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Effectiveness of measuring stress-free temperature in continuously welded rails by Rail Creep Method and Rail Stress Modules
Engineering Failure Analysis 104 (2019) 189–202
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Effectiveness of measuring stress-free temperature in continuously welded rails by Rail Creep Method and Rail Stress Modules
T
⁎
Nirmal K. Mandala, , Mitchell Leesb a b
Central Queensland University, Centre for Railway Engineering, Bruce Highway, Rockhampton, Queensland, Australia Aurizon Network Asset Management, Mackay, Queensland, Australia
S U M MA R Y
Continuously welded rail track has two critical parameters that affect track stability, namely longitudinal rail stress and lateral resistance of the track structure. Maintaining the stress free temperature of rail plays an important role in managing of rail stress focussing on lateral track stability. Railway companies are focusing on finding an effective means of minimising longitudinal rail forces through management of Stress Free Temperature. In this study, the effectiveness of commercially available rail stress monitors and the rail creep measurement methods to monitor rail stress in railway systems has been investigated. A field investigation of a section of track at Edungalba in Aurizon's heavy haul coal network has enabled observation of Stress Free Temperature through several testing methods. The investigation and analysis has led to insights into track behaviour and stress state. The study concluded that the conventional rail creep method is useful for general indications of track stress condition and follows a similar trend to rail stress monitor data. It was also concluded that accurate measurement of Stress Free Temperature throughout the entire network through installation of rail stress monitor modules may not be a cost-effective solution for the management of rail stress. However, the modules demonstrated an effectiveness to monitor and manage rail stress in targeted problem areas.
1. Introduction The management of rail stress continues to be a cause of concern and an area of rapid development across railway companies worldwide. The two most important parameters affecting the stability of Continuously Welded Rail (CWR) are longitudinal rail stress and the lateral resistance of the track. Longitudinal rail stress is difficult to measure and is a significant structural concern as high tensile forces can lead to breaks in the rail and high compressive forces can cause track buckles and, in worst case scenarios, derailments. In managing longitudinal rail stresses due to temperature changes, both actual rail temperature and stress free temperature (SFT) are important. The actual rail temperature is very sensitive to the surrounding environmental temperature and other conditions beside the rail. The SFT is the temperature at which there is no thermally caused longitudinal stress present in the rail. The railway companies lay rails on the track bed in such a way that there is no axial stress present in the rail at a set temperature. This temperature is called the Design Stress Free Temperature (DSFT) but the current SFT is not always the same as the DSFT. The drift of current SFT from DSFT depends on environmental, operational and geographical factors [1]. Rail restressing is the process of adjusting the length of continuously welded rail to re-establish the DSFT. The action of rail traffic over time causes some limited longitudinal displacement of rails, resulting in a drift of the actual SFT away from DSFT. The rail temperature, sensitive to ambient air temperature and related factors, is continuously changing and this also has small effects on SFT. Therefore it is most important to monitor both the longitudinal movement of the rails and their SFT to manage longitudinal rail stress effectively. Any difference of the actual rail temperature from the SFT will produce a longitudinal load along the rails [2]. It is necessary to ⁎
Corresponding author. E-mail address: [email protected] (N.K. Mandal).
https://doi.org/10.1016/j.engfailanal.2019.05.032 Received 27 August 2018; Received in revised form 13 May 2019; Accepted 29 May 2019 Available online 30 May 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Annual Rail Temperature (°C) vs DSFT [3].
keep the SFT as close as practically possible to the DSFT to minimise the extent of temperature generated rail stress with the aim of maintaining the safe balance between the maximum summer and minimum winter temperature extremes that can potentially cause buckles and pull-apart respectively (Fig. 1). Use of CWR has revolutionised the railway industry; however, it is a well-known risk that CWR is susceptible to failure caused by changes in temperature. Rails have a higher coefficient of thermal expansion relative to the underlying track structure, making them particularly sensitive to temperature fluctuations. Under normal operational conditions, railway rails are subject to vertical and longitudinal loads and lateral forces. Esveld [4] reports that these loads result in the following stresses in the rails:
• normal stresses which are determined primarily by temperature change; • bending stresses caused by the vertical train loading; and, • residual stresses due to manufacturing processes and contact stresses in the rail head. A typical track buckle derailment on the Queensland heavy haul network costs up to $1.2million in infrastructure repairs and rolling stock recovery [5]. A recent study [6] indicates that the railway company Aurizon is responsible for the movement of about 700,000 t of iron ore, coal and other freight across Australia each day. It also indicates that, because of extreme hot weather, track buckling (Fig. 2) has been an issue in Central Queensland as it has been throughout the world. According to US Federal Railroad Administration (FRA) safety statistics data, there were 2000 derailments between January 1995 to February 2012 and 126 between January 2010 to December 2012 in the USA alone [7]. This has a detrimental effect on the reliability of heavy haul transportation systems and the coal export industries, emphasising the need for effective rail stress management to avoid such occurrences and costs. A number of rail stress testing techniques have been implemented in a field investigation of a section of track in Aurizon's heavy haul coal network. This investigation was to assess track behaviour and discover trends that may assist in the active management of longitudinal rail stress. The aim of this study was to assist Aurizon and CQUniversity's Centre for Railway Engineering to better understand the effectiveness of using Salient Systems' rail stress monitors (RSMs) and the conventional rail creep measurement method by undertaking a trial at Edungalba, located approximately 87 km west of Rockhampton, Queensland (Figs. 3, 4). The wider objectives of this research are to evaluate different rail stress testing methods, implement a number of methods at the trial site to
Fig. 2. Thermal Stress Induced Buckle. 190
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Fig. 3. Aurizon Blackwater Heavy Haul Corridor map.
Fig. 4. Project site route map details with locations of RSMs.
determine stress condition, monitor and analyse RSM data to look for trends in SFT change, report on accuracy and reliability of testing techniques and monitor rail creep and use for comparison. The lateral track strength and the influence of track misalignment on lateral strength, along with rail temperature and its relationship with environmental temperature, are not considered in the paper. It is evident that SFT of rail plays a critical role in understanding the monitoring and management of rail stress. Railway managers are interested in the measurement of rail SFT because it is an important requirement of safety management standards. Evaluating the performance of measurement equipment and developing an overall strategy that railway managers can implement to comply with these safety standards is therefore a priority [8]. Many SFT measurement techniques have proved to be limited in their use and difficult to employ except by undertaking night time measurements, meaning railways generally rely on preventative measures to control buckling risk. SFT tends to be measured only in targeted problem areas due to the costs involved in the measuring process and the need to shut down revenue lines [9]. A failure mode detection system that continuously monitors rail stress in real-time as a 191
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replacement for intermittent manual measurements is strongly desired by railway companies. Such a system must accomplish the following [10]:
• Reliably and accurately determine the current SFT with minimum track occupancy and labour; • Provide a means of continuously monitoring so as not to have a window of missed condition when the SFT could swing outside of the safe range.
Given the safety requirements that make measurement of SFT so important, there have been many studies over recent decades that focused on various methods. Laser vibrometry [11], strain gauges [12] and other methods for measuring longitudinal rail stress and SFT are discussed in more detail in the next section (Section 2) along with measurement of lateral resistance of track [13]. Ahmad et al. [12] pointed out the daily change of SFT by 2–3 °C through field trials and proposed a simple track buckling management tool considering two major parameters: rail stress and track resistance. Other related studies on speed restrictions [14], buckling loads [15], post-buckling behaviour [16], and fastening strength on variation of SFT [17] are important. A thorough literature search reveals that a long term study on SFT is necessary for specific track structures under particular operational scenarios to maintain longitudinal rail stress in a safe range to prevent track buckles that compromise railway safety. This study focuses on achieving this outcome by using rail stress monitors and rail creep methods. Buckling behaviour and its analysis [18,19] is not included in this study. An earlier version of this paper was previously presented in a conference [1].
