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26 NDT accepted 20 June2003 Abstract Rail Flaw Detection has an important part to play in ensuring the safety of the worlds railroads. Recent accidents caused by broken rails have focused attention on the technologies that enable the detection of flaws in railroad rail. This paper reviews the technologies currently employed, along with examples of recent field applications. Some of the ongoing advancements and options for the future are also discussed. q2003 Elsevier Ltd. All rightsreserved. Keywords:Rail inspection; Ultrasonic testing; Electromagnetic testing; Fatigue defects; Roller searchunit 1.Introduction Rail Flaw Detection (RFD) is very much in the spotlight at present. If we look at rail testing today we see a very different picture than that presented five years ago. This sector of the industry has undergone many changes in all parts of the world. The Hatfield accident of October 2000 has played a major role in this. That is a terrible statement to make, but how often does that happen? It took a tragedy for people to understand the real value of railtesting. This paper will discuss the rail testing industry from the early days to what we see today. The progress of the work being performed for Railtrack (now Network Rail) will be presented along with some thoughts for thefuture. 2.Rail testing: from the earlydays Rail Testing really became a serious entity in the late 1920s when Dr Elmer Sperry, driven by the needs of the US railroad industry, developed the induction method for testing railroad rail 1. Over the years this technique was refined in the US and then in the 1950s ultrasonic testing emerged and started to become the mainstay of rail testing globally. This is still true today, even as more companies haveenteredtherail-testingarena.Some exceptionsto *Present address: School of Engineering and Applied Science, Aston University, Birmingham, UK. E-mail address:r.p.clarkaston.ac.uk (R.Clark). 27 this have been Sperry in the US, where the idea of complementarytestingtechniqueshasbeen developed, and in Russia where there are still many cars that use the magnetic induction technique only.The reportprepared by the Transportation Technology Center, Inc. (TTCI) for the Office of the Rail Regulator in October 2000 provides much useful background on the global rail testing industry today 2. The basic physicalprinciples ofthe two major techniques are as follows. In the case of ultrasonics, we are sending a beam of ultrasonic energy into the rail and looking for the return of reflected or scattered energy using a collection of transducers. The amplitude of any reflections together with when they occur in time can tell us about the integrity of the rail. Since defects are not totally predictable, we send in energy at several different incident angles in order to ensure that we maximize our chances of finding any detrimental features 3. The refracted angles generally used are 0, 37or 45 and 708. In addition, transducers are also positionedto lookacrossthe rail headfor longitudinaldefects such as vertical split heads and shear defects(Fig.1a).The induction technique is based on the physics of electromag- netic induction. An high amperage current is injectedinto the rail via brushes that make contact with the rail head. In effect the rail becomes part of an electrical circuit. If the current encounters a defect, thecurrentwilltravel around the defect (Fig. 1b). This distortion of the current flow is detected via a block of sensors that detect disturbances ofthe magnetic fieldassociated with the currentflow 4. 0963-8695/$- seefront matterq2003Elsevier Ltd.All rightsreserved. doi:10.1016/j.ndteint.2003.06.002 R. Clark / NDT (b) inductionprinciples. 3.Response to anaccident On March 18, 2001, the westbound California Zephyr, an Amtrak service from Chicago to the San Francisco Bay Area derailed in rural Iowa. The causea broken rail. One person was killed and 96 were injured. A detector car had been over the track a few weeks before the accident. A defect had been found and the railroad concerned had replaced the rail as determined by the Federal Railroad Administration (FRA) regulations. The problem was that the replacement rail used also had a defect in it. Within one week from the accident, the US rail testing companies had been mobilized to provide a manual testing capability totest the replacement rail on every subdivision of the railroad concerned. This incident was the first major use of manual rail testing on the North American freight railroads. The initial report related to this accident was published by the National Transportation Safety Board (NTSB) in March 2002 5. Following the preparation of a procedure 6 and adequate test trials on a 0.4 km test track in Danbury, CT, up to six Roller Search Unit (RSU) based portable rail detectors orwalkingstickswere put to work by Sperry. Other contractors contributed equipment and personnel as well. Subsequently, a more controlled schedule has been developed in co-operation with the railroad concerned and they now have a number of rail testersworkingyear round on a regional basis. The Portable Rail Detector (PRD) is shown in Fig. 2. The PRD makes use of thesame RSU as that employed on the test cars operating across North America. As the PRD is moved across therail surface, nine transducers operating at 2.25 MHz are simultaneously inspecting the volume of the rail.Toensure adequate coupling, a water tank isfittedtothe PRD and a copper tube directs the water flow 1 cm or so Fig. 2. Portable rail detector (NorthAmerica). in front of the moving RSU. Although water usage is not excessive, the tank may need filling once or twice in a typical 8-h day. The display is in the form of an A-scan on a conventional ultrasonic test instrument. The NTSB report recommended that the replacement rail testing should continue. Recommendation R-02-5 to the FRA states railroads should conduct ultrasonic or