Short pitch rail corrugation - cause and contributing factors

K.H. Oostermeijer

Holland Railconsult, Utrecht, the Netherlands Abstract In this paper a review of observations form test sites, varies surveys and studies on the subject of short pitch rail corrugation over the last 100 years is made. Based on this research, complemented by own surveys, the cause for short pitch corrugation formation is described as a sequence of events. The main and lower level contributing factors are identified that impact upon this sequence of events. The paper ends with a description of work that is being carried out as follow up. Introduction The first report of corrugation in the literature dates back to 1889 on the Midland line in Great Britain [16]. Both engineers from Britain and Germany had no explanation for this type of wear. The second report is from the United States in 1895. On a cable car track, corrugations were observed giving rise to very poor ride comfort and high noise levels [10]. It was believed that in the curve the cable exerted a force in lateral direction that caused the wheel to slip. There are also reports from the end of the 19th century from a cable railway in Great Britain. Here corrugations with a wavelength of around 50 mm were observed on both curves and tangent track. In the beginning of the 20th century, corrugations were already a serious problem at tramways. At a conference in 1910 the results of a questionnaire that was sent to 75 tramways administrations around the world were presented: 72 reported rail corrugations [10]. The German Railways were among the first railways to encounter excessive lengths of corrugated track, with figures up to 46% of the complete network. For this reason, corrugation was in the 1930's also referred to as the 'German disease'. Nowadays virtually all railway administrations suffer from rail corrugations and with increasing axle loads and train speeds the problems in terms of occurrence and maintenance cost are increasing. Definition of rail corrugation Rail corrugation is a wave type wear of the rail surface with a wide variety of wavelengths. In this paper, only short pitch rail corrugation is discussed. Corrugation1 shows as an undulation of the rail surface with typical wavelengths between 25 - 80 mm. On the surface of severely corrugated rails the shiny crests and darker troughs are easily recognised. The amplitudes can reach values of 50-100 㯀m. Figure 1 shows a typical example of a corrugated rail.

Figure 1: view on corrugation

1 For purpose of readability of this paper, short pitch rail corrugation is denoted as corrugation.

These corrugations give rise to high frequency vibrations in the range of 200 - 1500 Hz and cause an increase of noise emission of up to 15 dB(A) [1]. Due to these high noise levels, corrugations are often referred to as 'roaring rails'. Corrugations have a significant impact on the maintenance effort with an increase of cost up to 30% [23]. Although rail corrugation itself cause mainly noise nuisance and an increase of maintenance need, it also gives rise to the formation of particular types of rolling contact fatigue (RCF), such as squats and Belgrospi. The latter is a new type of RCF, found on high-speed tracks in Germany, (speed over 200 km/h) on the high rail in very wide curves with radii of 6.000 metres and on tangent tracks [16], [17]. Squats are rail defects that occur solely in straight line track and are often found in corrugated tracks [34].

Figure 2: corrugated rail with squats

Both types of defects characterise by cracks underneath the running surface of the rail than potentially cause pieces of rail to break out. For this reason, mitigation of corrugation problems is also a safety issue. Observations on rail corrugation Corrugations have been studied since the beginning of the 20th century. Many test sites have been made and many surveys have been conducted with a great variety of steel types, rail profiles and permanent way structures, all under a great variety of exploitation conditions. The main observations of these surveys are summarised below. Characteristics of rail corrugation - There appears to be no correlation between corrugation on the left and right rail [12], [35]. In case

the straight track is inclined in cross section, the higher rail is more heavily corrugated [26]. - Rail corrugations form a larger problem on lines with higher speeds [2], [13], [29]. - Corrugation peaks do not move along the rail [12], [19]. - Development of corrugation is not limited to track structures that provide a discretely support of the

rail. Also track structures that provide a continuous support of the rail in both vertical and lateral plane develop rail corrugations [27].

- Non corrugated rails prove to have more wear than corrugated rails [6], [20]. Type of steel - Steel from different manufacturers show a different corrugation growth rate [2], [6], [20]. - Siemens Martin steel corrugates less than Thomas steel [2], [28], [31]. Open hearth steel corrugates

less than acid Bessemer steel [12], [15]. - Rails with a lower tensile strength show less corrugation than higher strength steels [2], [5], [13].

Modern-day head-hardened rails however are reported to have a smaller propensity to form corrugations [14].