2. Measurement methods It is important to measure the SFT regularly to ensure that it remains within the DSFT zone for safe operation and to minimise the risk of track buckling [9]. The drift of SFT from the DSFT is due to several contributing factors involving traffic operations, changes of rail temperature and deficient track condition [20]. For many years the railway industry has been in search of an efficient and accurate method to safely and effectively measure the SFT of rail in track. Over the years, different methods of measuring the SFT in the rails have been developed and practiced within the railway industry. Gokhale & Hurlebaus [21] reported that many of these techniques are time consuming, difficult to implement, destructive and labour intensive. Various SFT measurement methods such as Rail Cut and Measure, Railframe equipment (Fig. 5), Rail Creep Monitoring (Fig. 6), Strain Gauges, Rail Stress Monitor modules (Fig. 7), Vertical Rail Stiffness Equipment (VERSE) (Fig. 8), D'stresen, SFTPro, Rail Scan, Magnetic Anisotropy and Permeability System (MAPS) and Acousto Elastic Effect (Ultrasonic) are popular in industrial applications. In this study, Rail Creep and RSM methods were employed to monitor the current SFT of left and right rails on straight and curved track at the Edungalba trial site (Fig. 4). Rail creep is the longitudinal movement of the rails that occurs when thermal and train dynamic forces exceed the longitudinal resistance to rail movement provided by the rail to sleeper fasteners and by sleepers being bedded into the track ballast. Conventional rail creep monitoring, although labour intensive, is considered to be the most simple and effective way to understand a track's rail stress behaviour at set locations. This method, along with the RSM technique, has been used at the trial site at Edungalba to monitor the change in SFT (Fig. 4). The RSM module is produced by Salient Systems and it uses strain gauge principles to measure rail SFT. A full bridge strain gauge circuit is installed in the RSM with a 45o rosette strain gauge element. It is a self-contained measurement system that is mounted on the web of the rail to measure the local longitudinal stress and temperature of the rail. This combination of longitudinal stress and rail temperature is used to calculate SFT at regular intervals which is then stored within the module or transmitted wirelessly in real-time.
Fig. 5. Railframe. 192
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Fig. 6. (a) Creepometer, (b) Laser rail creep monitoring.
Fig. 7. Rail Stress Monitor modules on site.
Fig. 8. Vortok Verse in use.
3. Measurement and data acquisition The field investigation was conducted at Edungalba within the narrow gauge heavy haul Blackwater coal system. This site was chosen based on the factors of geographical location, dynamic action of rail traffic, track construction, geometry, and specific site conditions. The railway track at the site was composed of both straight and curved track. Emphasis was also placed on a track that had evidenced an increased buckle risk. The site specific detailed characteristics of the project site are as follows:
• 26.5 tonne axle load • 60 kg/m Rail, Head hardened • Concrete Sleepers (685 mm spacing) • Fist Rail to Sleeper Fasteners • Crushed Blue Metal Ballast • Both Straight Track and 400 m Radius Curve 193
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Fig. 9. Edungalba field test setup [1].
• 1 in 92 gradient • Electrified overhead line equipment (OHLE). The following equipment was installed in track at the site (Fig. 9):
• 8 RSM modules (4 on curve, 4 on straight). • 15 connected strain gauges (both strain gauge rosettes and linear strain gauges). • Thermocouple to measure rail temperature. • Weather station. • RSM wayside monitoring equipment. This equipment and the data collected has been used in this Rail Stress Analysis project to increase the monitoring time of the site characteristics for use in long term comparisons. The establishment of field investigations required the need to follow all QR National/Aurizon safe working practices and the presence of a Protection Officer (PO) at all times staff were on site. The installation of measurement devices has been conducted in accordance with the relevant manufacturers' specifications. A total of eight RSM modules have been installed on straight and curved track at the trial site (Figs. 4, 9 and 10). The data collected from these RSM's have been used in the analysis. Rail stress information was measured and collected from the trial site and then analysed using the Rail Creep method and RSM modules to compare the two sets of SFT data. 3.1. Rail creep A total of eight rail creep surveys were conducted at approximately 4 weekly intervals in which the net longitudinal rail creep was measured over a 501 m track section. A laser device was used to measure longitudinal rail movement of reference marks placed on the head of the rail with respect to fixed trackside markers. The SFT based on the rail creep method has been calculated using the following relationship of thermal expansion:
TN = TN 0 +
ΔL Lα
(1)
where TN is the Stress Free Temperature (°C) at the time of rail creep measurement, TNO is the Design Neutral Temperature (°C), ΔL is the expansion or contraction of the rail (mm), α is the linear coefficient of thermal expansion (11.5 × 10−6/°C)), and L is the length of the rail section (mm). 3.2. RSM modules The trial of 8 Salient System RSM's (Fig. 7) has been evaluated by a thorough and detailed analysis of the recorded rail stress 194
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Fig. 10. Project site on down track looking toward Rockhampton.
information. The RSM's use an internal strain gauge to measure rail force combined with an integrated temperature sensor to measure rail temperature and calculate the reading of SFT. This is achieved through the relationships shown in Eqs. (2) & (3):
εL =
σL + α (TR − TN ) E
TN 0 +
(2)
εL σ = TN 0 + L + TR − TN α Eα
(3)
where εL is longitudinal strain, σL is the longitudinal stress in the rail (MPa), E is Young's Modulus of Elasticity (N/m2), α is the linear coefficient of thermal expansion, TN0 is the Design Neutral Temperature (°C), TN is the Stress Free Temperature (°C), TR is the Rail Temperature (°C). Data of particular interest has been:
• The drift of rail SFT from DSFT. • Trends in the change of SFT under a number of different climatic, operational and geometric conditions. The RSM's measure and record data at 10 minute intervals, allowing a detailed analysis of data to be undertaken. Fig. 11 shows the Stress-Net graphing page that allows the display of longitudinal rail stress, rail temperature and SFT in a graphical form. Whilst this software has been used to a certain extent, the main analysis and data representation has been conducted manually through Microsoft Excel. By conducting an independent data analysis, the accuracy of the data and testing methods have been checked. 4. Results and discussion 4.1. Introduction Over time, the SFT of the rail can drift from that of the initial DSFT due to factors involving traffic operations, temperature changes, and deficient track condition. Any difference of the actual rail temperature from the SFT will produce a longitudinal load. As pointed out previously, the two most important parameters affecting the stability of Continuous Welded Rail (CWR) are rail stress and the lateral resistance of the track. As advances in track buckling theory develop, railway companies can better manage maintenance practices and provide an adequate margin of safety against track buckling. Railway maintenance practices that help lessen the development of buckling prone conditions include rail laying, welding, and repair practices. These activities should help to maintain a low longitudinal rail stress condition, avoiding the loss of track stability that leads to track buckles. It was pointed out before that the techniques to monitor the SFT of the rail have proved to be limited and/or difficult to employ, hence railways generally rely on preventative measures to control buckling risk. SFT tends to be measured in targeted areas only due to the costs involved in the measuring process and the need to shut down revenue lines. Salient Systems was a global leader in realtime failure mode detection for the railway industry and stated that the only way to ensure a safe track condition was to continuously monitor the system. It is a desire for railway companies to find an effective and accurate means of measuring longitudinal forces 195
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Fig. 11. Salient System's Stress-Net Software home screen.
through SFT.
