# Effect of the Depth of Decarburized Layer in SKL15 Tension Clamp on Fatigue Strength
## 1 Introduction  
The tension clamp of a rail fastening system is a key element to tie the rail to the sleeper. The design of the tension clamp must prevent fatigue cracks because large numbers of tension clamps are installed in railway tracks and their failure significantly affects the safety of trains. However, the method of fatigue performance evaluation of tension clamps is not standardized; the manufacturer itself conducts a fatigue test and issues a performance report on the fatigue performance. The fatigue failure of tension clamps has been reported in several places. Recently, many SKL15 tension clamps, which are used in the DFF-300 fastening system in South Korea, have been reported to have experienced fatigue failures during service. Figure 1 shows a tension clamp, and its fatigue fracture surfaces, collected at the site of the failures. All tension clamps experienced the failure surface in the same position. The section with the fatigue failure occurred only in the section in which the curve radius was 1200 m or less among the total 12 curve sections in the route. As a result of an in-depth investigation of this fatigue failure, it was confirmed that the depth of the decarburized layer was deeper than that suggested by the manufacturer and excessive stress occurred during train passage [1].  
In the case of the tension clamp in the rail fastening system, heat treatment is performed to increase the strength of the metal, which could create the decarburized layer on the surface of the clamp. According to [PERSON] et al. [2], a decarburized layer occurred on the surface of the forged or quenched and tempered steel. This treatment caused decarburization on the surface, producing a soft layer with a lower strength than the base metal; that is,the decarburized layer. Generally, in the case of steel, the endurance limit, which is a stress range that does not cause fatigue cracking, is known to be about 30% to 60% of the tensile strength when a reverse bending load is applied [3]. However, as the carbon component, which is the main factor that determines the strength of steel, decreases, the decarburized layer has very low tensile strength compared to the base metal, so fatigue cracking occurs more easily than in the base metal. Thus, it is expected that the cracks generated in the decarburized layer will play a role in the initial crack, thereby lowering the fatigue strength of the base metal.  
The fracture of the tension clamp affects the driving safety of trains, in addition to the deterioration of the serviceability of the track system. Therefore, the early fracture of tension clamps due to the decrease in fatigue strength could result in serious accidents such as train derailments. For this reason, by accurately identifying the effect of the decarburized layer on the decrease in the fatigue strength of the tension clamp, it is possible to prevent the fracture of the tension clamp due to train passage. In this study, for a railway line in South Korea, the depth of the decarburized layer of SKL15, which recently failed due to fatigue, was measured, and the effect of the depth of the decarburized layer on fatigue strength was analyzed using linear elastic fracture mechanics (LEFM). Because the decarburized layer has a lower fatigue strength than the base metal, the depth of the decarburized layer is regarded as the initial crack. By comparing the stress intensity factor range (\(\Delta K_{l}\)) and fatigue threshold stress intensity factor range (\(\Delta K_{th}\)), we propose a maximum depth of decarburized layer at which the fatigue crack may not occur in the case of the route under investigation.