Indian Railway Comparative Study

8/10/2019 Indian Railway Comparative Study

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COMPARATIVE STUDY OF VARIOUS METHODS OF MEASUREMENTS OF

STRESS FREE TEMPERATURE IN CWR TRACK

Introduction

As on March 05, on IR, route kilometrage is nearly 63,465 kms out of which83% is laid on PSC sleepers and 77% of track is laid as Long Welded Rails on Broad

Gauge (main line).

Long welded rails are laid and fastened to the sleepers at rail temperature which

is called destressing Temperature or Td. This temperature is kept 5 to 10 Celsiushigher than the mean rail temperature (Which is the average of minimum and maximum

rail temperature).

If LWR could be complely constrained (ideal situation), then there would be no

change in its destressing temperature (initial laying tempature). However, because therail can not be fully constrained in all directions, elongation or contraction can occur

whenever track is subjected to train and environmentally induced loads, causing

temperature changes above or below Td. The following rail /track kinematics influenceLWR stress-free tempeature.

Rail Creep can occur through fasteners during train braking andaccelerating actions as well as due to tempature gradients along the rail. Lossof toe load of fitting can also cause movement of rail.

Curve Shifting can occur under repeated vehicles lateral loads and high

curving forces. Track Vertical Settlement can occur under repeated vertical wheel loads

and high impact loads.

Apart from the above, track maintenance operations (such as lining, lifting,

tamping, repairing defective / broken rails, etc.) which alter the longitudinal force state

in the rails also influence the stress free tempature.

Hence the effective rail temperature at which the rail would be stress free, can bedifferent from the Tdat any point of time and in fact, can also vary along the length of

LWR at same time.

The importance of correct knowledge of the stress-free tempature (SFT) of rails

in LWR tracks is demonstrated by the fact that every 10oC variation in tempature causes

LWR force variation of 16 t in 52 kg and 19 t in 60 kg track respectively. The

maximum and minimum tempature in a day varies up to 15oC and 25

oC during summer

& winter respectively. Thus the locked up localized stress towards higher / lower sidecan result in fractures / buckling of track when compounded with the thermal forces.

This underlines the need for correct assessment of stess-free tempature (SFT) of rail.

The safety of Long Welded Rails (LWR) track is vitally affected by locked up

thermal stresses, which play an important role in the phenomenon of buckling or

fractures and thus have important bearing on safety. At present, the IR do not have any


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equipment to objectively assess these stresses and therefore de-stressing operations are

carried out based upon the observation of movements at Switch Expansion Joint (SEJ).

Some countries have developed devices to measure stress, in long-welded rails

`in-situ' which makes it possible to monitor and take timely action to avoid rail

fractures. It has been decided to obtain expertise to measure rail stress `in-situ', in theinterest of enhanced safety. This will help in introducing the concept of "Need Based

Destressing" of Long Welded Rails ( LWR) instead of calendar based system. This willnot only save huge costs but also postpone the need to demand longer traffic block at

the specific time of the day.

Methods of measurement of stress-free tempature

Measuring the longitudinal stresses in rail will give an estimation of theremaining safety margin in a given track section. There are various methods of

measurement of stress in the track.

(1) Conventional methods These methods only allow measurement of the

difference in stress between two states. Such methods can only be used to determine theabsolute stress level if an additional xeroing measurement is carried out. This involves

cutting out a section of rail at the point to be measured and neutralizing the stresses in it.On IR, two methods have been developed i.e.

(a)Cut & Weld method developed by RDSO

An initial zero force state is needed.

A group of LWRs with similar maintenance conditions can be choosen

to know their actual SFT.

Destress these LWRs & record tdo.

Make a cut in Central portion at temp 200 below tdo.

Measure gap (g) at Cut after movement stabilizes and note rail temptobs.

Compute Rmob= AE2

g

ttobsdo

2)(

The gap at fracture gobs is:

gobs= AE2

mob

obsdcal

Rtt

2

)(

tdcal= tobs+

1

AE

xRgmobobs

(b)Cut & Weld Method (Developed by IRICEN)

Initial zero force state not needed.

Erect two marker pillars A& B distance L apart (L > BreathingLength) and transfer these marks A & B on to the rail foot.

Make a saw cut at Mark A at a temperature 20oC below original td.


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Remove fastenings in length L.

Measure gaps gaand gb with respect to masrker pillasrs A & B aftermovement stabilizes. The rail temperature is also recorded tobs.

tdcal= tobs +L

gg ba

Limitations of destructive methods:

Requires line blocks & time consuming.