- The corrugation wavelength is dependant on the steel material. Historical surveys report that pieces of rail from different manufactures in identical situation showed a different wavelength, even when welded together [2], [6], [23]. Surveys show that the corrugation wavelength at head-hardened rails is smaller than that at non head-hardened rails [24]. This is confirmed with more recent tests with confirmed identical track structure, subgrade and traffic conditions [14]. The head-hardened rail in

that case, developed corrugation wavelength between 20 - 40 mm whereas the 900A grade rails developed corrugation wavelength of 40 - 60 mm.

Metallurgical aspects - Plastic flow of the rail steel is bidirectional, dependent on the location of the contact patch: in the

direction of travel at the field corner and in opposite direction at the gauge corner [13], [30]. - Hardness of the steel at the crests is significantly higher than at the troughs [6], [10], [25], [30]. - The crystalline structure of the original rail steel is deformed under repeated loading [9]. The crystals

are transformed into smaller, distorted ones, with accumulated dislocations [30]. The cementite lamellae are broken up in fine particles. The deformed zone under the crest is approximately 50 times greater than at the valley [8].

- So called white etching layers (WEL) are often found on corrugation crests [3], [12], [20].

Train track interaction - Locations of corrugation crests coincide with slip of the wheel [1], [9]. - Historical sources report that corrugation first appeared when electrical traction was introduced and

steam traction was ended [10]. - The propensity for corrugation development is greater at the track that carries predominantly empty

or low loaded vehicles [32]. Observation analyses The described observations, combined with dedicated research have led to the definition of:

- the wavelength fixing mechanism - sequence of events that causes corrugation - main contributing factors - low level contributing factors

These aspects are elaborated in the following four chapters. Wavelength fixing mechanism The observations show that a great many aspects impact on the propensity of a rail to corrugate and on the actual appearance of the corrugation. Two groups of observations are of particular interest with respect to the wavelength fixing mechanism. These are:

- identical quality of rail made by two manufacturers showed different corrugation wavelength; - head-hardened rails showed a different corrugation wavelength compared to normal grade

rails. These observations were made at locations with identical track structure and exploitation conditions. On the basis of these observations it can only be concluded that the type of steel and its mechanical properties is the determining factor for the corrugation wavelength. Differences in steel production and steel treatments are known to influence the structure of the rail material and consequently the mechanical properties. The effect of production differences can be illustrated by the nitrogen content in the rail steel. Nitrogen makes the steel stronger and more brittle. Tests in Great Britain and Germany [13], [31] proved that the propensity of rails to corrugate increased with higher nitrogen content. This is the main explanation why open hearth steel is less susceptible to corrugations than acid Bessemer steel. Analysis in the Netherlands [20] showed that the smoothest rail had higher aluminium contents, indicating a low nitrogen content as the aluminium reacts with nitrogen to form aluminium-nitride. The fact that treatment of the rail has an impact is illustrated by heat treatment. Heat treatment realises a smaller interlamellar spacing of the steel structure that increases the hardness of the material.

Sequence of events As the wavelength fixing mechanism is found to be the rail steel, it is the crystalline structure of the rail steel that makes the difference. Hence the crystalline structure of the rail steel is of prime importance is the determination of the corrugation wavelength. The crystalline structure of rails is heavily distorted due to train traffic. A measure of this distortion is the number of dislocations in the atomic structure. The dislocation density in new steel crystals amount to 103 - 10 5 mm -2. The dislocation density of plastically deformed steel can reach values of 1010 mm -2. The increase in dislocation density that is caused by train traffic is clearly demonstrated in [30], see figure 3.

Figure 3: image quality map of new and used rail (from [30])

As dislocations can move through the atomic structure, the continuous plastic deformation will cause a dislocation pile-up when the dislocation meets a barrier such as a grain boundary or a sessile dislocation configuration2. The crystalline structure of the original material is the determining factor for location and number of these barriers. Once a barrier is met, dislocations pile-up behind the leading dislocation but do not combine as they are of the same sign, see figure 4. An increase in dislocation density in the atomic structure of the rail steel and the mutual interaction between dislocations results in hardened steel. These pile-ups therefore give rise to a non-uniform distribution of hardness along the rail, which will allow a wave like wear of the rail.

direction of shear force

sourcepile-up

barrier

direction of shear force

direction of shear force

sourcepile-up

barrier

direction of shear force

Figure 4: schematic representation of dislocation pile-up

Distortion of an individual crystal happens once the applied shear stress exceeds a value of 1/30 of the shear modulus of the rail steel. Such levels of shear force are reached in case of slip of the wheel. Site surveys show that locations of slip coincide with corrugation peaks [1], [9]. This is illustrated in figure 5, where dirt on the drying rail clearly marks the slip of the wheel at the corrugation crests.