other appropriate inspections to ensure that rail used to replace defective segments of existingrailisfreefrom internal defects. An extension of this process would be to incorporate ultrasonic rail testing stationsatrail recycling plants similar to those employed at new rail production plants. The response to Hatfield, although it evolved over a longer time period, saw the development of a PRD for the UK rail system (Fig. 3). This unit has recently started to become a more common feature on the UK rail network. Ergonomically, the unit is more attractive and lighter than thefirstmodel.Theelectronicspackagehasbeen developed to enable flexibility and use with different ultrasonic testing instruments. The mainunchanged feature though, is that of the RSU. The next logical step R. Clark / NDT&E International 37 (2004) 111118113 113 Fig. 3. Portable rail detector (Europe). is to develop a method of data storage that allows a complete record of the testing work performed to be available for post testreviewandarchive.At presentthe testing record is a filled out report based on the technicians interpretation of what is seen on the instrument screen andvisually on therail. The RSU is a more appropriate front end unit than the alternative slider probe arrangement, whether considering manual inspection or high speed inspection. The main benefits are the robustness, ability to conform to the rail geometry and most importantly a reliable and efficient transfer of energy into the rail being inspected. These featureshaveencouragedthemajorityofthe ultrasonic rail testing companies around the world to employ this technology. In the PRD case, the RSU houses nine ultrasonic transducers at three different angles, the aim being tofilltherailwith as much energy as possible at theoptimumincident angles. The unit has been developed with Railtracks support to particularly address the detection of gauge corner cracking (rolling contact fatigue) that contributed to the Hatfield accident 7. 4.Existing technology Fig. 4 shows the most recent technology to be deployed ontheUSrailroads.Thesystembringstogetherthe complementary ultrasonic and inductiontestingtechniques on a hi-rail platform. This provides the railroad with a high qualitytestandincreasedflexibilityofdeployment. Inthe past, induction has only really been possible on a railbound vehicle because of the size of the plant needed to generate the high currents injected into the rails. With developments in powersupplytechnology,the productionof a hi-rail based vehicle has proven feasible 8. To date, seven of these cars have been built and released sinceJanuary 2001. They operate at speeds of up to 32 km/h, although with the stop and confirm testing requirements in North America, there is always an operational trade-off between going forward faster andtheriskoflonger reversingmoveswhenaconfirmationis required. In addition to the above mentioned features, the vehicles have also seen the transition from a strip chartbased display to a B-scan based display. The B-scan is a more intuitive representation of the ultrasonic interactions within the rail and is easier for the operator to interpret. Within the B-scan based system, algorithms work to reduce the data presented for interpretation.Dataoverloadfor the operator canbe a significant problem with rail testing, but on the flip side, mathematics is not always good at dealing with the unusual features encountered on the railroad. There has to be a balance. Gates are employed throughout the rail cross- section and the data is analyzed based on algorithms developed from both lab and field tests, and many years of testing experience. Both the ultrasonic and induction data are presented on the same display in the correct alignment. Muchof the workdone inthe US is focused onthe heavy haul railroad environment. Although the train speeds are less, the rail conditions encountered on the close to 200,000 miles of track that make up the North American railroads are much more severe than on most of the passenger biased railroads of Europe. Thoughts on the challenges to the rail testing engineerhavepreviously beenpresentedand they Fig. 4. Hi-rail ultrasonic/inductionvehicle. R. Clark / NDT&E International 37 (2004) 111118114 114 continue to form the driving force for the development work that is performed all over the world 9. The International Heavy Haul Association (IHHA) has acknowledged the demanding environment of the heavy haul railroads and they have collected together much of that wisdom in a book published in 2001 and launched at the IHHA meeting in Brisbane, Australia in that year10. The International Railway Union (UIC) also has an ongoing initiative that is looking at rail testing globally under the titleRailDefectManagement.A few documents have been released so far 11,12 and a good overview was given by Lundgren et al. 13 at the Brisbane IHHA meeting. The reason for mentioning these initiatives is to highlight the global interest in rail testing today and to start to set the scene for the discussion of new technology that follows. In North America, by far the most common and problematical defects are transverse defects, weld defects and vertical split head defects. These defects constitute around 55% of the yearly detected defects by Sperry. They also constitute 75% of the notified failures received. A notifiedfailure is an instance where a rail has broken and the company has been informed of the occurrence. In many cases an investigation will be performed to try and identify the cause of the failure. The possible causes are manyeach situation presenting a source of further learning. On many occasions the defect may be classified as undetectable atthe time of test because it may have been too small or the surface condition of the rail may have presented additional noisethatmayhavemaskedthedefect.Also,thecauseof the broken rail may have been something such as a wheel flat. In these instances a latent defect likely to be