4.2. Rail creep results The rail creep survey indicated that both rails moved in the down direction (+) toward Rockhampton (Fig. 10) with net movement of 15 mm and 3 mm out of the track section for Right and Left rails respectively over a total period of 28 weeks. This is an expected result as this track is predominantly used for loaded coal traffic travelling in the down direction (Fig. 4). The longitudinal rail movement resulted in an overall SFT increase of 2.60 °C on the Right Rail and 0.52 °C on the Left Rail. The SFT change based on the rail creep method is shown in Fig. 12. Within Aurizon's Central Queensland Coal Network (CQCN), rail creep has, in most cases,
Fig. 12. SFT Trend from Rail Creep measurements. 196
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Fig. 13. Long Term Average SFT Overview – Straight and Curved Track.
shown to move in the direction of loaded rail traffic. This is due to the higher axle loading and dynamic forces induced on the track structure. The absolute longitudinal rail movement was a combination of creep through fastenings and sleeper movement within the ballast. It was found that the change in SFT varied more with a greater change in Rail Temperature between surveys, and there was a strong correlation between change in SFT and changes in lateral track alignment. 4.3. RSM results Of the 8 RSM modules employed in this study, two were found to be inaccurate and showed erratic SFT readings. The cause of this has been shown to be a failure in the module's internal strain gauge. The data analysis in this study has been restricted to reliable data which follows theoretically acceptable trends for the change in SFT on a stable track structure. The RSM data shows that the SFT is relatively stable on the straight track, while the curved track shows a decrease in SFT over the observation period (Fig. 13). The field observations showed the curved track has a trend of decreasing SFT by 3 °C to 5 °C over the cooler months of winter and is stable over summer (Fig. 14). This drop in SFT was caused by a combination of change in track lateral alignment and the predominant traffic direction contributing to increased rail creep. The rail temperature falls below the SFT in
Fig. 14. Curved Track Change of SFT with Rail Temperature. 197
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Fig. 15. Change of SFT with Rail Temperature over 10 days.
winter months, subjecting the rails to high axial tensile forces. These tensile forces, combined with the dynamic effects of coal traffic, are thought to have been the cause for this shift in lateral track alignment during cooler months. The axial compressive forces developed in hotter months were shown to be insufficient of themselves to restore the curved track to the original alignment. It was found that, on average, the SFT would rise by 2 °C and decrease by 1 °C from the mean stable SFT over the temperature extremes of a 24 hour period (Fig. 15). The reason for this change in SFT due to fluctuations in rail temperature can be explained by the resulting variations in thermal stresses which alter longitudinal rail force throughout the day. This is a phenomenon not commonly understood or documented. As Figs. 13 and 14 show the trend of variation of SFT for rails in curved track along with other information, considering the combined effects of rail temperature and longitudinal rail stresses due to rail temperature changes, RSM calculates SFT at regular intervals and then it is stored in the modules or transmitted wirelessly in real time. Analysing the real time data given by RSMs, longitudinal stresses and SFT in both straight and curved track can be monitored effectively. The differences in SFT between adjacent rails have been considered for straight track and curved track, as shown in Figs. 16 and 17 respectively. It was found that, on the straight track, the RSM's followed a similar trend (Fig. 16) and showed an average difference in SFT of 2.4 °C. It is evident from Fig. 17 that the left rail on the curve showed a steeper decrease in SFT compared to that of right rail. The average difference between rails on the curved track is 2.3 °C. The right rail is the high rail of the curve. The variation in SFT between adjacent rails in the curve is due to actual movement, not the small difference in radius. The high rail is subjected to lateral forces due to wheel flanging, and this is likely to be the cause of the reduced SFT difference. The fastenings were deemed of similar age and toe load for both rails. The SFT of the rail has been found to vary with temperature changes as discussed in earlier sections. Fig. 18 shows the typical SFT and Rail Temperature variations over a day in January 2012. It was found that, on average, the SFT would rise by 2 °C and decrease by 1 °C from the mean stable SFT read by the RSM module over the temperature extremes of a day. On a stable track, it can be said that the longitudinal, lateral and vertical movement of the rail is insignificant over the course of a day. As the RSM measures rail strain and converts this to stress to calculate SFT, the fluctuations observed are thought to be caused by the internal thermal stress developed in the rail due to the change in rail temperature. 4.4. Comparison between rail creep and RSM results The average difference between the SFT data obtained by the RSM's and the Rail Creep Method is 1.29 °C which is considered to be insignificant as the allowable RSM error limit is 20–3 °C. Limitations are placed on the conventional rail creep measurement method due to the use of measurement equipment which can result in variations in SFT. Due to the likelihood of inaccuracies caused by the non-uniformity of the track structure over the 501 m section, the rail creep method should only be used for a basic estimation of change in SFT. The change in SFT calculated from the Rail Creep method is shown in Table 1 for the observation period. As the RSM modules collect data at 10 min intervals, the SFT at the time each survey has been conducted is used for comparison. The average column is 198
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50
Variation in SFT between Adjacent Rails - Straight Track
48 46
SFT (°C)
44 42 40 38 36 34 32 30
SFT- 0795 (Right Rail - Straight Track) SFT- 0770 (Left Rail - Straight Track)
Date (DD/MM/YY)
Fig. 16. Variation in SFT between adjacent rails – straight track.
Variation in SFT between Adjacent Rails - Curved Track 40 38 36
SFT (°C)
34 32 30 28 26 24 22 20 22/03/2011
21/04/2011 SFT- 0796 (Left Rail - Curved Track) SFT- 0643 (Right Rail - Curved Track)
21/05/2011
20/06/2011
20/07/2011
Date (DD/MM/YY)
Fig. 17. Variation in SFT between adjacent rails – curved track.
taken as the track's SFT between the two rails as defined in the Civil Engineering Track Standards (CETS) applied by Aurizon. Fig. 19 shows plots of the SFT trend based on the RSM and Rail Creep calculation. For comparison, the stress state of the site was taken as the SFT provided by the RSM modules at the beginning of the rail creep measurement survey. It is evident that, while there are some discrepancies in the data, the overall trend of SFT is rather similar over the 8 month monitoring period. The overall average increase of SFT for the RSM's and Rail Creep Method is 2.53 °C and 1.56 °C respectively. Given that the average difference between the data obtained by the RSM's and the Rail Creep Method was 0.97 °C, this was considered to be insignificant. Fig. 19 is based on net change in SFT, considering the change in both sites over the 500 m sections.
199
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Change of SFT over a Day 60
Temperature (°C)
50
40
30
20
10
0 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 4/01/2012 0:00 2:20 4:40 7:00 9:40 12:10 14:30 16:50 19:10 21:40 0:00 Date/Time (DD/MM/YY) EST
2 per. Mov. Avg. (SFT- 0796) 2 per. Mov. Avg. (TEMP- 0796)
Fig. 18. Change in SFT and rail temperature over a day. Table 1 Change in SFT comparison of Rail Creep and RSM data. Δ SFT comparison (°C) Date
26-Jul-11 26-Aug-11 06-Oct-11 09-Nov-11 07-Dec-11 05-Jan-12 08-Feb-12 28-Mar-12 Total Over Survey
Left Rail
Average ( x Left, Right)
Right Rail
RSM
Rail Creep
RSM
Rail Creep
RSM
Rail Creep
– −0.01 1.36 −0.59 1.78 −0.40 0.18 −0.65 1.66
– 3.12 −1.74 0.00 −2.08 1.91 −0.69 0.00 0.52
– 3.05 −0.07 −0.65 3.08 −1.04 −1.48 0.52 3.40
– 2.95 0.87 0.52 −0.52 −0.52 −0.17 −0.52 2.60
– 1.52 0.64 −0.62 2.43 −0.72 −0.65 −0.06 2.53
– 3.04 −0.43 0.26 −1.30 0.69 −0.43 −0.26 1.56
4.5. Analysis According to QR National's CETS the following SFT tolerances are to be managed:
• SFTs of both rails need to be within −7 °C and +8 °C of DSFT for concrete sleeper track. • Adjacent rails of a track must not exceed a difference of 9 °C. • Maximum lateral movement of 50 mm over a 100 m length. • Total net rail creep of 50 mm over a 500 m section. This research over a 22 month observation period has shown that the testing site does not comply with these minimum infrastructure requirements. As discussed before, the SFT on the curved track has decreased by an average of 8 °C which is 1 °C outside of the SFT tolerances previously outlined by CETS. Note however that SFT tolerances have recently been changed to 38 ± 10 °C. Over the same 22 month period, the measured lateral track misalignment is 97 mm from the design alignment on the curve which is outside of the SFT tolerances. This drop of SFT on the curve results in a significant risk during summer months where the track has an increased tendency to buckle at a lower rail temperature than it would if the rail SFT was closer to the DSFT. Any variation in rail temperature from the SFT results in longitudinal forces in the rail. The buckling potential is related to the ability of the track structure to withstand these forces. The heavy track structure used within QR National's coal system does provide a greater lateral resistance compared to that of timber 200
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Fig. 19. SFT observation – Rail Creep vs RSM trend.
