Track Buckling Hazard Detection and Rail Stress Management

Transcription

1 Track Buckling Hazard Detection and Rail Stress Management Andrew Kish, Ph.D. President- Kandrew Inc. Consulting Services Peabody, MA, USA Phone: Ryan S. McWilliams Vice President-Technology & Business Development Salient Systems, Dublin, OH, USA Phone: Harold Harrison Salient Systems - Technology Consultant Friday Harbor, WA, USA Phone: h_sqd@salientsystems.com ABSTRACT A key aspect of managing the stress state of the railroad involves controlling longitudinal rail stresses to safe levels. Rail networks are therefore focused on reducing the risk of both buckled track and broken rail for safety and security enhancement. The efficient management of longitudinal stress induced thermal forces in continuous welded rail (CWR) is an important aspect of railroad safety and maintenance. Balancing such forces is a complex issue which requires intimate knowledge of many parameters which include track strength as well as environmental operating conditions. CWR track is prone to lateral buckling when the rails are under high compressive forces. Conversely, when high tensile forces dominate under cold temperature conditions, rail defect growth can cause rail fractures, weld failures and rail joints to pull apart. The magnitude of longitudinal thermal forces is governed by the rail neutral temperature (RNT) which is the rail temperature at which the net force is zero. It is well known that RNT is a highly variable parameter and that its measurement has proven to be both difficult and costly. This paper presents evolving and proven measurement technologies as well as new safety tools for longitudinal force management. The safety aspects include track buckling prevention through hazard identification diagnostics in terms of a yellow, orange and red buckling margin of safety (BMS) warning, and rail break detection through sudden stress discontinuity (interruption) measurements. The maintenance aspects include providing and applying data for rail defect/break repairs when readjusting RNT to safe values in various operation conditions, and providing data for curve realignment and stability improvement through more efficient RNT management. The applications are illustrated through case study examples. 1.0 Introduction Prevention of excessive longitudinal thermal forces in continuous welded rail (CWR) is an important track maintenance and safety issue. Thermal forces are generated when the rail temperature varies from the rail s neutral temperature (RNT) i.e. the rail temperature at which the longitudinal force is zero, which is typically the rail s installation or fastening temperature. (Note: common terminology also uses SFT and T N to designate RNT). Compressive forces are produced when the rail temperature is higher than the RNT, and tensile forces are produced when the rail temperatures are below the RNT. High compressive forces can cause track buckling, while high tensile forces can accelerate rail defect growth and cause rail joints to fail or pull apart in cold weather, with both being potential derailment causes. Longitudinal forces are controlled by installing and maintaining the rail at RNT levels within specified limits. However, it is a well known fact that the RNT will vary over time and that it typically shifts to a lower value that can increase the risk of track buckling. These lowered RNT values can result from rail repair or 1

2 other maintenance work undertaken during cold weather and from excessive rail movement. A key impediment to knowing these low and potentially dangerous RNTs is a lack of adequate measurement capability which has eluded researchers for the past three decades. There have been several measurement concepts/techniques proposed and evaluated over the years with mixed results. These included techniques based on various physics concepts as illustrated in Figure 1. RNT Measurement Concepts/Issues Concepts Researched Mechanical/electrical resistance strain gage Rail uplift Rail vibration Vibrating wire/filament Ultrasonic wave Acoustic wave Electromagnetic/acoustic wave (EMAT) Magnetic permeability: (Barkhausen noise) (Magnetostriction) X-ray diffraction Measurement Difficulties Accuracy Issues: sensitivity to rail microstructure, residual stresses, track parameters; hard to get force/rnt from stress; zeroes required Don t provide real-time, continuous data output Not nondestructive and not easily deployable No direct applicability to railroad maintenance and safety practices Figure 1 Summary of RNT Measurement Concepts The key compromising aspects being (1) measurement accuracy, (2) ease of deployment, (3) providing continuous data on a real time basis, and (4) having direct applicability to railroad maintenance and safety. Although several concepts partially fulfill these needs, the US Salient System s Rail Stress Monitor (RSM) and its supportive StressNet TM data base system have evolved to be the most responsive to the above needs. The RSM and its StressNet TM technology have been developed and tested over twenty-five years of R&D, and have been recently expanded to support specific industry needs to not only measure RNT but interpret it for practical application to enhance overall CWR safety and performance. More specifically the RSM and StressNet TM technology: Measures longitudinal force and RNT Detects rail breaks and track buckles Alerts on potential buckling hazards Enables more effective rail break/defect repairs Monitors rail joint condition In this paper several of these applications will be discussed with a particular emphasis on track buckling prevention through potential hazard identification diagnostics in terms of a green, yellow, orange or red buckling hazard warning. 2.0 The RSM-StressNet TM System (The RSM Technology) The rail-mounted hardware represents continuing evolving technology developments starting with a stress measurement circuit first developed at Battelle Columbus Laboratories in the early eighties [1, 2, 2