Stresses to be neutralized at the cut location.

Cutting & Re-welding costs

The railways are therefore interested in obtaining a non-destructive test methodthat would allow them to determine the longitudinal stress in a rail without cutting it.

The method of measuring the longitudinal force should have the followingcharacteristics.

Ability to measure the Longitudinal force in the rail.

Ability to measure the effective rail temp.

Ability to combine above two to give SFT.

To be able to compare the computed SFT with the original installed value.

To be able to operate continuously from train borne equipment.

The equipment should be sufficiently robust and weatherproofed towithstand the normal track and vehicle environment

The equipment must be inherently stable and not drift over long periods.

Data reduction should be performed automatically.

The system should be suitable for use with all types of rail and rail steel.

The system should be insensitive to temperature changes.

The recording car must not influence the measuring result.

The measuring system must be compatible with rails carrying return currentsand track circuit currents.

And the device to be developed shall meet the following requirements:

It should be portable.

It should be able to give absolute and accurate values.

The measurement should be independent of residual stresses in the rail.

Therefore, realizing the need to determine the general stress level in the LWR

and the difference in the stresses in the various portions of the same LWR, variousRailway Systems in the world made efforts to develop some methods of finding out the

stress level by way of Non-Destructive Instrumentation Device. The developmentalhistory of these measures is as follows:

(i) During early 80s in Japan, KANEKO INSTRUMENTS developed thesystem where two markings at a distance of one meter apart were made

on the web of the rail at the time of laying the rail / distressing i.e. when

the rail is stress free. Subsequently, exact distance between these two


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points was measured very very accurately along with the prevailing rail

temperature. The strain thus measured could be converted into stresslevel. Similar equipments were subsequently developed and adopted by

some of the European railway systems. This system has limitation thatinitial markings had to be done when the rail is stress-free and therefore

it was not possible to determine the stress in the rail without havingaccess to the zero stress level.

(ii) Monitor Rail Displacement method

Every 200m mark is made on the rail and its position W.r.t ground isrecorded.

The measurements are periodically repeated.

If the distance between two adjacent marks changes, it means thestress in the rail changes.

Track Segments whose distance between marks are shorter /longer

than before are compression/tension. The process is time consuming and not accurate.

(iii) Various laboratories in the United States tried to develop the systemusing the concept of strain gauging the rail. However, the system couldnot be developed to facilitate commercial applications.

(iv) In Europe itself during early 90s, heavy equipment was developed wherea on track rail car was used to pick up the rail, which has been released

by removing the clips, and subject it to bending deflection using ram inthe centre. The reactions at the support and load applied at the centre

were measured along with the relative deflection, which could beanalyzed to work out the stress level in the rail accurately. The

equipment was very costly and required blocking of the track as the

equipment was mounted on the rail car and had to be worked like a trackmachine.

(v) A simpler system was developed in UK where track was pulled up afterreleasing the clips up to 15 meters either side and stress in the rail level

worked out using the relationship between the pulling up force andtension in the rail. As in this method, reaction at the support cannot be

measured, therefore, the method is approximate when compared to themethod described in para (iv) above. However, in view of the simplicity

and the reasonable accuracy of this method, the same is being

extensively used by the various contractors of the European railroadsystem.

(vi) On similar line as in (v) above, PCE/C.Rly developed a procedure towork out the SFT of LWR. This non-destructive method of

determination of SFT involves pulling up of 30m unclipped rail length atcentre by application of vertically upward force and measuring the

corresponding deflection at prevalent rail temperature. This method

offers fast and low cost measurement, making SFT measurement more


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practical and financially viable. In this system, measurements can be

carried out quickly at much less cost compared to the conventional cutand weld method. Prevailing temperature of rail must be less than the

SFT so that the rail is in tension. Typical testing time will be about 30minutes for an experienced crew. This method is approved by Railway

Board to take up testing with equipment on trial basis.

The detailed procedure for working out SFT with this method is as

follows:


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Procedure of testing

(i) Insert 65 mm wedges at 2 locations at distance of 5 m at bothends of 30 m unclipped rail.

(ii) The head of rail at centre is clamped with lifting hook of stress

measuring equipment.(iii) Rail temperature is noted throughout the experiment.(iv) Initial sag or hog is measured at centre before applying upward

vertical force.(v) The tip of deflection measuring dial gauge is adjusted on top of

the rail and then force is applied progressively in controlled

manner.(vi) The reading of vertical load is noted at every 10mm interval of

upward deflection till a maximum reflection of 100mm isachieved at the cente over and above the sag or hog point.

