PSI - Issue 46

T.J. Gschwandl et al. / Procedia Structural Integrity 46 (2023) 17–23 T.J. Gschwandl et al. / Structural Integrity Procedia 00 (2021) 000–000

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maintenance of rails and turnouts (Stock et al. (2019)). The mechanisms of squat initiation and growth are still not fully clear; yet, there are several theories for the initiation of squats in rails and their propagation. (Al-Juboori (2020), Jörg et al. (2015), Naeimi (2020), Pal et al. (2013)) Squats can be classified as a Rolling Contact Fatigue (RCF) defect and they have first been observed in the 1950s in Japan as ‘black spots’ or ‘shelling defects’ (Ito and Kurihara (1965), Li et al. (2011), Nakamura et al. (1965)). Later, in the 1970s, defects called squats were detected in the British railway network and in France (Grassie (2011), P. Clayton and M. B. P. Allery (1982)). Thereby, the crack formation on the running band has a distinctive shape. Many describe the appearance ‘as if a heavy gnome sat or squatted on the rail leaving two similarly sized lobes behind’ (Grassie (2011)). Squats typically start with an initiating crack or defect and then continuously grow a few millimeters below the running surface. Generally, there are several stages of severity. In an earlier stage, the defect looks like a dark depression on the rail head. The final stage is the breakout of the material and hence the total failure of the rail (Li (2009)) – see Fig. 1.

Fig. 1 Top view of a squat rail defect in track, which started to grow from a V crack at the gage corner and propagated below the surface leading to a breakout of the material. The rolling direction of the train was from left to right (© Photo voestalpine Rail Technology GmbH). Hitherto, several factors, which promote squat formation have been determined. The three most relevant parameters are the crack initiation point, the residual stress state as well as the contact loading. Thereby, the initiation point marks the location on the rail head, where a crack is most likely to start. Furthermore, it denotes the position of a small crack with initial length and the crack angles with respect to the rail head and rolling direction. Another aspect is the contact loading of a rail, such as the static wheel load, the resulting contact patch or the traction direction. The third major influencing factor is the residual stress state of a rail. These factors drive damage within new rails in the course of their lifetime and all may significantly promote squats. Within this work, the focus is set on the residual stress state of pearlitic rails. Note that a rail already builds up residual stresses during the production process. First, stresses are induced during the cool down phase during production. Thereby, the stresses arise due to temperature differences in the varying rail sections. Following, the final bending and straightening process also induces further residual stresses. These factors enable the build-up of a distinctive stress distribution within the rail after manufacturing. Additionally, the residual stress situation within the rail is changing by operational loads. As a result, a compressive stress area forms in the rail head and shifts the prevailing tensile stress to lower regions of the head. Fig. 2 schematically shows the change of the residual stress curve from a new rail to a rail in service (Jörg (2010), Lichtberger (2010)). This work comprises the numerical evaluation of the stress state of a new rail with a Finite Element Analysis (FEA). Following, measurements of the residual stresses of a damaged rail with squats have been performed. For the measurement, two different methods, namely the contour method and the X-ray diffraction method, have been applied.