3 3], and culminating in today s expedient and railroad friendly ipod/iphone based data acquisition system. The system is based on current radio technology and novel antenna designs that allow for a seamless collection process in a hardened design capable of surviving normal freight railroad operations and maintenance with a power management system that allows 10+ year useful life, and the ability to collect the data with a variety of intermediate devices, including: Hand held ipod/iphone reader carried by a track inspector End of Train (EOT) device High rail inspection vehicle Wayside reader Figure 2 below shows the various uses and deployment options of the RSM-StressNet TM system. RSM Deployment Options: When and Where? When installing new rail When making rail break/defect repairs When destressing hot rail In curves to manage lateral stability At locations where fastener conditions are substandard At locations of high stress build-up (approaches to bridges, special track work, tunnels, etc.) In high tonnage lines to manage speed restrictions In high-speed passenger lines for RNT safety monitoring At joints for performance evaluation 3.0 RNT Monitoring and Rail Break Detection Figure 2 Deployment Options for the RSM Figure 3 is an example of a StressNet TM output of 5 years of RSM data at one location of Union Pacific test segment of newly laid CWR on a 2 degree (875 m radius) concrete tie track curve experiencing 230 MGTs of annual tonnage where both rails were instrumented with RSMs. The data show the RNT variation with time for the two rails (red for the low rail and green for the high rail) exhibiting the key features of: the overall RNT behavior trends, the daily variations (the shaded data and tooth-like spikes), the detection and influences of three rail breaks on the high rail (as shown by the sudden drop in RNT as denoted by RB1, RB2 and RB3), and the influence of track maintenance involving surfacing and lining. To get a more detailed look at a rail break scenario, Figure 4 shows an expanded view of RB1 and RB2 data taken by the RSM closest to the breaks. Further inspection of Figure 4 shows that the low rail (red data) is relatively stable in terms of RNT variation (ignoring the daily changes - tooth-like spikes). The high rail (green) data, however, show the dramatic influences of rail breaks as indicated by the sudden discontinuity in RNT. The influence of interim joint repairs is also evident in terms of large daily RNT shifts due to joint-gap movement with temperature as shown by A and B. Finally, the green dashed arrow shows the effects of joint welding and RNT restressing to the target RNT, and its effectiveness. Thus RNT knowledge and RNT monitoring during rail repair and readjustment is highly essential in CWR repair and maintenance, hence to more effectively managing the repair of broken and defective rail. Some inherent difficulties with these repair procedures are briefly outlined below in context of rail break mechanics. Daily RNT Variation 2 (875m) Curve Low Rail 50 Ne utra l Tempe ra ture ( F) 3 High Rail Cell Phone Outage & RSM Refurbishment Neutral Temperature (

4 Figure 3 Five Years of RSM Data on the Union Pacific Neutral Temperature ( F) RB1 A RB2 B Neutral Temperature ( C) 2 (875m) Curve Low Rail 10 High Rail 0 Calendar Days Figure 4 RNT Behavior in Rail Breaks 4.0 Rail Break Mechanics, Repair and Restressing A major reason for the reduced and possibly unsafe neutral temperatures is the difficulty in resetting the RNT to the desired target value after a rail break or after a rail defect removal. The fundamentals of rail break mechanics are discussed in [4, 5], while Figure 5 illustrates the basics. It is a well known that when the rail is broken or cut for defect removal the RNT drops to the temperature of the rail when the break/cut occurs. For example, in Figure 5 when the rail breaks at 40 F(4 C) its RNT is 40 F (4 C) at that location. Additionally in this case, the rail break/cut influence is experienced for a track length exceeding 635 ft (194m) on either side of the break/cut as indicated by L d. As shown in [4, 5], these L d s can be on the 4

