# A Fatigue Reliability Assessment for Rail Tension Clamps Based on Field Measurement Data
## 1 Introduction  
The rail fastening system firmly connects the rails to the sleepers and its performance affects the structural safety of rails, ride safety and quality, and railway noise. The modern rail fastening system deviates from simply clamping the rail and sleepers together by using tension clamps to resist the tension, compression, and rotation of the rails that derive from the train load or track alignment. A rail fastening system generally comprises a guide plate, tension clamp, and anchor bolt, as shown in Figure 1. Among these components, the tension clamp should be sufficiently durable because safety while driving a train cannot be assured if the clamps are damaged or fractured. However, except for the mechanical properties of the tension clamps provided by the manufacturer, experimental or analytical studies conducted on the tension clamps are insufficient [1; 2; 3]. The behavior of the rail fastening system can be affected by various factors, such as the track condition, track alignment, and initial construction quality. Due to insufficient consideration of these factors, tension clamps often fail earlier than the service life guaranteed by the manufacturer [1; 4; 5; 6]. Various studies [7; 8; 9; 10; 11; 12; 13; 14] have been performed to evaluate the fatigue damage to the rail fastening system through finite element analyses (FEA). [PERSON] et al. [7], [PERSON] et al. [8], and [PERSON] et al. [9] investigated the fatigue failure and fatigue life of rail clips using FEA. [PERSON] et al. [10] and [PERSON] et al. [11] analyzed the fatigue damage and life through FEA with the material and fatigue properties obtained by experimental studies. [PERSON] et al. [12] revealed a dynamic failure mechanism of rail fastening clip damage through FEA, and [PERSON] et al. [13] analyzed the effect of the decarbarized layer on fatigue cracks using finite element models. [PERSON] et al. [14] established a random dynamic model of vehicle-track coupling using finite element models and identified the effects of the wheel-rail randomload on the fatigue damage of rail fastening clips. However, the reliability levels of a rail fastening system that has been damaged by fatigue have been rarely reported, except for two papers [15; 16] in which the statistical parameters of the equivalent stress ranges were estimated using the finite element analysis. The reliability levels of rail fastening systems need to be estimated accurately by field measurement data to provide a foundation in the reliability-based design and maintenance of the rail fastening system. One year and five months after opening the urban railway in South Korea, 0.17% of the SKL15 tension clamps in the DFF-300 rail fastening system had been damaged. The SKL15 tension clamps were made from a spring steel, which had the minimum yield and tensile strengths of 1150 MPa and 1300 MPa, respectively [6]. As shown in Figure 2, the fracture surfaces of the damaged tension clamps showed the typical characteristics of a fatigue crack, which has an elliptical shape and a smooth surface. All of the cracked tension clamps were observed on the railroad with a curve radius (\(R\)) of 1200 m or less. Specifically, 79.1% of the damage occurred on the railroad with a curve radius of 800 m, and 94.5% of the cracked tension clamps were installed on the inner rail. The inner and outer rails can be divided into the inside and outside track gauges at the curved sections. Approximately 56% of the damage was found on the inside track gauge; Table 1 summarizes the statuses of the cracked tension clamps. Since the failure of the tension clamps is caused by fatigue, a fatigue reliability analysis was performed where most of the cracks occurred and the reasons for the tendency of fatigue cracks in the inner and outer rails were investigated. Figure 1: The DFF-300 rail fastening system: (a) An in situ photograph; (b) the main components. Figure 2: A fractured tension clamp.