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The squat, a kind of rolling contact fatigue occurring on the rail top, can excite the high-frequency vehicle–track interaction effectively due to its geometric deviations with a typical wavelength of 20–40 mm, leading to the accelerated deterioration of a track. In this work, a validated 3D transient finite element model is employed to calculate in the time domain the vertical and the longitudinal dynamic contact forces between the wheel and the rail caused by squats. The vehicle–track structure and the wheel–rail continua are both considered in order to include all the important eigencharacteristics of the system related to squats. By introducing the rotational and translational movements of the wheel, the transient wheel–rail rolling contact is solved in detail by a 3D frictional contact model integrated. The contact filter effect is considered automatically in the simulations by the finite size of the contact patch. The present work focuses on the influences of the length, width and depth of a light squat on the resulted dynamic contact forces, for which idealised defect models are used. The growth of a squat is also modelled to a certain extent by a series of defects with different dimensions. The results show that the system is mainly excited at two frequencies separately in the vertical and the longitudinal dynamics. Their superposition explains the typical appearance of mature squats. As a squat grows up, the magnitude of the excited vibration at the lower frequency increases faster than the one at the higher frequency. [ABSTRACT FROM AUTHOR]

Abstract: A phenomenological investigation of squat defects on rail grade R260Mn is performed. The surface-breaking 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. [Copyright &y& Elsevier]

Abstract: This paper presents evidences obtained by field monitoring, measurement and survey to show the validity of some numerical predictions about squat growth. The predictions concerns a postulated squat growth process, the relationship between the dynamic contact force and the corrugation-like wave pattern that often follows squats, the high frequency wheel–rail interaction related to squats, and the influence of tangential force on squat growth. The observations reveal signature tunes of squats which may be used for early detection of squats, show the necessity to include high frequency dynamic wheel–rail interaction in squat-related analyses, and provide evidence of relationship between rolling stock performance and squat initiation and growth. In validating the numerical results the model is also verified for its applicability to analyses of squat-related problem and other problems similar in nature. The model can be employed for the solution of three-dimensional frictional rolling contact problems. It can also be used for analyses of loading conditions of wheel–rail contact at short wave defects, and the associated damages such as wear, plastic deformation, fatigue and corrugation. [Copyright &y& Elsevier]