2 More information on dislocations can be found in [21].

Figure 5: wheel slip coincides with corrugation crests

Wheel slip on tangent track occurs when big steering moments are exerted to the wheel, hence when the equivalent conicity is temporarily high, e.g. in case of flange contact. The Klingel movement is disrupted. A relatively large lateral displacement of the wheelset is a necessity to reach high levels of equivalent conicity. Such a disruption of the Klingel movement is illustrated in both figure 5 and 6.

Figure 6: movement of the wheel on a rail To summarise, the sequence of events for the formation of corrugation is shown in figure 7.

dislocation formationand movement

exceeding shearstress limit of rail

disruption ofKlingel movement rail corrugationdislocation formation

and movementexceeding shearstress limit of rail

disruption ofKlingel movement rail corrugation

Figure 7: sequence of events that causes the formation of corrugation

Contributing factors The main contributing factors for the described sequence of events are found to be:

- rail steel - contact stress - equivalent conicity - dynamic gauge - ride behaviour

Rail steel The rail steel is found to be the wavelength fixing mechanism and is discussed in the previous. Contact stress An important condition to form corrugations is that the shear stress is exceeded. The stress in the contact patch therefore needs to be sufficiently high. Some studies also indicate that outside certain boundaries of contact stress, corrugation would not form or disappear [32]. This would indicate the existence of not only a lower boundary for the contact stress but also an upper boundary. Equivalent conicity The nominal steering abilities of the wheel rail system have a direct relation to the propensity of hunting of the wheelset. This is of course closely related to the bogie design as is included under 'ride behaviour'. It is known from cross border traffic, that trains that provide a comfortable ride in the one country can provide a considerable lower comfort level in the other country that has a different type of track comprising a different rail profile and rail inclination. Dynamic gauge The term 'dynamic gauge' needs explanation. The interaction between wheel and rail in studies to date take into account a fixed rail and a wheel that can move laterally with respect to the rail. In reality the rail head displaces when a wheel passes, either due to bending of the rail, due to rail roll or due to bending of the sleeper. This results in a temporarily gauge widening, hence the term 'dynamic gauge'. Analyses of rail defects in the Netherlands indicate a clear distinction between the number of defects and the type of sleeper [33]. In the Netherlands, three types of sleepers are most commonly in use, although nowadays only concrete monobloc sleepers are applied. These three types of sleepers are fitted with different types of fastening systems, see figure 8.

Figure 8: common sleepers at Dutch railways

These fastening systems have distinctive elastic behaviour. Hence the dynamic behaviour in lateral plane is distinguished. The track structure as such plays an important role in the aspect 'dynamic gauge'.

Ride behaviour An important aspect is the ride behaviour of a certain train on the track. This aspect comprises the dynamic ride characteristics of a particular train and takes into consideration the actual track alignment, including track geometry deterioration. The main contributing factors as described interact at distinct steps in sequence of events of corrugation formation as shown in figure 9.

rail steelrail steel

dislocation formationand movement

exceeding shearstress limit of rail

equivalent conicityequivalent conicity

disruption ofKlingel movement

contact stresscontact stress

rail corrugation

dynamic gaugedynamic gaugeride behaviourride behaviour

rail steelrail steel

dislocation formationand movement

exceeding shearstress limit of rail

equivalent conicityequivalent conicity

disruption ofKlingel movement

contact stresscontact stress

rail corrugation

dynamic gaugedynamic gaugeride behaviourride behaviour

Figure 9: sequence of events and main contributing factors Low level contributing factors The main contributing factors are fed by underlying aspects. These low level contributing factors are the aspects that can be tuned with the result that the formation of corrugation takes a changed period or the wavelength is altered. They are as the buttons of the radio. Much of the proposed hypotheses for the cause of corrugation formation can be categorised as a low level contributing factor. As an example accounts the vertical rail vibration. Much research has been done in this field [4], [15], [20], [22]. These researches show a relation between vertical track dynamics and vertical pin-pin resonance and the propensity to form corrugation. They also show that vertical track dynamics alone cannot explain all situations in which corrugations do form and in which not, nor can it explain the difference in wavelength at identical track structures and exploitation conditions but with different rails. Categorising this aspect as a low level contributing factor helps to understand the fact that a relationship can be proven, but various items remain open. Changing a single low level contributing factor will change the situation but not solve the complete problem.

