Track Structure for 25T Axel-Load

Influence of Residual Stresses

By: J. S. Mundrey
Formar Adviser, Civil Eng. Railways Board

In the year 2018 RDSO released a set of calculations about the Total Stress, that occurs in 60 kg rail section during the operation of 25 t axle-load rolling stocks.

It has been concluded that on 60 kg 90 UTS rails, 25 t axle-load, rolling stock operating even as low speed as 50 kmph cause Stresses, that exceed the maximum permissible yield strength of rails. At 100 kmph the excess is much higher.

The calculations bring out, that for running 25 t axle-load rolling stocks it will be necessary to adopt 60 kg 110 UTS rails, to ensure that the total stress in rails remain within the Permissible Yield Strength.

Earlier to the above calculations, the norms adopted by RDSO in determining Rail Stresses permitted the speed of 100 kmph for 25 t axle-load rolling stocks on 60 kg 90 UTS rails. The calculations made left enough safe margins between the Total Rail Stresses and the Maximum Permissible Yield Strength of Rails..

The volatility in the permissible speed of 25 t axle-load rolling Stocks has come, when RDSO, after due diligence, which included the stamp of Track Standards Committee, decided to jack up the value of Residual Stresses in Rail Stress Calculations from 6.00 kg to 24.5 kg more than four times, which is even more than double of the Bending Stresses due to wheel loads.

The implications of such an approach in the calculations of Rail Stresses on the Transportation Output of Indian Railways can be well imagined. Nowhere on Indian Railways, which include the newly constructed / under construction Dedicated Freight Corridors, 25 t axle-load rolling stocks can run even at a abysmal low speed of 50 kmph without infringing Rail Safety.

Surely, RDSO, with its vast reservoir of knowledge, would be well aware that all over the World Railways, which includes Heavy Haul Railways of Europe (iron ore lines of Norway/ Sweden, Australia and Africa), 60 kg 90 UTS rails continue to be used, permitting operation at a speed of 100-120 kmph. Their main emphasis is at present to prolong the life of the rail by Head Hardening, as the service life of the rails in track is getting governed by Rail/ Wheel Interaction at the rail head and NOT by the Fatigue Stresses in rail. With the vast improvements in rail making, where metallic/non metallic inclusions (hydrogen content) have been well controlled, rail failures attributed to Fatigue have come down drastically, even reducing the need of Ultrasonic Testing on that account. All Ultrasonic/Eddy Current Testing is presently directed towards detecting flaws emanating from Rolling Contact Fatigue on the rail head.

IR Rail Specification for 60 kg 90 UTS rails being not much different from European Railways, there can be no reason for limiting speed on Indian Railways, lower than that prevailing on those Railways.

What prompted RDSO to carry out such elaborate exercise on Rail Stresses, RDSO in their technical paper have brought out the reasons for carrying out the elaborate exercise on Rail Stresses on Indian Railways as reproduced below.

“The rail fractures are showing an increasing trend from 2012-2013 onwards; increasing from 698 in 2012-13 to 1552 in 2016-17. Total number of weld failures also remained very high. The data indicates that approximately 59% of rail fractures take place below 350 GMT, which indicates that the rails are loaded beyond their fatigue strength quite often resulting in to fatigue failure earlier than stipulated life. Further, 52% of rail failures have taken place in winter 4 months (period considered 16th Oct. -15th Feb.), which indicates that tensile is not adequate to cater for extreme combination of residual, dynamic and temperature stresses.”

Instead of deliberating deep into the issue and finding out the causes for high incidence of rail/weld fractures, RDSO took an easy course of putting all blame on presence of “Residual Stresses“ in the rails as rolled. The test reports produced by RDCIS Ranchi in 2016 about the residual stresses in rails became handy in this oversimplified diagnostic approach.

RDSO should have known that many Railways abroad, which include European Railways  and Australian Railways, have not experienced similar problems with 60kg 90 UTS rails, although operating in much harsher operating environment of high axle-load and higher speed. They, in their Rail Specifications, permit higher residual stresses of 250 MP, as compared to 190 MPa permitted in INR Specification.

On European Railways, the fatigue failure of rails have drastically come down after the stricter control is being exercised by Rail Rolling Mills in controlling metallic/ nonmetallic inclusions in rail steel, which also includes the hydrogen content.

RDSO have heavily relied on the observations made by Dr. Esveld, who in the latest edition of his book, ”Modern Railway Track“, has the following to comment on the role of residual stresses:

“The roller-straightening process as applied to date provides excellent straightness but introduces detrimental residual stresses of the order of 100 to 300 N/mm2, depending upon the yield strength of the rail steel. These should be added to the load stresses.