5 order of ft ( m) depending on the tensile force in the rail and on the rail/fastener s longitudinal resistance. Rail Break/Cut RNT Profile T Break =40 F(4 C); SFT PreBreak =100 F (38 C); Gap=3in (7.6cm) RNT( F) Pre-Break RNT = 100 F (38 C) RNT Locked in After Joint Closes Add 3 in Rail 60 T BR = 40 F 3 in 136#Rail Timber Ties RNT Profile Just After The Break/Cut L d =635 ft (194m) Distance from break (ft) Figure 5 Rail Break/Cut RNT Profiles The resulting rail gaps require closure by adding rail through a rail-plug insertion. Note that as shown in [4] closing the gap by pulling rail together is generally not an effective procedure since it might only restore the RNT to what it was in the rail prior to the break/cut, which may be much lower than the required target RNT. Adding rail through a plug insertion, however, does result in locking in the reduced RNT as indicated by the green RNT profile. It is this reduced RNT profile that is required to be readjusted before the onset of warm temperatures to avoid buckling prone conditions. The usual readjustment is made by cutting rail out and unfastening a prescribed length of rail at warmer temperatures. As indicated in [4, 5], the basic difficulty with this procedure is that the correct amount of rail to cut out and the correct unfastening lengths required are not known because the reduced RNT profile requiring adjustment is not known. The Salient RSM-StressNet TM System can be applied to provide this knowledge, thus promoting a more effective management of rail break/defect removal repairs in terms of RNT readjustment. Specifically, the RSM-StressNet TM System: Determines and monitors the RNT after plug/joint interim repair Alerts on when to return for the final repair/rnt readjustment to avoid a buckling prone condition (see Buckling Hazard description in next Section) Provides information on how to readjust to the desired RNT Provides information on effectiveness of the RNT readjustment One such application is shown in Figure 6 where the RSM data shows the specifics of rail break (RB3 shown in Figure 3) repair on the Union Pacific test site. The RSM measured RNT data clearly shows the interim joint s RNT of 60 F (16 C) to be readjusted. Without RSM/RNT measurement this would not be known, hence the amount of rail to cut out and unfasten would also be not known. RSM data also shows the effectiveness of the readjustment procedure and tracks the subsequent RNT condition. UP South Morrill Sub Rail Break on 2/8/06, Joint Repair and RNT Readjustment As Monitored By Rail Stress Module (RSM) 5 RNT Readjustment Rail Break 30 emp ( F) Joint Repair RNT 40 F (4 C) Joint RNT 60 F (16 C) 20 RNT and

6 Figure 6 RNT Monitoring and Readjustment during a Rail Break and Repair 5.0 Track Buckling Hazard Detection With the annual buckle derailments in the US ranging from at a cost of over $10M per year, the prevention of track buckle caused derailments remains a key industry goal. Railroad efforts are continuing to focus on improving buckling prevention practices and on to better identifying buckling prone conditions. To assist in these efforts Salient Systems has recently developed a Buckling Hazard Index which provides a warning on potential buckles in terms of a yellow, orange, or red alert. The yelloworange-red alerts are based on the track having a prescribed buckling margin of safety (BMS). The BMS is determined by the StressNet TM system and is based on the RSM measured rail and neutral temperature data coupled with the track s buckling strength evaluations. The latter is based on the Volpe/FRA CWR-SAFE model [6], a part of which has been incorporated into the StressNet TM system, referred to as BUCKLE. The BMS is a numerical temperature index on how close the track is to a critical condition in accordance with theoretical developments in [7, 8]. For completeness, a brief review of buckling safety concepts and BMS are provided below. 5.1 Buckling Safety Concepts for BMS Applications Figure 7 shows the fundamental safety concept and criterion for buckling prevention as detailed in [9], as well as applied by the UIC in line with [10, 11]. T Bmax Temperature increase above neutral Buckle Temperature increase above neutral Step 1 Step 2 Buckle T Bmin o Deflection o Deflection T Bmax 6 T Bmax Temperature increase above neutral Step 3 Buckle T Bmin Buckling regime Temperature increase above neutral Step 4 Buckle Buckling regime T Bmin Safe temperature increase