or steel sleeper track; this in turn increases its resistance to track buckling. The research project was innovative in the way that Aurizon (formerly called QR National) trialled the use of strain gauges for the measurement and management of SFT in their heavy haul railway network. The application of these methods led to new understanding of how SFT behaves and the accuracy of the commonly accepted rail creep and offset monitoring method. Monitoring SFT continuously over a two-year period is something that is still new for most railways around the world. This was a first for Australia's largest rail freight operator and the new findings changed the standard and the way the rail stress was managed. Since this project, the increased confidence in the laser rail creep method has resulted in this becoming a standardised way to measure changes in the stress and intervene before the rail stress state becomes unsafe. 5. Conclusion The field investigation has enabled the observation of SFT through a number of testing methods and conditions. The conventional rail creep method is useful for a general indication of the track's longitudinal stress condition and its results follow a similar trend to the RSM data. The accurate measurement of SFT throughout the entire network through the installation of RSM modules is not a cost-effective solution for the management of longitudinal rail stress. However, the modules demonstrated an effectiveness to monitor and facilitate the management of this stress in targeted problem areas. The RSM modules are especially useful for the monitoring of longitudinal rail stress in curves. The remote and frequent monitoring of SFT that these modules provide also makes them suitable for the management of hot weather speed restrictions. An error of just 1 mm in the setting of the rail gap during rail laying and any subsequent restressing procedure results in a variance in SFT of 0.87 °C over a 100 m section. This, combined with rounding of temperatures in the current stressing method, leads to an overall accuracy of approximately ± 3 °C dependent on the measuring equipment used. 6. Recommendations The following recommendations have been made as a result of the study: 1. A rail cut-and-measure procedure should be undertaken on both straight and curved track to help validate the accuracy of the readings obtained by RSM modules. A restress should be conducted on the curves to bring the SFT back to DSFT and within the tolerances outlined in track safety standards. 2. The relevant CETS outlines the minimum safety standard for monitoring and managing longitudinal rail stress. There is a need for a best practice manual to be developed which outlines how railway managers can more effectively monitor and manage this issue within the network. 3. The effect of maintenance activities on the track's SFT needs to be investigated within the coal rail network. This can be achieved by monitoring and analysing the data from RSM modules when maintenance works are conducted in the area. 4. The management of longitudinal rail stress should be targeted through correct installation and maintenance of rail to help reduce SFT variance due to inaccuracies in the welding procedure. 201
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5. It is recommended that curves be targeted and monitored to ensure that rail creep and lateral track misalignment do not contribute to a significant change in SFT over winter. 6. The practice of implementing hot weather speed restrictions should be reviewed following the findings of a slight increase in SFT at peak daily rail temperatures. This increase in SFT means there is less potential for track buckling at the highest rail temperatures than originally thought. 7. RSM modules can be used at targeted locations that are known to be problematic with regard to lateral track stability. Short and long term monitoring can enable an evaluation of the track longitudinal stress condition and how the track is behaving. Acknowledgements Tim McSweeney, Adjunct Research Fellow, CRE is thankfully acknowledged for his advice at many stages of this ongoing study. References [1] N.K. Mandal, M. Lees, An investigation into monitoring rail stress in continuously welded rails through stress free temperature, Conference of Railway Engineering, 16–18 May, Melbourne, Australia, 2016. [2] D. Vangi, A. Virga, A practical application of ultrasonic thermal stress monitoring in continuous welded rails, Exp. Mech. 47 (5) (2007) 617–623. [3] Q.R. Network, Rail Stressing Manual. QR National Technical Standard CEP026C, (2009). [4] C. Esveld, Modern Railway Track, 2nd ed, MRT Productions, Delft, The Netherlands, 2001. [5] V. Simpson, Track Buckle Cost. QR National, (2012). [6] I. Hoather, N.K. Mandal, Management of Rail Stress in a Modern Railway Maintenance Infrastructure, Conference of Railway Engineering. 16–18 May, Melbourne, Australia, (2016). [7] R. Phillips, C. Nucera, F.L. Scalea, M. Fateh, Field testing of prototype systems for the non-destructive measurement of the neutral temperature of railroad tracks, Proc. of the 2014 Joint Rail Conference, Colorado Springs, CO, USA, 2014 (April 2~4). [8] I. Marks, SFT Measurement, Queensland Rail, 2012. [9] S.S.N. Ahmad, Ensuring Track Safety and Reducing Unnecessary Train Speed Restrictions in Hot Weather by the Application of a Unified Track Stability Management Tool, Masters Thesis Centre for Railway Engineering, 2011 (CQUniversity, Rockhampton, Australia). [10] Salient Systems Inc, System and Method for Determining Rail Safety Limits. United States Patent US 7,502,670B2, (2009). [11] Tom Judge, Measuring stress to combat sun kinks, Railway Track & Structures, 2003, pp. 43–49. April. [12] S.S. Ahmad, N.K. Mandal, G. Chattopadhyay, J. Powell, Development of a unified railway track stability management tool to enhance track safety, J. Rail Rapid Transit. 227 (5) (2013) 493–516. [13] E. Kabo, A numerical study of the lateral ballast resistance in railway tracks, J. Rail Rapid Transit. 220 (2006) 425–433. [14] A. Kish, D.W. Clark, Improving hot weather speed restrictions for track buckling derailment prevention, Proc. of the IHHA 2015 Conference, 21–24 June, Perth, Australia, 2015. [15] I.N. Martinez, I.V. Sanchis, P.M. Fernandez, R.I. Franco, Analytical model for predicting the buckling load of continuous welded rail tracks, J. Rail Rapid Transit. 229 (5) (2015) 542–552. [16] G.P. Pucillo, Thermal buckling and post-buckling behaviour of continuous welded rail track, Veh. Syst. Dyn. 54 (12) (2016) 1785–1807. [17] P.J. Grabe, D. Jacobs, The effects of fastening strength on the variation in stress-free temperature in continuously welded rail, J. Rail Rapid Transit. 223 (3) (2016) 840–851. [18] G. Yang, M.A. Bradford, Thermal-induced buckling and postbuckling analysis of continuous railway tracks, Int. J. Solids Struct. 97-98 (2016) 637–649. [19] G. Yang, M.A. Bradford, On train speed reduction in circumstances of thermally-induced railway track buckling, Eng. Failure Anal. (2018), https://doi.org/10. 1016/j.engfailanal.2018.02.009 accepted for publication. [20] QR National, Measurement of Rail Stress-Free Temperature Manual. QR National Technical Standard CEP044, (2006). [21] S. Gokhale, S. Hurlebaus, Monitoring of the stress free temperature in rails using the acoustoelastic effect, AIP Conf. Proc. 975 (1) (2008) 1368–1373.