(vii) Now the vertical load is released progressively in controlled

manner and readings are again noted again at every 10mminterval of down ward deflection and experiment is closed after

recording of load at 0 deflection.(viii) Stress-free temperature for the given rail section is computed

with these parameters of central deflection (yc = wedge hight hog /sag + amount of lift given to rail by equipment i.e. 100 mm

normally or actual lift at final load) , vertical load (P) and

prevailing rail temperature (ti).(ix) For computation of SFT Monograms have been developed

relating central deflection and vertical loads for different types ofrail sections. From these monograms placed at Annexure I & II

the temperature difference (t) is read out for correspondingvalues of P and yc.

(x) For correction of Auxiliary Tension caused due to stretching ofrail during the experiment, the value of temperature correction isread out from the appropriate curves placed at Annexure III & IV.

(xi) SFT = ti + Temperature Difference (t) TemperatureCorrection.

Limitation of this method

(i) This procedure is valid for values of Temperature Differencemore than Temperature Correction so that rail is definitelyunder tension and prevailing rail temperature is certainly belowSFT.

(ii) LWR has to be divided into segments of 250 m length on bothLH and RH and the location at which the experiment is to bedone shall be locat ed at minimum 250m away from SEJ.

(iii) The experiment is to be carried out when prevailing temperatureis lower by approximately 15

oC than destressing temperature.

(iv) The experiment shall not be conducted on or near the LC orGirder bridge.


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Further developments in this field have taken place using

electromagnetic probe and ultrasonic waves as detailed below:

(1) Magneto Elastic Methods -

The magnetization of ferro-magnetic materials causes high-frequencyelectromagnetic and acoustic phenomena, known as Barkhausen noise. The

level of such noise is dependent on the stress condition. The measured signal

contains the pulses generated in the rail and has a noise-like spectrum. Theamplitude of this so called Magnetic Barkhausen Noise (MBN) depends

significantly on the longitudinal stress in the rail. Tension increases theamplitude of the MBN, whilst compression leads to decreases. The higher the

longitudinal stress, the higher the signal amplitude.

The measurement of stress using the Basrkhausen noise method utilizes the

relationship between the level of Barkhausen noise and the stress in the materialunder investigation. The magnetic Barkhausen noise appears on the surface of

the material under study and the relationship between the noise and the stress in

the elastic range is clear. The measurement of stresses in the material is affectedby residual stresses. There are two methods developed for measuring stress.

Magneto-elastic stress measurement using the Railscan This method isbased on the fact that there is a relationship between the masgneticcharacteristics of the rail and its stress state. However, this relationship is

affected by the quality of material used and variations in the residual

stresses set up during production have a major effect on the stresses. Thismethod only assumes that the material characteristics remain constant

within one single piece of rail and eliminates manufacturing stresses. Forthis purpose caliberation measurements need to be taken with the type of

rail to be studies. This is suitable for studying the distribution of the SFT

9which is dependent on position and time) in LWR track. This methodmakes it possible to cover several hundred meters of rail per hour taking a

measurement every couple of meters.

Magneto-elastic stress measurement using the Railtest This is suitable forstudying the stresses in the cross sections of the rail and hence also for

verifying the permanent stress resulting rom manufacture.

Based on this concept, several papers have been published and patents

developed by various engineers worldover, a few of which are detailedbelow:

(A) F Mau of TMG International, Australia during 6th International

Heavy Haul conference on 10th April 1997, presented a paper (placed at

Annexure-I) wherein a detailed procedure to develop a portableequipment to determine longitudinal thermal stresses in rail independent of

residual stresses was deliberated in detail. The equipment shall be based on


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overcome this problem by relating vertical and longitudinal stresses in the

web of the rail. To derive the SFT of a length of rail track, all that isnecessary is to measure the total longitudinal stress and total vertical stress.

The total vertical stress is the same as the vertical residual stress since thereis no constraint. With knowledge of the relationship between vertical and

longitudinal residual stress, the thermally induced stress can be deduced by asimple subtraction.

The measured thermally induced stress can be converted to a temperaturedifference by using the standard rail industry conversion of 2.4MPa per

0oC. Thus by dividing the thermally induced stress by the conversion factor

and measuring the actual rail temperature, it is possible to derive a non-destructive measurement of SFT.

Benefits of a MAPS SFT measurement

An objective for any practical SFT measurement is not only to be non-destructive, but also to be non-intrusive so thast traffic interruptions are

minimal. As MAPS measurements do not require rails to be unclipped or cut,and individual measurements can be made directly on the web of the rail, the

time required for taking the measurement is approximately 5 minutes. Thus this

method is trully non-destructive method for SFT measurements with widercoverage of the network without adversely affecting railway traffic.

