coupler before the macro-crack initiation, the stress-strain state must be calculated. However, stresses are calculated using analytical formulae, where changes to the geometric shape are evaluated using stress concentration factors. Given the complexity of the design of an automatic coupler, certain calculation errors may be assumed [9; 10]. The failure statistics for automatic couplers on the railway rolling stock have suggested that the failure of automatic couplers usually tends to occur at the transition between the head and the tail [11; 12]. The authors of the papers referred to above propose increasing the low-cycle fatigue resistance of the automatic coupler by selecting materials with higher yield strength \(\sigma_{y}\) or with higher plasticity (by increasing \(\psi\)). This method involves an alteration in the chemical composition of the material, i.e., it is based on the choice of steel grade and heat treatment. One work [11] proposes a damage accumulation model for an automatic coupler tail using the damage linear summation hypothesis for low-cycle fatigue.