7 Figure 7- Track Buckling Stability Response and Safe Temperature Concept Step 1 shows the track lateral deflection from an initial line defect with amplitude o with temperature increase. Step 2 exhibits the theoretically determined buckling equilibrium curve (shown in dashed) identifying the upper and lower critical temperatures ΔT Bmax and ΔT Bmin. Step 3 shows that the actual buckling occurs at a temperature in a buckling regime bounded by the upper and lower critical temperatures, and Step 4 identifies the lowest point on the buckling response curve as the safe allowable temperature increase. According to theoretical concepts [5], at temperatures below this value buckling should not occur, and the track s deflections are confined to the growth of the initial line defects with temperature. The safety criterion on track buckling prevention is based on this safe temperature value i.e. on the ΔT Bmin on the buckling response curve. Buckling prevention safety criterion then requires that rail temperature increase above neutral to be confined to values less than ΔT Bmin. Since ΔT Bmin is an analytically determined quantity based on theory and on track condition/parameter inputs, for additional safety considerations it is customary to add a safety factor on ΔT Bmin. For most heavy-haul applications this safety factor is typically 10 F (6 C) less than ΔT Bmin [9]. For the European rail network s safety factors refer to [10, 11]. It is important to note that the safe temperature increase value, ΔT Bmin, and the buckling regime s upper bound, ΔT Bmax, are highly track parameter/type/condition dependent, and are referenced to the track s neutral temperature. Hence for buckling hazard detection both the safe allowable temperature (as modified by any additional safety factors) and the track s RNT are required in accordance with the safety criterion that: thermal load < buckling strength, where the thermal load is based on the RNT, and the track strength is based on the safe allowable temperature. In line with the above, the buckling margin of safety (BMS) is defined as the reserve buckling strength in terms of the additional temperature or thermal load required to produce a buckling prone condition i.e. reaching the track s safety limit. This is graphically illustrated in Figure 8. T T Bmax T Bmin Safety Factor BMS = (T ALL T actual ) T ALL T actual Buckle Amplitude 7

8 5.2 Buckling Hazard Index Figure 8- Buckling Margin of Safety (BMS) Definition Salient s StressNet TM System has been recently upgraded to provide a buckling hazard warning based on the RSM s RNT output. The warning is based on a Buckling Hazard Index which provides a color indexed alert in terms of a green, yellow, orange, or red conditions. The green-yellow-orange-red alerts are based on having a prescribed buckling margin of safety (BMS) where the BMS is determined by the StressNet TM. This determination is based on the RSM provided rail and neutral temperature data, and on the track s buckling strength as computed by BUCKLE. The BUCKLE algorithm based on the Volpe/FRA CWR-SAFE model (3), and it is customized for easy railroad use by requiring only simple input parameters readily available to the railroad engineer. The program determines the allowable temperature increase, and based on the RSM measured rail temperature and RNT it computes a buckling margin of safety (BSM). Based on the calculated BMSs falling into prescribed safe, marginally safe or unsafe regimes in accordance with Table 1, alerts can be issued on a potential for an incipient buckle. Note that in Table 1 the green-yellow-orange-red BMS regimes are illustrative and provisional in line with current US practice, and are based on the RSM s real time output and StressNet s real time application for BMS determination. Automated triggers can be developed for specific railroad property or network needs. Table 1 - Provisional Criterion for BMS Application to Hazard Identification and Advisories BMS BMS Larger than 25 F (14 C) ACTION ACTION OK; no action Between 10 and 25 F ( 6 and 14 C) Caution; advisory Between 0 and 10 F (0 and 6 C) Slow order; readjust RNT Less than 0 Red flag; immediate attention 5.3 Buckling Hazard Warning Example and Case Study Based on RSM/StressNet TM data as outlined in Figures 2 and 3, buckling hazard evaluations were conducted for the Union Pacific at two RNT conditions for a maximum summer rail temperature of 140 F (60 C) application in line the green yellow-orange-red index definition. The results are shown in Figure 9 where the left-hand side shows the RNT profile, while the right-hand side the StressNet s BUCKLE output providing the BMS and the hazard index. At RNT avg=103 F(40 C) a high BMS=50 F (28 C) is calculated showing the green condition at a rail temperature of 140 F (60 C) i.e. that at the track is not buckling prone till =190 F (88 C). At a lower RNT avg = 75 F (24 C) BUCKLE output indicates a BMS=22 F (12 C) or a yellow alert i.e. that the track is not vulnerable to buckling till =162 F (72 C). Based on such outputs the railroad can determine and apply remedial action as needed. For the Union Pacific, this test provided information on not needing summer slow orders. For additional hot-weather speed restriction concepts and applications refer to [12]. 8 Is there a buckling hazard at T RAIL = 140 F (60 C)? RNT avg =103 F(40 C)