Procedure

Measure total longitudinal stress and total vertical stress.

Total vertical stress is same as the vertical residual stress since there is

no constraint.


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With the knowledge of the vertical and longitudinal residual stress, thethermally induced stress is deduced by simple subtraction.

The measured thermally induced stress can be converted to a tempdifference by using the std rail industry conversion of 2.4 MPa per oC.

So, by dividing the thermally induced stress by the conversion factor and

measuring the actual rail temp, temp it is possible to derive SFT

2.Ultrasonic Methods:-

(A)A Technical Report prepared by DM Egle & DE Bray of Oklahomaduring 1974-75. In this Report, a procedure is described for developing aPortable Stress Measurement System. The basis of measurement is the

acousto-elastic effect or the stress induced changes in the speed of

ultrasonic waves. A general arrangement of the proposed system formeasuring stress changes is shown in the Figure below:


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The speeds of various wave modes in a rail are dependent on initial

material properties, temperature as well as applied stress. Because thevariations in wave speed with initial properties and temperature are not

negligible, a reference wave speed must be established for each individualrail if the acoustoelastic effect is to be feasible for measuring absolute

stresses.

Results of the acoustoelastic studies showed that a bulk longitudinal wave

propagating parallel to the applied force exhibited the greatestacoustoelastic effect. Of the guided wave modes, the speed of those

consisting predominately of longitudinal waves propagating nearly

parallel to the applied load showed the greatest sensitivity to longitudinalstress. The fact that different guided wave modes have different

sensitivities to applied stress suggests that a measuring systemincorporating two wave types and measuring the relative difference in

velocity in a particular rail may allow making absolute stress

measurements. A more restricted although potentially useful approachmight be to take measurement data with a portable instrument at a

specific location in a particular rail while in an unstressed condition. Dataobtained after the rail was in operation could then be used to determine

the relative stress change. As a result of this work, it is concluded that a

portable rail stress measurement unit based on the acoustoelastic effect isfeasible in principle.

(B) A Technical paper presented by J. Szelazek, Warszawa (Annexure-V)

The basis for this study is also the accousto-elastic effect.

The difference in time of flight pulse polorized along &perpendicular longitudinal force in the rail, averaged value ofresidual stresses & rail material texture.

Using Longitudinal wave only, one cant measure absolute value ofLongitudinal force in the rail.

Therefore, limitation of this method that stress changes in chosenlocation only can be measured as the acoustic properties of the

material are constant, residual stresses are also constant and theonly factors influencing measured time of flight are temp & L-Stress in the rail.

Absolute values of forces can be measured by first takingmeasurement on a thermally stress free rail i.e at the time of laying

/destressing operation and then later at the same location.

In 1992 the measurement of stresses introduced into the CWRduring track laying. It was shown that in the neutral temperature

when thermal stress is assumed to be zero, in some locations on the

track the bughudinal forces acting in each of the rails can reach

more then 6 kN [3].


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Figures presented below show result of other field measurementsperformed with portable DEBRO-30 device.

Fig. 6 shows thermal stress versus rail temperature measured onvarious CWR Line denoted as A shows the changes of stress

observed during one day and measured on the straight CWR Theslope of the straight line is equal to 2,5 MPa/C what is equal to

theoretical predictions.

It was assumed that rail neutral temperature was 22C. Lines

denoted as B and C show stress changes observed during muchlonger time period.The slope of the straight line is equal to 2,5

MPa/C what is equal to theoretical predictions. It was assumed

that rail neutral temperature was 22C. Lines denoted as B and Cshow stress changes observed during much longer time period.

Stress changes presented as line B were measured close to the roadcrossing and as line C - in the distance from the road crossing [13].

For line B stress-temperature dependence is equal to 2,6 MPa/Cand for line C - only 1.9 MPa/C. It means that depending on

location on CWR the same temperature changes can result in

various stress changes. Much higher stress scatter on line Bcomparing to lines A and C is probably due to other than

temperature factors influencing stress state.

Fig. 7 presents stress changes measured during track repair. Therail were cut, short piece of rail was removed and rails were welded

together. The aim of the operation was to reduce compressive force

in the CWR.

It can be seen that the operation reduced compressive stress alongthe 112 m track section and the average stress change is equal to 43MPa. Variation in stress increment along 150 m long section was

caused probably by variations in the rail temperature. Some parts ofthe track were in the shade, other in the sun.