Fig. 1 summarizes the residual stresses measured by ORE D 156 for new grade R 260 (900A) and R 260 Mn (900 B) rails and used rails. The figure shows that the residual stresses in the rail head change from tension into compressive stresses due to rolling-out effect caused by the wheel-rail forces. The residual stresses are limited in the European Standard to 250 N/mm2 in the centre of the rail foot.” 

Fig. 1: Residual Stress due to Roller Straightening; Source: C. Esveld Modern Railway Track (redesigned by F.A. Wingler)

Although Dr. C. Esveld has advocated the addition of residual stresses to the load stresses, he has not commented on the need for the change in UIC Rail Specification to reduce the norms for residual stresses below the 250 N/mm2 value.

It has to be noted that none of the advanced railways have taken any precipitous action in switching over to higher UTS rails on account of residual stress values, as being proposed on Indian Railways. This is on account of the fact:

  1. There are wide variations in the residual stress and strains in the cross-section of the rail and along the length of the rail, leading to a very complex arrangement of residual stresses, not fully mapped and understood.
  2. Residual stress values undergo a major change during service as indicated in the diagram brought out by Dr. Esveld in his book.
  3. The residual stress values are the least in the head of the Rail, where majority of the rail fractures, that occur in service, have their nucleus.
  4. At the centre of the rail foot, where the value of the residual stress is the highest, there has hardly been any case of the rail fracture having originated from there.
  5. On advanced railways, the fatigue failures of rails have been well controlled by better rail making, a flaw less system of handling, transport, laying and maintenance of rails. For them a well straightened rail is more important for their high speed routes than permitting a little higher value of residual stresses.

Australian Track Manual has the following observation to make on the present day Test Methods used to determine the Residual Stresses in Rails.

Test Procedures being used to determine the residual stresses in rails such as: The web cut method, strain gauging or the crack arrest tests may be used to obtain quantitative information about the magnitude of residual stresses or their effect on the behaviour of the rail. Data obtained from such tests should be considered as INDICATIVE only, as the rails, that are naturally in use and have provided SATISFACTORY SERVICE PERFORMANCE, CAN FAIL THE PROPOSED ACCEPTANCE CRITERIA”!!

RDSO will be well advised to refer to the “TRACK LOADING FUNDAMENTALS“, Technical Monograph No. 12. by C.W. Clarke, which forms the basis of present day calculations of Track Stresses. At page 3 it states as under:

“It is not surprising, therefore, that many engineers feel that the oretical analysis of track stresses is at the most approximate, and that practice without waiting for theory has determined the dimensions of track components for any particular application. No calculation of track stress of deformation can be regarded as exact. The variables involved are numerous, but a usable analytical treatment is of great value and for determining the probable track stresses produced by any new design of vehicle.”

RDSO may also be well advised to note that if the present values of Residual Stresses are taken into account, then the Track Stress Calculations would indicate that INDIAN RAILWAYS, so long, have been operating under UNSAFE CONDITIONS, which surely is not true. In fact, the satisfactory operation of the past on tracks consisting of different rail sections and the rolling stocks with varying axle-loads and speed should form the basis for determining speed potential of new rolling stocks running on heavier rail sections and well designed stronger track structure.

Centre for Advanced Maintenance Technology (CAMTEC) Gwalior has made an interesting study on the incidence of Rail/ Weld fractures on some of the IR routes, where wagons with 25 t axle-load are operating, and published a study report in 2018. Their study indicates that most of the problems on these routes are on account of Poor Service Life of Turnouts (CMS Crossings), Glued Insulated Joints and Rail Welds. Rail fractures are rare. They have not indicated as to whether the nucleus of fractures was in the head or foot.

They have mentioned that the adoption of higher UTS rails will not be a solution, as that would lead to a higher number of Weld Fractures. According to them, general adoption of stronger track structure with durable formation, adequate clean ballast cushion, stronger fastenings, particular the rail pads, better rail welding and systematic rail grinding will provide a better solution.