9 Figure 9 Example of Buckling Hazard Warning 6.0 Conclusions Track buckling hazard detection and rail stress management largely depend on the track s RNT condition. Although techniques to measure and monitor RNT with sufficient accuracy have been lacking, a promising evolving technology based on continuous and real time RNT measurement system has been developed. The applications of this system s measurements have been extended to cover several rail longitudinal stress management aspects, including rail break detection, improved rail break/defect removal repairs, and to a track buckling hazard detection and warning capability, all aimed at providing for more efficient CWR safety and maintenance strategies. 7.0 References [1] H. D. Harrison, Evaluation of Methods for Measurement of Longitudinal Rail Force in Unloaded Track, Battelle Columbus Laboratories, Technical Memo, Contract No. DOT-FRA-9162, Task 2, 1981 [2] H. D. Harrison, Strain Gage Based Longitudinal Rail Stress Measurements at FAST, Battelle Columbus Laboratories, Technical Note, Contract No. DOT-FRA-9162, October 12, 1983 [3] D. R. Ahlbeck, H. D. Harrison, Monitoring Rail Neutral Temperature and the Long Term Stability of Track, presented at the American Society of Mechanical Engineers, Annual Winter Meeting, San Francisco, California,

10 [4] A. Kish, Destressing/Restressing for Improved CWR Neutral Temperature Management, in Guidelines to Best Practices for Heavy Haul Railway Operations: Track, International Heavy Haul Association publication, 2009, pp to 6-73 [5] A. Kish, G. Samavedam, Improvements in CWR Destressing for Better Management of Rail Neutral Temperature, Paper presented at 2005 Transportation Research Board Annual Conference - January 2005, and in Journal of the Transportation Research Board #1916, pp 56-65, 2005 [6] A. Kish, W. Mui, CWR-SAFE, Software and User s Guide, February 2001, US DOT/Volpe Center [7] A. Kish, Track Lateral Stability, in Guidelines to Best Practices for Heavy Haul Railway Operations: Track, International Heavy Haul Association publication, 2009 pp to 1-84 [8] A. Kish, G. Samavedam, Dynamic Buckling of Continuous Welded Rail Track: Theory, Tests, and Safety Concepts, Transportation Research Record No. 1289, Rail: Lateral Track Stability 1991 [9] A. Kish, G. Samavedam, Track Buckling Prevention: Theory, Safety Concepts and Applications, DOT/FRA/ORD Final Report, October 2004, in FRA publication [10] A. Kish, et al, Laying and Maintenance of CWR Track, Union of International Railways (UIC) Code 720, ERRI D202/RP#10, Chapter 6, April 1999, Utrecht, The Netherlands [11] Union of International Railways (UIC) Code 720R, Laying and Maintenance of CWR Track, 2nd Edition, December 2002 [12] A. Kish, D. Clark, Track Buckling Derailment Prevention through Risk-Based Train Speed Reductions, 2009 AREMA Annual Conference, Chicago, September