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Effectiveness of measuring stress-free temperature in continuously welded rails by Rail Creep Method and Rail Stress Modules
T
⁎
Nirmal K. Mandala, , Mitchell Leesb a b
Central Queensland University, Centre for Railway Engineering, Bruce Highway, Rockhampton, Queensland, Australia Aurizon Network Asset Management, Mackay, Queensland, Australia
S U M MA R Y
Continuously welded rail track has two critical parameters that affect track stability, namely longitudinal rail stress and lateral resistance of the track structure. Maintaining the stress free temperature of rail plays an important role in managing of rail stress focussing on lateral track stability. Railway companies are focusing on finding an effective means of minimising longitudinal rail forces through management of Stress Free Temperature. In this study, the effectiveness of commercially available rail stress monitors and the rail creep measurement methods to monitor rail stress in railway systems has been investigated. A field investigation of a section of track at Edungalba in Aurizon's heavy haul coal network has enabled observation of Stress Free Temperature through several testing methods. The investigation and analysis has led to insights into track behaviour and stress state. The study concluded that the conventional rail creep method is useful for general indications of track stress condition and follows a similar trend to rail stress monitor data. It was also concluded that accurate measurement of Stress Free Temperature throughout the entire network through installation of rail stress monitor modules may not be a cost-effective solution for the management of rail stress. However, the modules demonstrated an effectiveness to monitor and manage rail stress in targeted problem areas.
1. Introduction The management of rail stress continues to be a cause of concern and an area of rapid development across railway companies worldwide. The two most important parameters affecting the stability of Continuously Welded Rail (CWR) are longitudinal rail stress and the lateral resistance of the track. Longitudinal rail stress is difficult to measure and is a significant structural concern as high tensile forces can lead to breaks in the rail and high compressive forces can cause track buckles and, in worst case scenarios, derailments. In managing longitudinal rail stresses due to temperature changes, both actual rail temperature and stress free temperature (SFT) are important. The actual rail temperature is very sensitive to the surrounding environmental temperature and other conditions beside the rail. The SFT is the temperature at which there is no thermally caused longitudinal stress present in the rail. The railway companies lay rails on the track bed in such a way that there is no axial stress present in the rail at a set temperature. This temperature is called the Design Stress Free Temperature (DSFT) but the current SFT is not always the same as the DSFT. The drift of current SFT from DSFT depends on environmental, operational and geographical factors [1]. Rail restressing is the process of adjusting the length of continuously welded rail to re-establish the DSFT. The action of rail traffic over time causes some limited longitudinal displacement of rails, resulting in a drift of the actual SFT away from DSFT. The rail temperature, sensitive to ambient air temperature and related factors, is continuously changing and this also has small effects on SFT. Therefore it is most important to monitor both the longitudinal movement of the rails and their SFT to manage longitudinal rail stress effectively. Any difference of the actual rail temperature from the SFT will produce a longitudinal load along the rails [2]. It is necessary to ⁎
Corresponding author. E-mail address: [email protected] (N.K. Mandal).
https://doi.org/10.1016/j.engfailanal.2019.05.032 Received 27 August 2018; Received in revised form 13 May 2019; Accepted 29 May 2019 Available online 30 May 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Annual Rail Temperature (°C) vs DSFT [3].
keep the SFT as close as practically possible to the DSFT to minimise the extent of temperature generated rail stress with the aim of maintaining the safe balance between the maximum summer and minimum winter temperature extremes that can potentially cause buckles and pull-apart respectively (Fig. 1). Use of CWR has revolutionised the railway industry; however, it is a well-known risk that CWR is susceptible to failure caused by changes in temperature. Rails have a higher coefficient of thermal expansion relative to the underlying track structure, making them particularly sensitive to temperature fluctuations. Under normal operational conditions, railway rails are subject to vertical and longitudinal loads and lateral forces. Esveld [4] reports that these loads result in the following stresses in the rails:
• normal stresses which are determined primarily by temperature change; • bending stresses caused by the vertical train loading; and, • residual stresses due to manufacturing processes and contact stresses in the rail head. A typical track buckle derailment on the Queensland heavy haul network costs up to $1.2million in infrastructure repairs and rolling stock recovery [5]. A recent study [6] indicates that the railway company Aurizon is responsible for the movement of about 700,000 t of iron ore, coal and other freight across Australia each day. It also indicates that, because of extreme hot weather, track buckling (Fig. 2) has been an issue in Central Queensland as it has been throughout the world. According to US Federal Railroad Administration (FRA) safety statistics data, there were 2000 derailments between January 1995 to February 2012 and 126 between January 2010 to December 2012 in the USA alone [7]. This has a detrimental effect on the reliability of heavy haul transportation systems and the coal export industries, emphasising the need for effective rail stress management to avoid such occurrences and costs. A number of rail stress testing techniques have been implemented in a field investigation of a section of track in Aurizon's heavy haul coal network. This investigation was to assess track behaviour and discover trends that may assist in the active management of longitudinal rail stress. The aim of this study was to assist Aurizon and CQUniversity's Centre for Railway Engineering to better understand the effectiveness of using Salient Systems' rail stress monitors (RSMs) and the conventional rail creep measurement method by undertaking a trial at Edungalba, located approximately 87 km west of Rockhampton, Queensland (Figs. 3, 4). The wider objectives of this research are to evaluate different rail stress testing methods, implement a number of methods at the trial site to
Fig. 2. Thermal Stress Induced Buckle. 190
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Fig. 3. Aurizon Blackwater Heavy Haul Corridor map.
Fig. 4. Project site route map details with locations of RSMs.
determine stress condition, monitor and analyse RSM data to look for trends in SFT change, report on accuracy and reliability of testing techniques and monitor rail creep and use for comparison. The lateral track strength and the influence of track misalignment on lateral strength, along with rail temperature and its relationship with environmental temperature, are not considered in the paper. It is evident that SFT of rail plays a critical role in understanding the monitoring and management of rail stress. Railway managers are interested in the measurement of rail SFT because it is an important requirement of safety management standards. Evaluating the performance of measurement equipment and developing an overall strategy that railway managers can implement to comply with these safety standards is therefore a priority [8]. Many SFT measurement techniques have proved to be limited in their use and difficult to employ except by undertaking night time measurements, meaning railways generally rely on preventative measures to control buckling risk. SFT tends to be measured only in targeted problem areas due to the costs involved in the measuring process and the need to shut down revenue lines [9]. A failure mode detection system that continuously monitors rail stress in real-time as a 191
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replacement for intermittent manual measurements is strongly desired by railway companies. Such a system must accomplish the following [10]:
• Reliably and accurately determine the current SFT with minimum track occupancy and labour; • Provide a means of continuously monitoring so as not to have a window of missed condition when the SFT could swing outside of the safe range.
Given the safety requirements that make measurement of SFT so important, there have been many studies over recent decades that focused on various methods. Laser vibrometry [11], strain gauges [12] and other methods for measuring longitudinal rail stress and SFT are discussed in more detail in the next section (Section 2) along with measurement of lateral resistance of track [13]. Ahmad et al. [12] pointed out the daily change of SFT by 2–3 °C through field trials and proposed a simple track buckling management tool considering two major parameters: rail stress and track resistance. Other related studies on speed restrictions [14], buckling loads [15], post-buckling behaviour [16], and fastening strength on variation of SFT [17] are important. A thorough literature search reveals that a long term study on SFT is necessary for specific track structures under particular operational scenarios to maintain longitudinal rail stress in a safe range to prevent track buckles that compromise railway safety. This study focuses on achieving this outcome by using rail stress monitors and rail creep methods. Buckling behaviour and its analysis [18,19] is not included in this study. An earlier version of this paper was previously presented in a conference [1].
