why rails crack

The Arup Journal 3/2005 37 Background On 17 October 2000, a high-speed Intercity train from Leeds to London on the East Coast Main Line derailed at Hatfield, Hertfordshire (Fig 1), resulting in four deaths and over 30 injuries. It soon became clear that a break in the high rail on a 1500m curve was to blame. Investigations concluded that there were numerous cracks on the running surface, and when one of these penetrated to the base, multiple fractures of the railhead led to disintegration over some 30m (see Fig 2 for nomenclature). The potential for gauge corner cracking (GCC), as the phenomenon was initially called, to lead to railhead break-up made the then UK rail infrastructure operator, Railtrack, begin a rigorous system-wide inspection programme. Few in the UK had ever heard of GCC and no-one knew why it suddenly occurred. The search began for the cause. Why rails crack Gauge corner cracking on the British network: Investigation Robert Care Steve Clark Mark Dembosky Andy Doherty Gauge corner cracking caused the fatal Hatfield rail crash. Few had ever heard of the phenomenon, and no-one knew why it was suddenly occurring. Arup, with TTCI, was asked to investigate the cause. Arup’s role: November 2000 to date Phase 1 The large number of locations damaged by GCC led to many temporary speed restrictions, and a major effort to replace affected rails, switches, and crossings (S&C). All this caused train delays and cancellations, and early work was directed towards treating the symptoms rather than addressing the causes. The cracking at Hatfield was caused by rolling contact fatigue (RCF), something familiar in branches of engineering focused on bodies in rolling contact, such as bearings. Such bodies can damage each other in several ways, depending on the severity of the contact pressure and the shear or tearing forces in the contact area, which for most trains in Britain is about 15mm in diameter. Damage can be in the form of surface cracks, or the wearing away or plastic flow of the materials themselves. The RCF was in turn blamed on a wide range of issues, ranging from recent alterations to the structure of the British rail industry, to changes in the materials used on the railway, to the overall condition of the track infrastructure. To help Railtrack gain control of the situation, Arup was appointed jointly with the Transportation Technology Center Inc (TTCI) to identify and investigate all possible causes of the problem. This was a major task, given the lack of robust information, the contractual arrangements in the industry, and the poor relations between the parties involved in the privatized and fragmented British rail industry. The root cause could be rail metallurgy, maintenance and renewal techniques, working practices, vehicle design/maintenance, wheel condition, some or all of these in combination, or any one of several others. Arup/TTCI organized an intensive Phase 1 campaign to identify all the potential causal factors, quantify the variables associated with them, and assess the likelihood of each as a major contributor. This was done in November/December 2000, and an interim report was issued before that Christmas. Many measurements, inspections, and analyses were conducted, and several possible causes were rejected. No single ‘smoking gun’ could be identified. Phase 2 Starting in January 2001, Phase 2 quantified the relative contribution to the initiation and growth of RCF of the factors identified in Phase 1, with the ultimate goal of designing and implementing a control strategy to minimize and manage its incidence. 1. The Hatfield crash. 10100_Arup 29/11/05 2:35 pm Page 37 The Arup Journal 3/200538 The Arup/TTCI team co-ordinated a programme of measurements from track and vehicles across the industry: wheel and rail profiles, track forces, wheel impact loads, and wheel truing machine tolerances. In all, over 4000 wheel profile measurements and 2000 rail profile measurements were taken. Scores of RCF sites were investigated, and the computer modelling analyses of vehicles on specific types and designs of track were numbered in the thousands. The team also initiated a programme of laboratory research to understand the contribution of rail metallurgy and vehicle loadings to the growth rate of the cracks, including both small- and large-scale test regimes. The work performed in 10 months by the Arup/TTCI team (with the co-operation and support of the British railway industry) covered what would normally be a three to five-year major research effort. Through these studies Arup/TTCI led the development of a control strategy for use by professional engineers and railway group managers responsible for decision-making and directing railway asset maintenance, renewal, and operations. The control strategy recommends the actions that these groups need to take across the wheel-rail interface to discharge their accountabilities and responsibilities