found at the next test may become a catastrophe when subjected to the impact of a train wheel withflats. The hi-rail ultrasonic/induction equipment has been carefully monitored as it has entered service. To date the positives have far outweighed the negatives. Being more sophisticated in its totality, the early days saw many teething troubles from generators to test carriage manipu- lation. The engineers had many challenges presented to them. Now, though the problems seem to be bottoming out as the trucks and the design have stabilized. One railroad has analyzed the performance of the vehicles as part of a Six Sigma Quality Project. The cars have been put on the demandingcoalterritoryof Appalachiamountains, curves, severely worn rail and very variable weather conditions. The analysis has shown that since the introduc- tion of the new vehicles, the instances of rail head defect rail failures have dropped significantly (an over 50%reduction). The test vehicles have been finding more defects and at an earlier point in the defects growth cycle. This helps to emphasize the power of the complementary techniques on a toughtotestterritory14.Thesystemhasalsoresultedin fewer stops to do manual verification and a 60% increase in testing speed in most cases. An analysis of the defects detectedononeofthemainNorthAmerican freight railroads over the course of a year showed that 20% of the defects marked indicated only with induction. This helps reinforce the value of complementarytechniques. 5.Work for Railtrack (now NetworkRail) In the latter half of 2001, Sperry was awarded a contract by Railtrack to build and operate an ultrasonic test vehicle on the UK rail network. Tothat end, 2002 saw much activity aimed at achieving that target. The instrumented carriage (UTU2) was fitted out in Derby in the UK and commis- sioned at the Old Dalby test track in late 2002 (Fig.5). The ultrasonic test system build and mechanical design and build work for the deployment of the RSUs were performed in the US. The test system is B-scan based and much the same as that used on the hi-rail ultrasonic/induc- tion cars. The mechanical design proved to be a challenge. Conscious of the need to comply with the vehicle acceptance rules in the UK and the tighter clearance demands, the design has been developed from that used on a high-speed vehicle already operating in Europe, most specifically in Sweden, Norway and Germany. The RSU housing has been redesigned to meetthe clearance requirements. A model of one quadrant of the RSU deployment arrangement mounted under the vehicle is shown in Fig. 6. The B-scan display is similar to that shown in Fig. 7 without the windows for the induction data. These windows were removed and the ultrasonic display was re-configured to make use of the available space. For ease of training and use the software has been developed in a Windows environment. The vertical dimension of each window relates to the vertical position of the indication source in the rail. The horizontal dimension of each window relatesto the longitudinal position of the indication in therail. The third part of the vehicle work has been the opportunity to design a new calibration rail for Railtrack. Drawing on experience and the requirements/suggestions of other railroads, a new design has been produced for Railtrack.The rails have been installed in the Old Dalby Fig. 5. Network Rail test vehicle. R. Clark / NDT&E International 37 (2004) 111118115 115 Fig. 6. Mechanical arrangement on UTU2. test track and have been used in the commissioning work. It is hoped that the design will be adopted by other railroads such that the rail testing community can work towardssome form of international standard. The next step in the UK will be the introduction of a new all ultrasonic hi-rail vehicle similar to one recently developed for use in Germany. This vehicle uses the same B-scan based test system and a carriage derived from that used for the hi-rail ultrasonic/induction vehicle inthe USA (Fig. 8). Again the stringent vehicle acceptance criteria have dictated the design steps taken. This carriage has room for both ultrasonic RSUs and eddy current sensors. 6.Technology development Rail testing has never seen large amounts of develop- ment funding. It seems that it is only when an accident occurs that money becomes more readily available. When this occurs though, it is perhaps even more important that wedemandcarefuluseofthatmoneyand co-ordinate Fig. 7. B-Scan display. 1 Fig. 8. European ultrasonic hi-railvehicle. the development efforts in a coherent fashion.Weoftenjump on the new is obviously better bandwagon without thinking. Yes it is good to invitenew ideas and people tothe table, but we must always be careful that they do trulyhave something to offer. Also, there is often a tendency to forget the people who have made railtesting theirlife.These people work for companies that have invested timeand money in moving theindustry forward when the needfor improvements has appeared less of a priority. Different countries havedifferentapproaches todevelopment,butthe accident scenario I have just described seems commoninthemajorityof cases. The majority of the technology development in the USis currently performed by the rail testingservice suppliers. The railroad challenge has recently received a higher profile in the wider arena 15,16, but many of the new investigators are in the early stages of projects. Oneof the main problems that few researchers grasp at the outset is how the industry works. The variables that haveto be dealt with aremoredemandingthanthey realize and the path from the lab to thefieldis difficult.Raildoes not particularly lend itself to being used inexhaustivetrialsaheadoftheserviceimplementation,so developments often reach the field withmuchwork still to be done. The Rail Defect Test Facility (RDTF) atthe TTCI in Pueblo, CO is an attempt to help reduce this hurdle. 2 钢轨探伤：概述和未来发展的需要钢轨探伤：概述和未来发展的需要 Robin Clark * 斯佩里铁路