A small, lightweight portable & battery operated ultrasonic devicesequipped with multitransducer probeheads for L-Subsurface waves,

it is possible to monitor stress changes in the LWR

Fig. 6. The dependencies stress increment - rail

temperature measured with subsurface 1ongitudinal waveon various tracks.

Fig. 7. Stress changes along the rail due to repair(destressing) operation.


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(3)A patent developed by Austill & Robert J of U.S.A (Annexure-VI)

The procedure described in this patent to determine SFT is based on thecomparison of the length of an unconstrained piece of steel with a

demarcated portion (similar length) of a railway track rail which isconstrained and thereby reflected at most temperatures because of the trackstie into the earths surface. The demarcated portion of rail is calibrated to be

substantially the same length as the gauge bar (unconstrained piece of steel)when both are at the same temperature and neither is under axial load. The

difference in length is an analog of an associated rail characteristics such as

the number of degrees that the rails instantaneous temperature is above orbelow the rails SFT. The length comparisons are made with precision

devices.

The Inventor of the patent claims that the following are the advantages of the

process described in the patent:

(i) The process of determining SFT does not require calculations ormental steps.

(ii) The SFT can be determined at any temperature or time of the year.

(iii) The process for determining the SFT will permit track maintenancepersonnel to monitor changing SFTs in order that they may adjust

them as a planned maintenance activity before they reach a hazardous

level.

(iv) The process for determining the SFT can also be used in conjunctionwith a SFT adjustment operation in order that the correct amount ofrail to add or remove to attain the desired new SFT.

(v) The process of determining the SFT is such that the vulnerable,high-precision components are portable and can be taken away from

a site when not in use.

(vi) To perform the required measurement over a comparatively longdistance in order to average-out or eliminate the effects of localanomalies that would reduce accuracy is measurement was made in a

short distance.

(vii) To infer the amount of temperature-related deflection in aconstrained CWR rail by comparing a denoted portion of its length toan unconstrained stress free metal gauge bar.

(viii) To provide a process for determining the SFT which can be installed,calibrated and used by typical track maintenance personnel without

requiring them to undergo special training.


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(4) Remote Monitoring (Annexure VII)

Using Advantechs ADAM data acquisition controller, workers can wireless monitorrailway conditions via the Longitudinal Stress Detector, which can be mounted directly

on the track.

Data Traks of Fort Collins have developed the Longitudinal StressDetector (LSD).

Using wireless data acquisition technology from Advantech Automation,the LSD is mounted directly to the track where it records temp & stress

levels as frequently as once an hour.

This data can be downloaded and analyzed by the P.way Engineers

Comparative study of various methods of stress measurements in LWR:

1.Cut & weld method Simplest but destructive Requires lines block of more duration.

2.Monitor Rail Displacement method Time consuming.

Not accurate, actual SFT cant beworkout. Widely used technique in practice today.

3.Resistence Strain Gauges Tried in USA. System could not be developed for

commercial application

4.Semi Non destructive Rail lift uptechnique

Against the provision of maintenancepractice of LWR

Time consuming & requires line block


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5.MBN Method Sensitive to rail material magneticproperties, electric current in rail and

structure variation.

Fairly accurate Residual Stresses in rail can be

eliminated in the calculation of L-thermalforces & SFT.

6. Ultrasonic method(Debro Ultrasonic

equipment)

It gives information about relativechanges in rail Longitudinalstress.

Requires no special surface treatment. It is rapid and reproducibility is good. Accuracy is generally poor. It is used to access Longitudinal residual

stresses on rail.

To accesses thermal stresses in LWR, itis necessary to make a first measurement

in the rail in the absence of any forcesapplied externally or due to temperature.

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References:

1. European Rail Research Institutes Report No.ERR/D/173/RP/12.2. European Rail Research Institutes Report No.ERR/D/202/RP/9.3. 6th International Heavy Haul Conference (Strategies beyond 2000), 10 th April

1997.4. 8thInternational Heavy Haul Conference, 2005.5. Confidential Report dt. 13.9.2005 released by CTEs C.Rly on Non-destructive

Measurement of stresses in LWR.

6. Paper presented by David Bateman, London on Rail Stress & Rail StressManagement during a Seminar held on 31 March 2004 (Published in Permanent

Way Institution Journal June 2004 Vol. 122).

7. Interim Report No. FRA-OR&D-76-270 dt. July 1976 prepared by D.M. Egle &D.E. Bray for U.S. Department of Transportation.

8. Various Internet sites.

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