for effectively managing RCF. Phase 3 Phase 2 ended in October 2001, but Arup/TTCI continued to provide wheel-rail consulting services to Railtrack as required. Phase 3 began in January 2002 and continues to the present for Railtrack’s successor, Network Rail. Its highlights have included: • developing a method to include the effects of RCF and remedial rail grinding on Network Rail’s long-term maintenance and renewal projections • initiating RCF monitoring programmes on the Great Western Region and the London, Tilbury, and Southend network run by the train operating company c2c • a major programme to develop remediation methods for poor rail adhesion caused by autumn leaf fall • dynamic modelling to investigate the tendency for the new Desiro trains being introduced on the Southern Region to cause RCF, and monitoring the effects of their introduction • a preliminary investigation into the feasibility of using acoustic methods to detect defective bearings on high speed trains. Phase 4 By January 2003 Railtrack had been replaced by Network Rail, partly as a result of the financial crisis that RCF had inflicted on the industry. Network Rail realized that the British railway industry was addressing its wheel-rail interface issues rather haphazardly, and asked Arup/TTCI to assist with industry-wide wheel-rail technology management. This Phase 4 ran formally until March 2004, and some parts continue today. The first task was to identify and review current and planned work packages throughout the industry and for each to determine the sponsor, supplier, remit, links with other packages, planned deliverables, whether a final report was produced, and the extent to which the deliverables had been achieved. Additionally, each work package was related to the control strategy produced in Phase 2. From this, gaps and overlaps in the programmed work packages were identified and addressed. Further, the programme was to provide encouragement and assistance to turn research effort into tangible results. Other accomplishments included developing a Southern Region defence plan to mitigate any effects of new vehicle introduction, establishing a ‘Golden Mile’ section of track where rail best practices are strictly observed, investigating track-friendly bogies, and producing a Passenger Rail Best Practices Handbook (still in progress). Arup/TTCI was instrumental in designing a new organizational structure to provide the technical backup for the VTI SC (Vehicle Train Interaction Systems Committee), an industry-wide body established initially under the Strategic Rail Direction of translation Direction of rotation Flakes/ cracks Microstructure laminates Longitudinal shear force Normal force Contact patch Ratchetting strain deformation Rail material Crown area of the rail Gauge corner Field side of the rail Rail headGauge face Rail web Rail base INSIDE OF TRACK OUTSIDE OF TRACK 5. Contact patch pressure and forces causing surface elastic deformation of the rail microstructure, leading to RCF. 4. Head checking on a 1300m curve. 3. GCC on a 900m curve high rail. 2. Typical rail section nomenclature. 10100_Arup 29/11/05 2:35 pm Page 38 The Arup Journal 3/2005 39 Authority to continue the co-ordination of anti-RCF research. VTI SC replaced the committee WRISA (Wheel Rail Interface System Authority) proposed in Arup/TTCI’s report in December 2000. In early 2003, Network Rail and Arup/TTCI proposed a hypothesis summarizing current understanding of the causes of RCF, and then co-ordinated its presentation to the industry and the establishment of an experimental strategy to test it. Finally at the instigation of Arup/TTCI and First Group (owner of several train operating companies), an opportunity was taken to input anti-RCF approaches into the design of a new fleet of trains for the trans-Pennine routes. This resulted in significant changes to that train design, incorporating the latest thinking on RCF reduction. RCF: a natural consequence of bodies in rolling contact Though the railhead cracks were initially ascribed to GCC (Fig 3), most of it should more appropriately be described as ‘head checking’ (Fig 4) in that the cracks developed nearer the rail centreline, usually 15-25mm from the gauge face. GCC is found on the gauge corner itself. GCC and head checking are both subsets of the more general phenomenon of RCF. To illustrate the basic process (Fig 5) the rail is shown as comprising adjacent plates or laminates. Since the pressure and forces are high enough to cause plastic distortion of the laminates near the surface, an accumulation of metal dislocation occurs with each wheel pass until cracks occur at the surface. (RCF inspectors often run their fingers across the railhead and can feel the crack edges.) At first the cracks are quite short and at a shallow angle, but some may elongate and turn into a steep angle, following a path similar to the laminate boundaries (Fig 5). This