Rolling Contact Fatigue of Rails: what remains to …

1 Rolling Contact Fatigue of Rails: what remains to be done? R A Smith Imperial College of Science, Technology and Medicine, London Introduction On 17 October 2000, a high-speed train, travelling at 185 km/hour, en-route from London King's Cross to Leeds, derailed when taking a 1460m radius curve, some 17km north of King's Cross. The rear eight vehicles of the eleven vehicle train were derailed, two coaches were turned almost on to their sides, whilst a third coach, a buffet car was very badly damaged. Of the 182 people on the train, four were killed and 70 suffered injuries, four of them seriously [1]. It rapidly became apparent that the derailment was caused by a fractured rail on the outer line of the curve. Of particular concern was that beyond the first fracture, the next 35 metres of rail had broken into 300 pieces, and some 44.

2 Metres further on, another length of about 54 metres was similarly fragmented. It was clear that the original and subsequent fractures had largely been triggered from Fatigue cracks existing in the rail : although the term gauge corner cracking . was used as the first description, head checking and the more generic Rolling Contact Fatigue (RCF) were used later. These events caused detailed inspections to be made of tracks throughout Britain in the days that followed. Many sites were located where cracks were visible on the surface of rails. Speed restrictions, some as low as 8 km/hour, were quickly introduced. Many trains were cancelled; schedules were revised to double or treble normal journey times and even then were unreliable.

3 Motorways became choked as people switched their journey to roads and internal domestic flights became overbooked. When a journalist wrote, no other railway accident in British history, or, I would guess, any other country's history has led to the degree of public anger, managerial panic, political confusion, blame and counter- blame that came in the wake of the Hatfield crash. In fact, outside wars and nuclear accidents, it is hard to think of any technological failure which has had such lasting and widespread effects [2], he was not exaggerating. It is not the purpose of this paper to enter into the debate about the extent to which the privatisation of Britain's railways, effective from Spring 1997, which replaced a single entity vertically integrated railway system, with a fragmented arrangement of over 100 major payers, played a part in these events [3].

4 It is sufficient to state here that the custodianship of the railway infrastructure was placed in the hands of Railtrack, a company who wished to be known by there own publicity as the heart of the railway'. Instead, we will concentrate on only the technical crisis surrounding the Rolling Contact Fatigue of rails. 2. The Fatigue problem generally It is worth mentioning some of the key points which have arisen from over 150. years of research into the Fatigue of metals. Fatigue began to be recognised as a specific failure mode when the early railways began to suffer failures of axles, wheels, rails, boilers and other components. Much impetus for investigations into Fatigue stemmed from the first railway accident to cause a major death toll, which occurred near Versailles in 1842 [4] when the axle of a locomotive broke.

5 During the next two decades, the great German railway engineer W hler, demonstrated that cyclic stress ranges determined Fatigue lives and that for steels at least, a Fatigue limiting stress existed, below which Fatigue lives were infinite. The so called S/N curve, relating stress range to cycles to failure and the Fatigue limit, still remain the basis of design against classical Fatigue . The mechanism of Fatigue has been unravelled during the 20th century, particularly in the last fifty years. It is now known that Fatigue is caused by the initiation and growth of cracks. The quantification of crack growth has become possible through the use of fracture mechanics, although the quantification of the initiation stage remains rather tentative.

6 It is well established that Fatigue initiation usually occurs at a free surface, aided by some kind of stress concentration feature. Circumstances can arise which produce non-propagating cracks. An example is that of a crack initiated at a particularly severe stress concentrating feature, which then stops as it grows out of the zone of high local stresses into a bulk stress field which is insufficient to carry it forward. It has become apparent that similar circumstances are important in RCF. Fatigue is often only one of several simultaneous deterioration mechanisms, for example, corrosion, creep or wear. The conjoint action of these mechanisms, is frequently more complicated than a linear superposition of modes. There are many standard tests on Fatigue of which Suresh [5] is an excellent modern example.

7 In a practical sense, application of our knowledge of Fatigue to real situations has been hampered in two ways. First, and perhaps at first sight, surprisingly, on many occasions we do not have sufficient knowledge of the actual service loads to which a particular component is subjected, nor do we know the critical location in an often complex shape where the most severe conditions for crack initiation exist. This means that similitude between data generated in the laboratory and service conditions is frequently difficult to achieve. If this is coupled with the natural statistical scatter of the Fatigue process, it can mean that Fatigue life predictions are frequently in error by more than an order of magnitude. The second major problem concerns the management of Fatigue , that is the continuation in service of parts known to contain cracks and the calculation of 3.

8 Residual safe lives. This process necessarily involves the detection and sizing of cracks in components. Although several methods for this exist, they are not always easy to apply and the results they produce can be ambiguous. In particular, the ability of non-destructive testing techniques to penetrate below the surface to measure the depth of progression of cracks into the bulk of the material is still limited and the whole area of non-destructive examination is considered by many to be an art rather than a science. Rolling Contact Fatigue of rails The stress concentration feature which causes initiation of RCF cracks is the Contact between the wheel and the rail . Conditions under the Contact patch are always severe and the yield stress of the rail steel is always exceeded, on at least a microscale, due to the surface roughness of the wheel and the rail .

9 It follows that irreversible events take place at every passage of every wheel. The term permanent way' is a misnomer, because it is changing continuously. The irreversibilities of each wheel passage, result in both a wear and a Fatigue process and the resultant life of the rail is a competition between these two failure processes. The stresses generated under the Contact are complex and governed by the detail of the wheel/ rail geometry near the Contact patch, the position of which is determined by, inter alia, curving behaviour, vehicle suspension characteristics, and, of course, existing conditions of the wheel and rail . Both traction forces and radial curving forces increase shear stresses in the Contact zone and hence the propensity to initiate cracks.

10 The phenomenology and sequence of development of RCF cracks in rails is shown schematically in Fig 1. Many observations have confirmed that cracks develop towards the direction of motion, initially inclined at a shallow angle of about 15 o to the head of the rail . When the cracks reach a depth of typically 10mm, the angle steepens to about 70o and the cracks then propagate through the rail until failure. During the shallow angle growth, flakes of material may detach themselves from the head of the rail , but the rail danger of a broken rail is obviously a result of the turned-down crack, and, therefore, this phase must be avoided if possible. The stages of the development of the cracks can be decomposed into separate phases; each controlled by separate elements of the various overall stress fields: Immediately under the surface, material is sheared in the direction of motion, ductility is exhausted by cyclic ratchetting and inclined cracks are formed along the shear planes.