Keywords: railway, rail, straightness, quality control, noise, vibrations, ballast.

Rail manufacturing, even with the most advanced technologies, leads to bars which inevitably have a residual deviation from the perfectly straight shape. These deviations, that should not be confused with the phenomenon of corrugation that happens in service mainly in tight curves, have wavelengths longer than 1 m and can result in high groundborne vibrations, ballast settlement as a result of excessive superstructure vibrations, excessive stresses in the bogies, booming noise at rather low frequency. Rails are produced by hot rolling followed by cooling (either natural or artificially accelerated) and a final straightening with a set of rollers. Straightening, which induces noticeable residual stresses and may have negative effect on fatigue and fracture properties [1,2], is requested because differential cooling between the rail head, the rail web and the rail foot inevitably leads to distortions that can not be compensated during track lay down.

Checking the rail shape during the straightening process is not easy, as long as the rail must be checked "on the fly" while passing through the rollers and the rail surface is contaminated by iron oxides (calamine). Current international standards [3] define the maximum allowed deviations from a straight line but without any reference to possible periodicities (or "waviness") of the rail.

Straightness can be controlled only in service, with the rails installed in the track with the track unloaded, as measuring cars induce a global track vertical displacement (in the order of 1 mm) that is normally much larger than the original geometrical defect of the rail (in the order of 0.2~0.5 mm).

The paper describes a methodology that allows, by the simple use of a portable trolley, to check the straightness of rails in service identifying, if any, periodicities in the rail shape. The method developed consists of two main phases: the collection of reliable data [4] and their analysis. The second step consists of an original signal processing procedure, that uses simple and well established techniques, that allowed to precisely identify the various components of waviness. Results from different measuring campaigns are critically compared showing that the residual waviness after straightening depends on each rolling plant.

1

C. Betegón Biempica, J.J. del Coz Díaz, P.J. García Nieto, I. Peñuelas Sánchez, "Nonlinear analysis of residual stresses in a rail manufacturing process by FEM", Applied Mathematical Modelling, 33, 34-53, 2009. doi:10.1016/j.apm.2007.10.015

2

K.H. Lo, P. Mummery, D.J. Buttle, "Characterisation of residual principal stresses and their implications on failure of railway rails", Engineering Failure Analysis, 17, 1273-1284, 2010. doi:10.1016/j.engfailanal.2010.03.001

3

EN 13674-1:2003+A1, "Railway applications - Track - Rail - Part 1: Vignole railway rails 46 kg/m and above", 2007.

4

A. Bracciali, F. Piccioli, T. De Cicco, "Measurement and analysis of mid wavelength rail irregularity", Proceedings of Railway Engineering 2009 Conference, London, 24-25 June 2009.

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