The complete sequence of events, main contributing factors and low level contributing factors, together with their interrelation is shown in figure 11.

rail steelrail steel

steel type

rail inclination profile

wear

rail roll

wear

chemicalcomposition heat treatment residual rail stressmetallurgical

aspects

dislocation formationand movement

exceeding shearstress limit of rail

track geometrydeterioration

equivalent conicityequivalent conicity

disruption ofKlingel movement

axle load

train speed

rail profile

contact stresscontact stress

contact patch

characteristicwavelength rail

continuity railsurface

rail surfaceirregularities

joints

vertical railvibration

horizontal railvibration

friction levelwheel rail interface

gauge

dynamic trackcharacteristic

rail corrugation

track alignment

dynamic gaugedynamic gauge

dynamic ridecharacteristics

diameter

wheelrail

rail fasteningfoundation

wear

ride behaviourride behaviour

rail steelrail steel

steel type

rail inclination profile

wear

rail roll

wear

chemicalcomposition heat treatment residual rail stressmetallurgical

aspects

dislocation formationand movement

exceeding shearstress limit of rail

track geometrydeterioration

equivalent conicityequivalent conicity

disruption ofKlingel movement

axle load

train speed

rail profile

contact stresscontact stress

contact patch

characteristicwavelength rail

continuity railsurface

rail surfaceirregularities

joints

vertical railvibration

horizontal railvibration

friction levelwheel rail interface

gauge

dynamic trackcharacteristic

rail corrugation

track alignment

dynamic gaugedynamic gauge

dynamic ridecharacteristics

diameter

wheelrail

rail fasteningfoundation

wear

ride behaviourride behaviour

Figure 11: causation of rail corrugation and contributing factors Further research With the establishment of the sequence of events that causes corrugation and the identification of the contributing factors, further research focuses on two aspects: - Although a clear relation between rail steel and corrugation wavelength is present, the exact relation

between crystal size and wavelength, which lay in a very different order of magnitude, needs to be defined. Results from this aspect will enable rail manufacturers to optimise their product.

- As the track structure is an important aspect of the 'dynamic gauge', this aspect will be further research with the aim to define an improved track structure that has a lower propensity for corrugation development.

References [1] Alias, J.: "Characteristics of wave formations in rails"; Rail International, November 1986, page 17-

23. [2] Bäseler W.: "Wellen und Riffeln"; Eisenbahntechnische Rundschau, April 1953, page 171 - 176. [3] Baumann G.: "Untersuchungen zu Gefühestrukturen und Eigenschaften der 'Weißen Schichten' auf

verriffelten Schienenlaufflächen"; dissertation Technical University of Berlin, ISBN 3-89574-309-7, 1998.

[4] Clark R.A., Dean P.A., Elkins J.A., Newton S.G.: "An investigation into the dynamic effects of railway vehicles running on corrugated rails"; Journal of Mechanical Engineering Science 24, #2 1982, page 65 - 76.

[5] Delgado L.P.: "Experience obtained concerning the undulatory wear of rails"; Bulletin of the IRCA, June 1958, page 963 - 990.

[6] Dreßler A.: "Neue Theorie über die Ursache der Riffelbildung auf den Fahrflächen der Eisenbahnschienen"; Eisenbahntechnische Rundschau, October 1954, page 433 - 442.

