# Effect of the Depth of Decarburized Layer in SKL15 Tension Clamp on Fatigue Strength
## 4 Conclusions  
The depth of the decarburized layer of the tension clamp in which the fatigue crack occurred was measured, and the depth of the decarburized layer at which a fatigue crack could occur was confirmed via a LEFM-based method. Through this study, the following conclusions were drawn.  
1. As a result of measuring the depth of the decarburized layer of eight tension clamps with fatigue failure via the hardness measurement method, the distribution of measured depth ranged from 0.17 to 0.36 mm. All but one had a decarburized layer depth greater than 0.2 mm, which is the maximum allowable decarburized layer depth suggested by the manufacturer, and the fatigue failure occurred even in the tension clamp with a decarburized layer depth thinner than the allowable depth.  
Because the fatigue strength of the decarburized layer is lower than that of the base metal, the result of crack growth analysis based on linear elastic fracture mechanics under the assumption that the decarburized layer is an initial crack successfully confirmed whether the fatigue crack occurred according to the depth of the decarburized layer. The stress state at the location of the fatigue crack occurring due to the passage of a train was calculated by finite element analysis, and \(\Delta K_{th}\) for the stress state at that location was 1.74 MPa\(\surd\)m. In the case of the measured minimum depth of the decarburized layer of 0.17 mm, \(\Delta K_{l}\) generated by the passage of a train was 3.65 MPa\(\surd\)m. Thus, it was well explained that the fatigue failure occurred despite the fact that the depth of the decarburized layer was thinner than the manufacturer's allowable depth of the decarburized layer.
3. By measuring the depth of the decarburized layer of the tension clamp where the fatigue crack occurred and calculating \(\Delta K_{l}\), it was confirmed that it was impossible to prevent the occurrence of fatigue cracking because the depth of the maximum allowable decarburized layer suggested by the manufacturer is too thick. The maximum allowable depth of the decarburized layer of SKL15 on the route on which the survey was conducted should be equal to or less than 0.04 mm. However, the stress range at the crack location was also excessive, so stress range should be controlled.
4. The maximum allowable depth of the decarburized layer suggested above is applicable only to the SKL15 tension clamp of the corresponding route and cannot be applied to routes with different operating conditions or to other types of tension clamps or rail clips. However, the approach used in this study could be used to determine the allowable depth of the decarburized layer of other types of tension clamps or rail clips.  
Conceptualization, Y.-C.P.; methodology, Y.-C.P.; validation, H.-B.S.; formal analysis, C.-B.A.; investigation, Y.-C.P., H.-B.S.; resources, M.K.; writing--original draft preparation, Y.-C.P.; writing--review and editing, H.-B.S.; visualization, C.-B.A.; supervision, H.-B.S.; project administration, M.K.; funding acquisition, H.-B.S. All authors have read and agreed to the published version of the manuscript.  
This work was funded by Incheon National University (International Cooperative) Research Grant (2016-2216).  
The authors declare no conflict of interest.