tends to occur when the surface length approaches 30mm and makes a rail fracture far more probable. A useful way to depict the potential for the pressures and forces required to produce RCF is the ‘shakedown diagram’1 developed by Bowers and Johnson of Cambridge University (Fig 6). The actual location of the ‘crack initiation’ boundary is a function of the metallurgical properties and the friction at the contact patch. Fig 6 also shows a region of increasing wear that is a function of the force. This implies that when the rate of wear exceeds the rate of crack growth, cracking doesn’t propagate before it is worn away by the passage of wheels. The shakedown diagram shows that: • RCF can be a natural consequence of rolling contact. • RCF occurrence depends on the contact patch pressure and forces. • RCF initiation and growth are influenced by the metallurgical and frictional properties at the contact patch. Research Initially it was thought that RCF in the form of head checking and GCC was caused by quasi steady-state wheel-rail contact forces, but not any longer. The intense investigations from late 2000 through 2003 showed there to be at least three separate modes of RCF initiation and growth on the British system: • Mode 0: steady state, generally occurring on tight curves • Mode 1: bi-stable contact, generally occurring on medium curves • Mode 2: convergent motion, generally occurring on shallow curves, straight track, and S&C. These RCF modes are likely to be similar on other railways. In all cases, the behaviour of the wheel-rail interface clearly can lead to contact pressures and tangential shear stresses that exceed the shakedown limit of the metal at or just below the surface of the railhead. The following observations were made in the field and laboratory: • Almost all RCF in the British system occurs on curves and usually on the high rail (Fig 7). Fig 8 shows an early estimate of the probability of RCF on the British system as a function of curve radius. This shape is largely true of most British Limit moves right as friction drops Limit moves up with hardness Increasing wear Po/K Tc = Ft/Fn Squats No crack initiation Crack initiation Po: Contact patch peak Hertzian pressure K: Yield stress in shear Ft: Contact patch normal force Fn: Contact patch shear force Cant Outer rail (high rail) Inner rail (low rail) 0.35 Probability of RCF 1500m Radius of curvature 6. A shakedown diagram shows graphically the relationship between contact conditions and the properties of the rail material. Since wear is generally a non-linear function of Tc, cracks eventually give way to wear. (Squats are shallow internal rail defects caused by railhead head surface abnormalities of wear and stress.) 8. Approximate distribution of RCF on the British system as a function of curve radius. 7. Wheel contact on a curve with the high rail and low rail. 10100_Arup 29/11/05 2:35 pm Page 39 The Arup Journal 3/200540 main line routes, and very similar to distributions in the Netherlands. • On the British system, the highest probability of RCF is on c1500m radius curves (Fig 8). • Most RCF on straight track is associated with S&C. RCF in switches is often found in the same relative position and is thought to be associated with events or ‘triggers’ that tend to change the rolling radius difference (eg sudden changes in alignment or gauge or the switch/stock rail transition). • In shallow curves, where wheel/rail contact is on the gauge shoulder of the rail, the RCF is observed as head checks. In tight curves, where wheel/rail contact is near the gauge corner, the RCF appears as GCC. • Field observations typically show bogies with stiff suspensions contacting near the gauge corner and softer suspensions contacting higher on the railhead. • Laboratory studies have shown that the angle of the observed surface cracks matches well the ratio of the lateral and longitudinal forces (Fig 9). The most compelling evidence comes from WRISA-sponsored research2 that compared very detailed field observations with estimates of contact patch location, pressure, and forces. To date, six RCF sites have been studied in detail and a very high correlation has been found between RCF predicted by shakedown diagrams and observation. These studies were time-consuming and required very detailed RCF maps, both wheel and rail profiles, and specially processed track geometry data. All this information plus a mathematical model of the vehicles traversing each site was combined in a vehicle dynamics simulator. Estimates of contact pressure and forces were then projected into shakedown diagrams (Fig 10). In parallel, a product called the Whole Life Rail Model (WLRM) produced an alternative predictor of RCF using the same outputs from vehicle simulators, but not shakedown theory. The WLRM predictor asserts that not only force is necessary to create cracks but also energy expenditure. The algorithm combines the longitudinal and