[7] Esveld C.: "Modern railway track", ISBN 90-800324-3-3; 2001. [8] Feller, H.G., Warf, K.: "Surface analysis of corrugated rails"; Wear 144, 1991, page 153 - 161. [9] Fink M.: "Phys.-chem. Vorgänge zwischen Rad und Schiene"; Glasers Annalen, September 1951,

page 207 - 210. [10] Fink, M.: "Die Entstehung der Schienenriffeln, der Stand der Riffelforschung nach rund 60 Jahre";

Glasers Annalen, November 1953, page 342 - 350. [11] Fink, M.: "Die Entstehung der Schienenriffeln, der Stand der Riffelforschung nach rund 60 Jahre";

Glasers Annalen, December 1953, page 373 - 380. [12] Frederick C.O.: "A rail corrugation theory"; CM1986, Budapest, Hungary, page 181 - 211. [13] Frederick C.O., Bugden W.G.: "Corrugation research on British Rail"; Symposium on rail

corrugation, Berlin, Germany, June 1983, page 181 - 211. [14] Girsch G., Heyder R.: "Testing of HSH-rails in high speed tracks to minimise rail damage"; CM2003,

Gothenburg, Sweden, 10 - 13 June 2003, page 265 - 272. [15] Grassie S.L., Kalousek J.: "Rail corrugation: characteristics, causes and treatments"; Journal of Rail

and Rapid Transit 207, 1993, page 57 - 68. [16] Grohmann, H.D.: "Beschädigungsarten an der Schiene verursacht durch den Betrieb";

Internationales symposium 'Schienenfehler', Brandenburg, Germany, 16-17 November 2000, page 2.1 - 2.12.

[17] Grohmann, H.D., Hempelmann K., Groß-Thebing A.: "A new type of RCF, experimental investigations and theoretical modelling"; Wear 253 (2002) page 67 - 74.

[18] Haarmann, A.: "Die Baustoffe der Spurbahnen"; Stahl und Eisen, 2 January 1913, page 1 - 12. [19] Heller W., Schweitzer R.: "Untersuchung und betriebliche Erprobung neuartiger naturharter

Schienenstähle", Techische Mitteilungen Krupp, 37 (1979), page 79 - 87. [20] Hiensch M., Nielsen J.C.O., Verheijen E.: "Rail corrugation in The Netherlands - measurements and

simulations", Wear 253, 2002, page 140 - 149. [21] Hull, D.; Bacon, D.J.: "Introduction to dislocations"; ISBN 0-7506-4681-0, fourth edition 2001. [22] Knothe K., Valdivia A.: "Riffelbildung auf Eisenbahnschienen - Wechselspiel zwischen

Kurzzeitdynamik und Langzeit-Verschleißverhalten"; Glasers Annalen 112, #2 1988, page 50 - 57. [23] Krabbendam G.: "Is rail corrugation due to internal stresses?"; Bulletin of the IRCA, March 1958,

page 411 - 428. [24] Krabbendam G.: "Speurtocht naar de oorzaak van golfslijtage"; Spoor- en tramwegen, nr. 13, June

1956, page 197 - 199. [25] Monnet M., Palmé M.: "Contribution a l'étude des causes de l'usure ondulatoire des rails", Revue

général de chemins de fer, February 1951, page 84 - 87. [26] Oberhoffer P.: "Ursachen der Riffelbildung auf straßen bahnschienen"; Stahl und Eisen, 18 August

1921, page 1137 - 1141. [27] Oostermeijer K.H.: "Causation of rail corrugation and contributing factors", WCRR 2003,

Edinborough, Great Britain. [28] Rosemeier H.: "Untersuchungen über Schienenriffeln"; Die Bundesbahn 28, 1954, page 216 - 228. [29] Sato Y., Matsumoto a., Knothe K.: "Review on rail corrugation studies"; Wear 253, 2002, page 130 -

139. [30] Satoh Y., Iwafuchi K.: "Crystal orientation of serviced rail with rolling contact fatigue damage";

CM2003, Gothenburg, Sweden, 10 - 13 June 2003, page 139 - 145. [31] Spieker W., Köhler H., Kühlmeyer M.: "Untersuchungen über die Riffelbildung auf Schienen in

Versuchsstrecken unter üblichen Bedingungen des Fahrbestriebes"; Stahl und Eisen 91, 23 December 1971, page 1470 - 1487.

[32] Srinivasan M.: "Prevention and cure of rail corrugation"; Railway Gazette, March 1971, page 97 - 101.

[33] Watson A.S.: "RCF on Dutch railways: application of sampling analysis"; AEATR-T&S-2002-203 (availability restricted).

[34] Werner K.: "Rollkontaktermüdungsrisse in Schienen infoge Rayleighwellen im Rad"; Eisenbahntechnische Rundschau 48, May 1999, page 297 - 303.

[35] Widmayer H.: "Measurements on the corrugation trial tracks of the DB"; Symposium on rail corrugation, Berlin, Germany, June 1983.