2. Measurement methods It is important to measure the SFT regularly to ensure that it remains within the DSFT zone for safe operation and to minimise the risk of track buckling [9]. The drift of SFT from the DSFT is due to several contributing factors involving traffic operations, changes of rail temperature and deficient track condition [20]. For many years the railway industry has been in search of an efficient and accurate method to safely and effectively measure the SFT of rail in track. Over the years, different methods of measuring the SFT in the rails have been developed and practiced within the railway industry. Gokhale & Hurlebaus [21] reported that many of these techniques are time consuming, difficult to implement, destructive and labour intensive. Various SFT measurement methods such as Rail Cut and Measure, Railframe equipment (Fig. 5), Rail Creep Monitoring (Fig. 6), Strain Gauges, Rail Stress Monitor modules (Fig. 7), Vertical Rail Stiffness Equipment (VERSE) (Fig. 8), D'stresen, SFTPro, Rail Scan, Magnetic Anisotropy and Permeability System (MAPS) and Acousto Elastic Effect (Ultrasonic) are popular in industrial applications. In this study, Rail Creep and RSM methods were employed to monitor the current SFT of left and right rails on straight and curved track at the Edungalba trial site (Fig. 4). Rail creep is the longitudinal movement of the rails that occurs when thermal and train dynamic forces exceed the longitudinal resistance to rail movement provided by the rail to sleeper fasteners and by sleepers being bedded into the track ballast. Conventional rail creep monitoring, although labour intensive, is considered to be the most simple and effective way to understand a track's rail stress behaviour at set locations. This method, along with the RSM technique, has been used at the trial site at Edungalba to monitor the change in SFT (Fig. 4). The RSM module is produced by Salient Systems and it uses strain gauge principles to measure rail SFT. A full bridge strain gauge circuit is installed in the RSM with a 45o rosette strain gauge element. It is a self-contained measurement system that is mounted on the web of the rail to measure the local longitudinal stress and temperature of the rail. This combination of longitudinal stress and rail temperature is used to calculate SFT at regular intervals which is then stored within the module or transmitted wirelessly in real-time.
Fig. 5. Railframe. 192
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Fig. 6. (a) Creepometer, (b) Laser rail creep monitoring.
Fig. 7. Rail Stress Monitor modules on site.
Fig. 8. Vortok Verse in use.
3. Measurement and data acquisition The field investigation was conducted at Edungalba within the narrow gauge heavy haul Blackwater coal system. This site was chosen based on the factors of geographical location, dynamic action of rail traffic, track construction, geometry, and specific site conditions. The railway track at the site was composed of both straight and curved track. Emphasis was also placed on a track that had evidenced an increased buckle risk. The site specific detailed characteristics of the project site are as follows:
• 26.5 tonne axle load • 60 kg/m Rail, Head hardened • Concrete Sleepers (685 mm spacing) • Fist Rail to Sleeper Fasteners • Crushed Blue Metal Ballast • Both Straight Track and 400 m Radius Curve 193
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Fig. 9. Edungalba field test setup [1].
• 1 in 92 gradient • Electrified overhead line equipment (OHLE). The following equipment was installed in track at the site (Fig. 9):
• 8 RSM modules (4 on curve, 4 on straight). • 15 connected strain gauges (both strain gauge rosettes and linear strain gauges). • Thermocouple to measure rail temperature. • Weather station. • RSM wayside monitoring equipment. This equipment and the data collected has been used in this Rail Stress Analysis project to increase the monitoring time of the site characteristics for use in long term comparisons. The establishment of field investigations required the need to follow all QR National/Aurizon safe working practices and the presence of a Protection Officer (PO) at all times staff were on site. The installation of measurement devices has been conducted in accordance with the relevant manufacturers' specifications. A total of eight RSM modules have been installed on straight and curved track at the trial site (Figs. 4, 9 and 10). The data collected from these RSM's have been used in the analysis. Rail stress information was measured and collected from the trial site and then analysed using the Rail Creep method and RSM modules to compare the two sets of SFT data. 3.1. Rail creep A total of eight rail creep surveys were conducted at approximately 4 weekly intervals in which the net longitudinal rail creep was measured over a 501 m track section. A laser device was used to measure longitudinal rail movement of reference marks placed on the head of the rail with respect to fixed trackside markers. The SFT based on the rail creep method has been calculated using the following relationship of thermal expansion:
TN = TN 0 +
ΔL Lα
(1)
where TN is the Stress Free Temperature (°C) at the time of rail creep measurement, TNO is the Design Neutral Temperature (°C), ΔL is the expansion or contraction of the rail (mm), α is the linear coefficient of thermal expansion (11.5 × 10−6/°C)), and L is the length of the rail section (mm). 3.2. RSM modules The trial of 8 Salient System RSM's (Fig. 7) has been evaluated by a thorough and detailed analysis of the recorded rail stress 194
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Fig. 10. Project site on down track looking toward Rockhampton.
information. The RSM's use an internal strain gauge to measure rail force combined with an integrated temperature sensor to measure rail temperature and calculate the reading of SFT. This is achieved through the relationships shown in Eqs. (2) & (3):
εL =
σL + α (TR − TN ) E
TN 0 +
(2)
εL σ = TN 0 + L + TR − TN α Eα
(3)
where εL is longitudinal strain, σL is the longitudinal stress in the rail (MPa), E is Young's Modulus of Elasticity (N/m2), α is the linear coefficient of thermal expansion, TN0 is the Design Neutral Temperature (°C), TN is the Stress Free Temperature (°C), TR is the Rail Temperature (°C). Data of particular interest has been:
• The drift of rail SFT from DSFT. • Trends in the change of SFT under a number of different climatic, operational and geometric conditions. The RSM's measure and record data at 10 minute intervals, allowing a detailed analysis of data to be undertaken. Fig. 11 shows the Stress-Net graphing page that allows the display of longitudinal rail stress, rail temperature and SFT in a graphical form. Whilst this software has been used to a certain extent, the main analysis and data representation has been conducted manually through Microsoft Excel. By conducting an independent data analysis, the accuracy of the data and testing methods have been checked. 4. Results and discussion 4.1. Introduction Over time, the SFT of the rail can drift from that of the initial DSFT due to factors involving traffic operations, temperature changes, and deficient track condition. Any difference of the actual rail temperature from the SFT will produce a longitudinal load. As pointed out previously, the two most important parameters affecting the stability of Continuous Welded Rail (CWR) are rail stress and the lateral resistance of the track. As advances in track buckling theory develop, railway companies can better manage maintenance practices and provide an adequate margin of safety against track buckling. Railway maintenance practices that help lessen the development of buckling prone conditions include rail laying, welding, and repair practices. These activities should help to maintain a low longitudinal rail stress condition, avoiding the loss of track stability that leads to track buckles. It was pointed out before that the techniques to monitor the SFT of the rail have proved to be limited and/or difficult to employ, hence railways generally rely on preventative measures to control buckling risk. SFT tends to be measured in targeted areas only due to the costs involved in the measuring process and the need to shut down revenue lines. Salient Systems was a global leader in realtime failure mode detection for the railway industry and stated that the only way to ensure a safe track condition was to continuously monitor the system. It is a desire for railway companies to find an effective and accurate means of measuring longitudinal forces 195
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Fig. 11. Salient System's Stress-Net Software home screen.
through SFT.
