On the mechanism of squat formation on train rails – Part I: Origination

May 11, 2015 in

Journal Paper


Author(s)
Michaël Steenbergen
Rolf Dollevoet


ISSN International Journal of Fatigue 47 (2013) 361–372
DOI http://dx.doi.org/10.1016/j.ijfatigue.2012.04.023

Theme(s)




Journal
International Journal of Fatigue


Publisher: Elsevier
Publishing date: May 11, 2012

Keywords
Rail crack, rolling contact fatigue (RCF), Shear stress, squat, White etching layer

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Abstract

A phenomenological investigation of squat defects on rail grade R260Mn is performed. The surfacebreaking crack pattern, which is either linear or branched (V-shaped), shows a typical position and orientation in the running band. Particular characteristics are asymmetry of this pattern, with the presence of a leading and a trailing branch, and crack reflection or deviation at the running band border. Bending tests reveal a 3D internal crack pattern, with a pair of crack planes or ‘wings’ enclosing a wedge at the surface. Microstructural analysis of the rail upper layer shows metallurgical principles of crack initiation: delamination and transverse fracture of white etching material at the surface. This analysis moreover reveals a 3D anisotropic texture of the upper layer under combined bi-directional tangential surface stresses. Mechanical interpretation of the crack morphology shows that the leading or single branch of the surface-breaking crack pattern is a shear-induced fatigue crack, following the anisotropic microstructure when growing into the rail. The trailing crack of a branched squat is explained as the result of a subsequent transverse, wedge-shaped brittle failure mechanism of the surface layer of the rail, developing within the actual elliptical Hertzian contact patch – or the envelope of potential contact ellipses at the leading crack position. It is driven by the transverse shear loading towards the rail gauge face.