lateral creep with the corresponding creep forces to predict both RCF and wear potential (Fig 11). Estimates from both the shakedown limit and WLRM predictor have been consistently good, particularly when inadequacies in the process are accounted for: • Only a limited number of wheel profiles are used in the model. • Past history of track geometry is not well known. • Vehicle models assume nominal parameters. • Vehicle models use significant simplifications in modelling the contact patch and its location. • The friction history of the sites is not well known. In spite of these inadequacies, the studies to date have provided a credible explanation for the mechanism that produces RCF, reasonable tools to predict it, and valuable insight into the primary vehicle and track factors that produce conditions likely to initiate RCF. The same studies have shown that many combinations of vehicles and track, each well within its own current safety standards, could frequently produce RCF, replicating field observations. 2500 1500 1000 500 0 C O N TA C T S TR E S S M N m 2 00 0.1 0.2 0.3 0.4 TANGENTIAL FORCE COEFFICIENT Minimum track alignment, measured rail profiles 50% track alignment, measured rail profiles Perfect track alignment measured rail profiles National RCF limit 00 100 200 300m DISTANCE ALONG TEST SITE FIELD SIDE GAUGE SIDE 9. Surface crack angle as a function of contact patch longitudinal and lateral forces. 10. Shakedown diagram showing the effect of track alignment on a Class 43 locomotive as it traverses an S&C unit. Points produced by modelling above the ‘notional’ limit correspond to RCF observed in the field. 11. WLRM RCF index for curve high rail; the black contours are RCF locations on railhead from field observations. 10100_Arup 29/11/05 2:35 pm Page 40 The Arup Journal 3/2005 41 Modes of RCF generation Of the three modes of RCF initiation and growth, steady state mode (Mode 0) is usually thought of as RCF and best describes GCC and head checking in curves. The wheel-rail contact forces that cause RCF by this mode are generated by vehicle curving behaviour in a specific range of curve radii and operating conditions. This is thought to occur in tighter curves with radii of 1200m and less. Bi-stable contact mode (Mode 1) describes RCF occurring when the wheel-rail interface operates in a region of instability, where small changes in wheelset lateral shift generate large changes in rolling radius difference. This is thought to occur when the wheel profile wears so that there is conformality* in the flange root area in the wheel, and the wheel-rail contact position occurs there - generally at the gauge corner/gauge shoulder area and under conditions of high conicity**. This is thought to occur in curves with radii of 1200m - 2000m; however, it appears to overlap into curves of tighter and shallower radius. Convergent motion mode (Mode 2) describes RCF occurring when changes in track lateral position and wheelset lateral shift cause the wheel flange to converge on the rail gauge face, even though flange and gauge face may not necessarily come into contact. This type of behaviour is thought to occur in moderate curves with radii of 2000m and higher, and in straight track. Again, there is assumed to be an overlap of this behaviour into curves of tighter radius where Mode 1 would normally be expected. These locations of RCF cracks on the railhead fit with the dynamic wheel-rail interface behaviour expected in the three modes (Fig 12). The quasi-steady state behaviour of vehicles in these ranges of curvature with respect to shakedown are shown as well. In all cases, RCF in the British system is due to excess wheel/rail forces primarily caused by the axle shifting relative to the rail too far to one side or the other. This is true on curved track, straight track, or S&C. In tight curves, the mechanism tends to be steady state, while in S&C and moderate curves and straight track the mechanism is transient. The second part of this paper, to be published in the next edition of The Arup Journal, explains how this happens in each mode. References (1) JOHNSON, K. Contact mechanics. Cambridge University Press, 1985. (2) DEMBOSKY, M, and BAKER, P. Great Western Zone RCF pilot study modelling report. WRISA, 2003. * Close match between the shape of the wheel and the shape of the railhead, so that very small lateral shifts between the wheel and the rail can result in significant shifts of location of the contact patch, from the head to the shoulder of the rail. ** Rail wheels are shaped like a cone to facilitate cornering, although the taper is not uniform. As the wheel tread moves towards the flange the taper flattens out, increasing the ‘conicity’. Acknowledgements The authors wish to acknowledge the contribution of many people in the British railway industry to the research that underlies this paper. Gauge Field Low Mid High 0.35 1500mRailhead Fc Tc Probability of RCF Ideali