4.2. Rail creep results The rail creep survey indicated that both rails moved in the down direction (+) toward Rockhampton (Fig. 10) with net movement of 15 mm and 3 mm out of the track section for Right and Left rails respectively over a total period of 28 weeks. This is an expected result as this track is predominantly used for loaded coal traffic travelling in the down direction (Fig. 4). The longitudinal rail movement resulted in an overall SFT increase of 2.60 °C on the Right Rail and 0.52 °C on the Left Rail. The SFT change based on the rail creep method is shown in Fig. 12. Within Aurizon's Central Queensland Coal Network (CQCN), rail creep has, in most cases,
Fig. 12. SFT Trend from Rail Creep measurements. 196
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Fig. 13. Long Term Average SFT Overview – Straight and Curved Track.
shown to move in the direction of loaded rail traffic. This is due to the higher axle loading and dynamic forces induced on the track structure. The absolute longitudinal rail movement was a combination of creep through fastenings and sleeper movement within the ballast. It was found that the change in SFT varied more with a greater change in Rail Temperature between surveys, and there was a strong correlation between change in SFT and changes in lateral track alignment. 4.3. RSM results Of the 8 RSM modules employed in this study, two were found to be inaccurate and showed erratic SFT readings. The cause of this has been shown to be a failure in the module's internal strain gauge. The data analysis in this study has been restricted to reliable data which follows theoretically acceptable trends for the change in SFT on a stable track structure. The RSM data shows that the SFT is relatively stable on the straight track, while the curved track shows a decrease in SFT over the observation period (Fig. 13). The field observations showed the curved track has a trend of decreasing SFT by 3 °C to 5 °C over the cooler months of winter and is stable over summer (Fig. 14). This drop in SFT was caused by a combination of change in track lateral alignment and the predominant traffic direction contributing to increased rail creep. The rail temperature falls below the SFT in
Fig. 14. Curved Track Change of SFT with Rail Temperature. 197
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Fig. 15. Change of SFT with Rail Temperature over 10 days.
winter months, subjecting the rails to high axial tensile forces. These tensile forces, combined with the dynamic effects of coal traffic, are thought to have been the cause for this shift in lateral track alignment during cooler months. The axial compressive forces developed in hotter months were shown to be insufficient of themselves to restore the curved track to the original alignment. It was found that, on average, the SFT would rise by 2 °C and decrease by 1 °C from the mean stable SFT over the temperature extremes of a 24 hour period (Fig. 15). The reason for this change in SFT due to fluctuations in rail temperature can be explained by the resulting variations in thermal stresses which alter longitudinal rail force throughout the day. This is a phenomenon not commonly understood or documented. As Figs. 13 and 14 show the trend of variation of SFT for rails in curved track along with other information, considering the combined effects of rail temperature and longitudinal rail stresses due to rail temperature changes, RSM calculates SFT at regular intervals and then it is stored in the modules or transmitted wirelessly in real time. Analysing the real time data given by RSMs, longitudinal stresses and SFT in both straight and curved track can be monitored effectively. The differences in SFT between adjacent rails have been considered for straight track and curved track, as shown in Figs. 16 and 17 respectively. It was found that, on the straight track, the RSM's followed a similar trend (Fig. 16) and showed an average difference in SFT of 2.4 °C. It is evident from Fig. 17 that the left rail on the curve showed a steeper decrease in SFT compared to that of right rail. The average difference between rails on the curved track is 2.3 °C. The right rail is the high rail of the curve. The variation in SFT between adjacent rails in the curve is due to actual movement, not the small difference in radius. The high rail is subjected to lateral forces due to wheel flanging, and this is likely to be the cause of the reduced SFT difference. The fastenings were deemed of similar age and toe load for both rails. The SFT of the rail has been found to vary with temperature changes as discussed in earlier sections. Fig. 18 shows the typical SFT and Rail Temperature variations over a day in January 2012. It was found that, on average, the SFT would rise by 2 °C and decrease by 1 °C from the mean stable SFT read by the RSM module over the temperature extremes of a day. On a stable track, it can be said that the longitudinal, lateral and vertical movement of the rail is insignificant over the course of a day. As the RSM measures rail strain and converts this to stress to calculate SFT, the fluctuations observed are thought to be caused by the internal thermal stress developed in the rail due to the change in rail temperature. 4.4. Comparison between rail creep and RSM results The average difference between the SFT data obtained by the RSM's and the Rail Creep Method is 1.29 °C which is considered to be insignificant as the allowable RSM error limit is 20–3 °C. Limitations are placed on the conventional rail creep measurement method due to the use of measurement equipment which can result in variations in SFT. Due to the likelihood of inaccuracies caused by the non-uniformity of the track structure over the 501 m section, the rail creep method should only be used for a basic estimation of change in SFT. The change in SFT calculated from the Rail Creep method is shown in Table 1 for the observation period. As the RSM modules collect data at 10 min intervals, the SFT at the time each survey has been conducted is used for comparison. The average column is 198
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50
Variation in SFT between Adjacent Rails - Straight Track
48 46
SFT (°C)
44 42 40 38 36 34 32 30
SFT- 0795 (Right Rail - Straight Track) SFT- 0770 (Left Rail - Straight Track)
Date (DD/MM/YY)
Fig. 16. Variation in SFT between adjacent rails – straight track.
Variation in SFT between Adjacent Rails - Curved Track 40 38 36
SFT (°C)
34 32 30 28 26 24 22 20 22/03/2011
21/04/2011 SFT- 0796 (Left Rail - Curved Track) SFT- 0643 (Right Rail - Curved Track)
21/05/2011
20/06/2011
20/07/2011
Date (DD/MM/YY)
Fig. 17. Variation in SFT between adjacent rails – curved track.
taken as the track's SFT between the two rails as defined in the Civil Engineering Track Standards (CETS) applied by Aurizon. Fig. 19 shows plots of the SFT trend based on the RSM and Rail Creep calculation. For comparison, the stress state of the site was taken as the SFT provided by the RSM modules at the beginning of the rail creep measurement survey. It is evident that, while there are some discrepancies in the data, the overall trend of SFT is rather similar over the 8 month monitoring period. The overall average increase of SFT for the RSM's and Rail Creep Method is 2.53 °C and 1.56 °C respectively. Given that the average difference between the data obtained by the RSM's and the Rail Creep Method was 0.97 °C, this was considered to be insignificant. Fig. 19 is based on net change in SFT, considering the change in both sites over the 500 m sections.
199
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Change of SFT over a Day 60
Temperature (°C)
50
40
30
20
10
0 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 3/01/2012 4/01/2012 0:00 2:20 4:40 7:00 9:40 12:10 14:30 16:50 19:10 21:40 0:00 Date/Time (DD/MM/YY) EST
2 per. Mov. Avg. (SFT- 0796) 2 per. Mov. Avg. (TEMP- 0796)
Fig. 18. Change in SFT and rail temperature over a day. Table 1 Change in SFT comparison of Rail Creep and RSM data. Δ SFT comparison (°C) Date
26-Jul-11 26-Aug-11 06-Oct-11 09-Nov-11 07-Dec-11 05-Jan-12 08-Feb-12 28-Mar-12 Total Over Survey
Left Rail
Average ( x Left, Right)
Right Rail
RSM
Rail Creep
RSM
Rail Creep
RSM
Rail Creep
– −0.01 1.36 −0.59 1.78 −0.40 0.18 −0.65 1.66
– 3.12 −1.74 0.00 −2.08 1.91 −0.69 0.00 0.52
– 3.05 −0.07 −0.65 3.08 −1.04 −1.48 0.52 3.40
– 2.95 0.87 0.52 −0.52 −0.52 −0.17 −0.52 2.60
– 1.52 0.64 −0.62 2.43 −0.72 −0.65 −0.06 2.53
– 3.04 −0.43 0.26 −1.30 0.69 −0.43 −0.26 1.56
4.5. Analysis According to QR National's CETS the following SFT tolerances are to be managed:
• SFTs of both rails need to be within −7 °C and +8 °C of DSFT for concrete sleeper track. • Adjacent rails of a track must not exceed a difference of 9 °C. • Maximum lateral movement of 50 mm over a 100 m length. • Total net rail creep of 50 mm over a 500 m section. This research over a 22 month observation period has shown that the testing site does not comply with these minimum infrastructure requirements. As discussed before, the SFT on the curved track has decreased by an average of 8 °C which is 1 °C outside of the SFT tolerances previously outlined by CETS. Note however that SFT tolerances have recently been changed to 38 ± 10 °C. Over the same 22 month period, the measured lateral track misalignment is 97 mm from the design alignment on the curve which is outside of the SFT tolerances. This drop of SFT on the curve results in a significant risk during summer months where the track has an increased tendency to buckle at a lower rail temperature than it would if the rail SFT was closer to the DSFT. Any variation in rail temperature from the SFT results in longitudinal forces in the rail. The buckling potential is related to the ability of the track structure to withstand these forces. The heavy track structure used within QR National's coal system does provide a greater lateral resistance compared to that of timber 200
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Fig. 19. SFT observation – Rail Creep vs RSM trend.