Steps taken by Advanced Railways for reducing rail failures and for obtaining longer Service Life from Rails:

European Railways have been running their goods trains with an axle-load of 22.5 t at 120 kmph for many years. They are planning to run 25 t axle-load trains .Over the years they have upgraded their tracks by:

  • Creating a foolproof Track Drainage Systems, particularly in station yards by providing underground self cleaning drains.
  • Formation Treatment for stability, durability and achieving easy track stiffness gradient thereby reducing the dynamic augment considerably.
  • Ensuring adequate Clean Ballast Cushion.
  • Concrete sleepers and Fastening System with an assured long service life.
  • A responsive fully Mechanised Track Monitoring and Maintenance System, particularly adopting a regime of timely rail grinding to manage Rail Contact Fatigue (RCF) Defects. They have no plan to switch over to heavier or higher UTS rails.  Wherever there is a problem of excessive rail wear-on curves or on other locations, Head Hardened Rails will provide the right solution. Provision of Under Sleeper Pads has given a big relief to stresses in rails, sleepers and ballast, and they are being increasingly adopted. An extract from an article presented by Dr. Peter Veit, Head Technical University Graz, Austria, at the 10th International Heavy Haul Association Conference is reproduced below, which sums up the approach of European Railways on the question of appropriate rail design for Heavy Haul Routes. “As the UIC 60 rail is seen as the proper solution in terms of profile, the steel grade became more important. Due to high axle-loads in combination with continuously high powered locomotives, rail surface failures (RCF) and especially so called Head Checks are already common problems. While intensive rail grinding can reduce likeliness of rail breakages, the crack growth can only be limited by using higher steel grades. Higher steel grades, compared to the standard steel grade R260 (900), LEAD TO LOWER DUCTILITY causing eventually rail breakages on the rail footing in the disadvantageous situations. Head Hardened Rails with a steel grade of R 350 HT for the Rail Head are a proper solution for this problem, as the steel grade in the Rail Foot is keeping at R 260. In the past Head Hardened Rails have been used for minimising lateral wear of the rail head in small radii. However, Head Hardened Rails reduce the crack growth due to the phenomenon of head checking by 50%, and justify therefore easily the comparable low additional investment costs compared to standard rails. Thus new field of application occur for Head Hardened Rails at high traffic loads, in general up to radii of 3000 m.”

In Dr. Peter Veit`s presentation, two important points need to be noted for IR considerations:

In determining rail grade, no importance is given to the RESIDUAL STRESSES VALUES, although European Rail Specifications permit a residual stresses values of as high as 250 N mm2 ,as compared to 190 N permitted by IR. Rail Surface Defects are the main considerations.

The provision of Head Hardened Rails provide a better technical and economical solution compared to Higher UTS rails, when confronted with the issues of higher axle-loads, smaller radii and increased traffic density.

This write up will be in-complete if the phenomenon of Residual Stresses in Rails is not dealt with in proper perspective.

Residual Stresses in Rails are internal stresses, that exist independently of externally applied force or thermal force and are induced during the manufacturing of rails.

Origin of Residual Stresses: Rails have a bulbous head, a slender web and a substantial tapered foot. During cooling after the rail is shaped by rolls, heat loss from any point is determined by the shortest distance to the coolest outer surface. Hence, heat loss is the fastest in the web and the edge of the foot, which protrude up from the cooling bed and slowest in the head. This differential cooling generates Residual Stresses.

When the rails are being shaped by the rolls, it is white hot so relatively little stress causes plastic strain to occur;  the E value being at much reduced level .As the rail cools, the outer skin increases in strength and contracts, compressing the still soft inner steel and plastically deforming it. In the process, the outer steel is placed in tension and residual stresses are generated.

As cooling proceeds, there is variation in E-values between Head, Web and Foot because the slender web cools more quickly. Variation in stress and strains follow, and the result is a very complex arrangement of residual stresses.

Because the head shrinks differently, relative to the web and the foot, the rail gets into a sagged shape.

The rail is therefore passed through the rollers to stretch the head and compress the foot and thus straighten it in the vertical plane. Typically, the consequence is a high longitudinal stress on the top of the rail head coming to Zero at about 20 mm from the top compressive stresses in the web and again tensile stress in the foot.

Consequently, most rails contain residual stresses, that are generated in the two operations cooling and roller straightening.

Fig. 2: Longitudinal Stress in Rail; Source: Rail Industry Safety and Standard Board, Australia

Controlling Residual Stresses: The straighter the rail is rolled and more controlled the cooling, the less the residual stresses will be. While the rail manufacturers make considerable efforts to optimise both, most rails still have quite complex presence of residual stresses, which vary along its length.

Consider the Rail as it passes through the Roller Straightener: The lead and the tail end are relatively free to deflect under the force of the rolls, most of the rail length- particularly the section within the rolls is constrained by the rail each side.

