IntroductionIt is reported by researchers that the contact of the rail operates in a very arduous environment. This is usually characterized by extreme physical efforts thus reducing the operational efficiency of the system as a whole (Pook, 2007).
In the process of manufacturing, testing and installation or even operation, the surfaces are exposed to possible damage (P. C Paris, 1961). The rail’s surface may contain solid contaminants that can cause damage especially when rolled over by the wheels. Track ballast, wear debris, particles used for braking and other transported products are some solids that put the train at risk (Dwyer-Joyce, 2003).
Therefore, it is very necessary to explore the ability of the railway wheel or even other track sections to withstand or tolerate damage and surface indention. This initiates the concern engineers to systematically analyze railway track fatigue since this is also a progressive and localized process that severely damages various structures or cause fatal accidents if not keenly monitored. This kind of analysis is usually attributed to materials subjected to cyclic loading (Stephens, 2001).Railway track fatigues are not some news especially to the engineers and even to the world since they have sealed their place in history.
For instance, back in 1842, a train that was reportedly carrying revellers from Versailles derailed and caught fire with the resulting conflagration mutilating the dead beyond recognition. Further reports indicated that this derailment was indeed as a result of broken locomotive axle thus sensitizing experts on the significance of stress concentration and other related effects from repeated loading. In 1998, Eschede train disaster which is arguably the world’s deadliest train accident killed over 100 people yet it was very possible to eliminate such an occurrence since it was just but a single fatigue crack pronounced on a single wheel that eventually made the train derail at certain points when it failed. The past decade has seen railway infrastructure increasingly suffer from the damages due to this contact fatigue where wheels and rails under high loads roll or slide between their contacts.
This therefore emphasizes on the importance of developing a detailed report on fatigue and the possible solutions to prevent reoccurrence of such fatal accidents in the future and in particular, protect this company’s reputation.Railway track systems have pre-stressed concrete sleepers as the major component. Structural engineers have in the recent past improved on the construction and even designs that have permissible flexural stress constrains thus, dynamic load effects from wheel-road interactions are analyzed using dynamic impact factor since those interactions can be treated as a quasi-static load. These allowable stresses are used to eliminate crack initiations.
Irregularities resulting from dipped rails or even flat wheels are closely monitored using the frequently recorded data on the several impact events. This is necessary because the irregularities are notably very uncertain and therefore a consistent, persistent and reliable data that is only possible through frequent monitoring. It is imperative to note that expensive and excessive maintenance result from the cracking developed in the concrete sleepers due to these irregularities.In the study of railway track fatigue, the size of particles that find their way in the wheel-rail contact becomes a crucial issue hence the necessity to study a balance force of the frictional force between the surfaces to strike the estimated risks involved.
The coefficient of friction and of course the diameter of the wheel may entrench particles as large as 100mm into the system. Usually, brittle materials get fractured beneath the wheel while ductile materials undergo plastic deformation. Laboratory experiments carried out by company engineers involving samples of solid particles pressed between numerous samples of rail steel in hydraulic loading machines reveal that surface damage is not only related to the size and type of material but also associates this damage to the way of deformation of the material and its dispersion as soon as it enters the contact zone (Gao, 2001). Consequently, the use twin disk machines to study abrasion process on the rail as caused by solid particles indicate that the particles are fractured as the wheel rolls over them.
In the process, some fragments entrained into the contact scratch the rail surface due to micro-slip exhibited by the rolling surfaces (Grieve, 2001). Sophisticated numerical analysis indicate that even a single scratch could cause very powerful vibrations on the wheel-rail surface and is also an initial step essential in rail corrugation whose wavelength depends mainly on natural frequencies of the track as well as wheel-set curving velocity.Track vibrations with natural frequencies, either in the vertical or lateral directions resulting into rail corrugation, are then studied in detail in the company laboratories. This involves the modification of Kalker’s rolling contact theories with non-Hertzian form to calculate and analyze frictional work density especially on the contact zones of the wheel-rail surface.
Frictional work density is then utilized to derive material loss per unit area, thus accounting for the lost surface material (Clayton, 1996). From the numerous calculations carried out by the engineers, repeated rolling of the wheel or continuous slip of the rail that result into accumulated wear has also been revealed to be responsible in changing the rolling direction and orientation of the wheel.From all the tests and experiments continuously done by the engineers, it is only the disc machine that shows no serious signs of surface dents on condition that no solid particles are entrained within the contact surfaces. This would otherwise create very high wear of the system as well as see the onset of fatigue earlier than anticipated (Grieve, 2001).
Recent technology has seen the development of three dimensional representation of the rail-wheel representation that improves the analysis of fatigue in a more realistic manner. These new methods do involve advanced geometry for translation of two dimensional cracks to those of the three dimensional form while retaining the speed of the latter. Experts also suggest a possibility of improving this model in the future so that it is possible to simultaneously analyze different conditions while observing the effect they have on crack growth rate (Dwyer, 2003). Continuously welded rails experience thermal and residual stresses that will require future models for better and accurate analysis.
Further experiments also indicate that larger cracks are subject to bending of the rail. In this kind of scenario, multiple train wheels add the effect of contact stresses hence causing the bending. Continued bending of the rail will enhance the development of the multiple cracks which do influence one another’s growth. It is thus essential that recent work and investigation focuses on the multi-crack models in the rail.
Current research carried out in various private and public institutions emphasize on the significance of understanding various processes in railway track fatigue and the factors affecting them so that powerful solutions are found. ReferencesStephens, Ralph I. (2001). Metal Fatigue in Engineering (Second edition ed.
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