or steel sleeper track; this in turn increases its resistance to track buckling. The research project was innovative in the way that Aurizon (formerly called QR National) trialled the use of strain gauges for the measurement and management of SFT in their heavy haul railway network. The application of these methods led to new understanding of how SFT behaves and the accuracy of the commonly accepted rail creep and offset monitoring method. Monitoring SFT continuously over a two-year period is something that is still new for most railways around the world. This was a first for Australia's largest rail freight operator and the new findings changed the standard and the way the rail stress was managed. Since this project, the increased confidence in the laser rail creep method has resulted in this becoming a standardised way to measure changes in the stress and intervene before the rail stress state becomes unsafe. 5. Conclusion The field investigation has enabled the observation of SFT through a number of testing methods and conditions. The conventional rail creep method is useful for a general indication of the track's longitudinal stress condition and its results follow a similar trend to the RSM data. The accurate measurement of SFT throughout the entire network through the installation of RSM modules is not a cost-effective solution for the management of longitudinal rail stress. However, the modules demonstrated an effectiveness to monitor and facilitate the management of this stress in targeted problem areas. The RSM modules are especially useful for the monitoring of longitudinal rail stress in curves. The remote and frequent monitoring of SFT that these modules provide also makes them suitable for the management of hot weather speed restrictions. An error of just 1 mm in the setting of the rail gap during rail laying and any subsequent restressing procedure results in a variance in SFT of 0.87 °C over a 100 m section. This, combined with rounding of temperatures in the current stressing method, leads to an overall accuracy of approximately ± 3 °C dependent on the measuring equipment used. 6. Recommendations The following recommendations have been made as a result of the study: 1. A rail cut-and-measure procedure should be undertaken on both straight and curved track to help validate the accuracy of the readings obtained by RSM modules. A restress should be conducted on the curves to bring the SFT back to DSFT and within the tolerances outlined in track safety standards. 2. The relevant CETS outlines the minimum safety standard for monitoring and managing longitudinal rail stress. There is a need for a best practice manual to be developed which outlines how railway managers can more effectively monitor and manage this issue within the network. 3. The effect of maintenance activities on the track's SFT needs to be investigated within the coal rail network. This can be achieved by monitoring and analysing the data from RSM modules when maintenance works are conducted in the area. 4. The management of longitudinal rail stress should be targeted through correct installation and maintenance of rail to help reduce SFT variance due to inaccuracies in the welding procedure. 201
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5. It is recommended that curves be targeted and monitored to ensure that rail creep and lateral track misalignment do not contribute to a significant change in SFT over winter. 6. The practice of implementing hot weather speed restrictions should be reviewed following the findings of a slight increase in SFT at peak daily rail temperatures. This increase in SFT means there is less potential for track buckling at the highest rail temperatures than originally thought. 7. RSM modules can be used at targeted locations that are known to be problematic with regard to lateral track stability. Short and long term monitoring can enable an evaluation of the track longitudinal stress condition and how the track is behaving. Acknowledgements Tim McSweeney, Adjunct Research Fellow, CRE is thankfully acknowledged for his advice at many stages of this ongoing study. References [1] N.K. Mandal, M. Lees, An investigation into monitoring rail stress in continuously welded rails through stress free temperature, Conference of Railway Engineering, 16–18 May, Melbourne, Australia, 2016. [2] D. Vangi, A. Virga, A practical application of ultrasonic thermal stress monitoring in continuous welded rails, Exp. Mech. 47 (5) (2007) 617–623. [3] Q.R. Network, Rail Stressing Manual. QR National Technical Standard CEP026C, (2009). [4] C. Esveld, Modern Railway Track, 2nd ed, MRT Productions, Delft, The Netherlands, 2001. [5] V. Simpson, Track Buckle Cost. QR National, (2012). [6] I. Hoather, N.K. Mandal, Management of Rail Stress in a Modern Railway Maintenance Infrastructure, Conference of Railway Engineering. 16–18 May, Melbourne, Australia, (2016). [7] R. Phillips, C. Nucera, F.L. Scalea, M. Fateh, Field testing of prototype systems for the non-destructive measurement of the neutral temperature of railroad tracks, Proc. of the 2014 Joint Rail Conference, Colorado Springs, CO, USA, 2014 (April 2~4). [8] I. Marks, SFT Measurement, Queensland Rail, 2012. [9] S.S.N. Ahmad, Ensuring Track Safety and Reducing Unnecessary Train Speed Restrictions in Hot Weather by the Application of a Unified Track Stability Management Tool, Masters Thesis Centre for Railway Engineering, 2011 (CQUniversity, Rockhampton, Australia). [10] Salient Systems Inc, System and Method for Determining Rail Safety Limits. United States Patent US 7,502,670B2, (2009). [11] Tom Judge, Measuring stress to combat sun kinks, Railway Track & Structures, 2003, pp. 43–49. April. [12] S.S. Ahmad, N.K. Mandal, G. Chattopadhyay, J. Powell, Development of a unified railway track stability management tool to enhance track safety, J. Rail Rapid Transit. 227 (5) (2013) 493–516. [13] E. Kabo, A numerical study of the lateral ballast resistance in railway tracks, J. Rail Rapid Transit. 220 (2006) 425–433. [14] A. Kish, D.W. Clark, Improving hot weather speed restrictions for track buckling derailment prevention, Proc. of the IHHA 2015 Conference, 21–24 June, Perth, Australia, 2015. [15] I.N. Martinez, I.V. Sanchis, P.M. Fernandez, R.I. Franco, Analytical model for predicting the buckling load of continuous welded rail tracks, J. Rail Rapid Transit. 229 (5) (2015) 542–552. [16] G.P. Pucillo, Thermal buckling and post-buckling behaviour of continuous welded rail track, Veh. Syst. Dyn. 54 (12) (2016) 1785–1807. [17] P.J. Grabe, D. Jacobs, The effects of fastening strength on the variation in stress-free temperature in continuously welded rail, J. Rail Rapid Transit. 223 (3) (2016) 840–851. [18] G. Yang, M.A. Bradford, Thermal-induced buckling and postbuckling analysis of continuous railway tracks, Int. J. Solids Struct. 97-98 (2016) 637–649. [19] G. Yang, M.A. Bradford, On train speed reduction in circumstances of thermally-induced railway track buckling, Eng. Failure Anal. (2018), https://doi.org/10. 1016/j.engfailanal.2018.02.009 accepted for publication. [20] QR National, Measurement of Rail Stress-Free Temperature Manual. QR National Technical Standard CEP044, (2006). [21] S. Gokhale, S. Hurlebaus, Monitoring of the stress free temperature in rails using the acoustoelastic effect, AIP Conf. Proc. 975 (1) (2008) 1368–1373.
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