Any Testing Method aimed at detecting residual stresses must recognise the variation at rail ends inherent in roller straightening. For example, if the rail is cut, the residual stresses at the cut end will be different to those at the original end, and any longitudinal stresses become vertical stresses at a cut end.

The stress picture is further complicated by the fact that the amount of straightening force required will vary between rails and indeed between different parts of the same rail. Residual Stresses will vary accordingly.

Testing Methods, presently used, determine the residual stress values on a rail cut sample. They may be of some use to compare the various rail rolling processes, but the results cannot be used to evaluate the fatigue strength of the rails, as brought about in the Australian Manual, quoted in the earlier paragraphs.

There is no test so far available, which can tell the influence of residual stresses on the fatigue strength of the rails in a non destructive way and in the actual operating environment.

In Conclusion: We may state as under:

  • 60 kg 90 UTS rails are good enough for Indian Railways for use on 25 t axle-load routes.
  • The rails have a greater reserve with them, when used on Dedicated Freight Corridors, where the modern Track Structure consisting of well designed formation adequate ballast cushion and continuously welded and long rolled rails are being provided. The track work has been carried out with the deployment of Track Laying Trains, free of any damage to rails and other track components during handling, transport and laying of track. Latest technology has been used for welding and destressing, resulting in the track quality of the highest order. Modern track monitoring and maintenance systems are planned to work on predictive mode, where the track defects will be attended too much before the track needs any corrective maintenance. On DFCC, 60 kg 90UTS rails should continue to serve well, even at higher axle-load of 32.0t t in view of the latest technological inputs that have gone into its track construction.
  • It will be preposterous to equate the DFCC track works with some heavy mineral routes, where old rolling stock is being overloaded much over its carrying capacity to move higher volume of traffic. Even on those routes, where formation, ballast and track geometry conditions are below average, there are hardly any failures of rails on fatigue account, as brought out by the study carried out by CAMTECH. Most of the track problems there have occurred at turnouts, glued insulated joints, welds and with the rubber pads of the fastening system.
  • RDSO would be aware that 60 kg 90 UTS rails are used on many of the World heavy haul lines, which include Iron ore lines in Australia, where wagons with much higher axle-loads are successfully under operation.
  • World over, with the Advancement in Rail Making Technology ,with a better control on metallic/non metallic inclusions and the adoption of highly sensitive instruments for the testing of the rails at the mills, the structural failures of rails in service have come down drastically. All emphasis is now to control the rail wear and RSFD, which are now being better managed by adopting well designed Grinding Regime and switching over to Head Hardened Rails, where justified.
  • Higher UTS rails become a great liability, where a fool proof system for their mechanized handling, transportation and laying is not adopted, resulting in permanent damage to rails, Welding of higher UTS Rails, both the flash butt and alumino thermic is another weak link in their adoption.
  • It appears quite preposterous to blame the High Incidence of Rail Fractures to the illusionary and internationally little understood phenomenon of residual stresses in rails. Till recently, RDSO in its calculations is combining the residual stresses in rails with other unknown stress, giving a total weightage of 4.0 kg/mm2, and the operation of all new rolling stock was decided on that basis. Sudden jacking of their value to 24.00 kg, based on some tests conducted on isolated rail pieces, is only deceiving one self, particularly when none of the Developed Railways have adopted this obscure path in evaluating their rails.
  • RDSO would be well aware of the damage that higher UTS rails suffer, during their handling, transport and laying with the present day manual working, still prevalent on many of the new constructions on Indian Railways. These problems will multiply, if some hasty decision is taken on the switch over to 110 UTS rails. Their Welding will create further problems, difficult to handle.
  • Indian Railways can achieve a much increased service life from 60 kg 90UTS rails, even under 25 t axle-load wagons operating at 100 kmph, if sufficient Attention is given in the Construction and Maintenance of Track Structure (both Substructure and Superstructure)  as is being done on advanced railways.
  • Finally from the above narrative, one can well imagine the dangerous territory, that IR will enter, causing immense problems, both technically and financially, once it decides to switch over to 110 UTS RAILS. Hopefully such major decision will not be taken on such an obscure Phenomenon of Residual Stresses of Rails.

Disclaimer: Expert Speaks is a knowledge feature of ‘Railways Year Book’ and publishes articles by noted Railway Experts which have extensive experience in their respective field. However, the opinion expressed in the article published are that of author himself and Railways Year Book doesn’t endorses the views expressed in any of the articles published herein.