RESEARCH & FACTS

Project research included the search for all relevant parameters in the corrugation process, the definition of macroscopic and microscopic wheel/rail interaction forces during curve negociation,the mathematical modelling based upon appropriate multi body models and eventually the design and validation of applicable solutions. Several papers were published on the subject and a PhD thesis was written. Some basic information on corrugation, according to selected items, is given below :

What is rail corrugation?
How big is the corrugation problem?
What is the exact cause of rail corrugation?
Are there different types of corrugation?
Can corrugation be cured or prevented?
Why is it important to measure corrugation/grind quality and how?
What are the consequences of corrugation?

What is rail corrugation?

  • Rail corrugation is one of the most serious and expensive problems experienced by transit systems. Rail corrugation is a frequently occurring rail wear pattern that arises mostly in curves, although that it’s also possible on tangent track.
  • Rail corrugation is considered with wavelengths between 20 mm and 200 mm.
  • The wear pattern results in reduced rail and wheel lifetime and it can lead to urgent safety measures (rail replacement).
  • At the wayside, wheel/rail noise due to corrugated rail contains a distinctive tonal character, and the noise may be heard and identified at large distances from the track. Noise from corrugated rail is particularly severe and may be a source of significant community reaction.

How big is the problem?

About 40% of all transit tracks are prone to corrugation development.

Generally, all track geometry (mostly curves but also tangents) are equally prone to corrugation development

The yearly expenses related to corrugation curing in Europe alone are estimated to be above 60 million €. Beside that cost, rail renewal works are also a major cause for traffic disturbance.

What is the exact cause of rail corrugation ?

The passage of a bogie in small curve leads to important dynamic forces between wheel and rail in all directions: tangent, axial and vertical. These forces are due to:

  • important friction between wheel and rail due to curving (tangential and radial friction forces)
  • Rail and wheel imperfection (including welds).



The amplitude of these forces is potentially increased by
  • flange friction at high rail;
  • misalignment of wheelsets in bogie;
  • rail support stiffness variations;
  • important slip between wheel and rail due to difference in wheel diameter between wheels of same wheelset;
  • existing corrugation;
  • poor bogie steering.
These forces are modulated by resonance frequencies (and anti-resonances) of track and wheelset:
  • P2 low frequency anti-resonance (non suspended mass bouncing on track spring): wheel/rail bending mode;
  • axle torsion mode (especially 2nd order torsion);
  • sleeper resonance (e.g. Stedef block (mass) bouncing on rail pad spring);
  • axle bending/wheel lateral mode;
  • ...

This leads to rail corrugation in different forms: excessive corrugation & wear corrugation.

Are there different types of corrugation?

The dynamic forces between rail and bogie lead to rail corrugation in two different forms: Wear corrugation & excessive corrugation:

Wear corrugation

The friction forces create a wear pattern mainly on the lower rail (highest axle load) with a wavelength corresponding to one of the track/wheelset resonances:

  • stiff track leads to long wavelengths (corresponding with P2 anti-resonance)
  • soft track without sleepers (soft rail fixation systems, soft continuously supported track) leads to short wavelengths (corresponding with 2nd torsion resonance-longitudinal wear) (not observed in this project)
  • soft track with booted sleeper and resilient undersleeper pads (cfr. Stedef track) leads to short wavelengths (corresponding with sleeper/rail pad resonance-lateral wear)
  • track with guard rail can lead to short wavelength corresponding with lateral wheelset resonance (lateral wear)

Under normal conditions, corrugation amplitude growth is moderate: amplitude of 0.05 mm in 1 year time, but amplitude grows exponentially!

Wear corrugation on stiff track is more aggressive than wear corrugation on soft track.

Wear corrugation on soft track corresponding with the axle torsion resonance can be reduced by using high rail gauge face lubrication.

Wear corrugation on booted track can be very aggressive, especially in small curves and with high axle loads. It can be reduced by using high damped resilient rail pads and/or by using high rail gauge face lubrication

The track dynamic behaviour (especially in vertical direction) has a far greater influence on the development of most corrugation types in comparison with the wheel or vehicle dynamic behaviour. In most cases, corrugation wavelengths correspond with vertical track anti-resonance and/or with lateral track resonance frequencies. Vertical track resonances in the ‘sensitive’ frequency domain (vehicle speed dependent) can be avoided by using high damped embedded rail or very soft rail fixation. Horizontal track resonances can be shifted outside the ‘sensitive‘ frequency band (corresponding with 2cm-20cm wavelength) by using very rigid or very soft lateral rail fixation.

Excessive corrugation

The curving forces generate plastic flow of rail, plastic bending of rail or contact fatigue with important corrugation pattern on inner and outer rail.

This type of corrugation is associated with:

  • P2 resonance excitation: rather stiff track with variations of rail support stiffness leading to low frequency excitation;
  • >LI>very high axle loads (case of plastic flow) or use of low inertia rail (case of plastic bending)
  • important amplification of curving forces due to one of the following factors:
    . misalignment of wheelsets in bogie;
    . important difference in wheel diameters for wheels of the same wheelset;
    . very poor bogie steering.

Corrugation can grow up to 1 mm in 6 weeks of time. Excessive corrugation can only be cured by eliminating the cause: e.g. force amplification factor; reducing the tolerances on wheel diameters.

Can corrugation be cured or prevented?

Measures, which reduce wear rate of all types of wear corrugation, are:

Use of head hardened rail: HHR instead of e.g. 900A grade rail will decrease the wear rate with factor 2. Standard carbon rail is twice as likely to develop corrugation compared to alloyed or heat treated rail, and this corrugation will have a tendency to be more severe

  • Use of very resilient rail fasteners. In order to be effective the vertical dynamic fastener stiffness has to be below 10kN/mm. The use of softer rail pads or under sleeper pads has only a secondary effect on corrugation (change of wavelength). In this project it has been demonstrated that there is no difference between normal rail and HHR in terms of corrugation amplitude growth when very resilient rail fasteners are used. Read more
  • use of continuous vertically very soft and horizontally very rigid embedded rail (tram application). Read more
  • use of resilient wheels (new wheels): effective against very short pitch corrugation when specially designed not to exhibit resonance frequencies in the sensitive frequency domain (for tram and metro application this is situated between 250 Hz and 350 Hz).
  • lubrication: top of rail or high rail gauge face (in function of type of lubrication) Read more
  • Excessive corrugation can only be cured by eliminating the cause, e.g.: force amplification factor: reducing the tolerances on wheel diameters, increase of the rail inertia when too low...
  • Monitor and measure wheel profiles and top of rail profiles for irregularities which can be responsible for corrugation development Read more
General remarks

Attention: harder rail and lubrication in combination with high axle loads could lead to an increase of rolling contact fatigue.

In case all resonance excitation (track and wheel set) is avoided during curve negotiation, rail wear will still occur due to the friction forces. This rail wear (quasi random patterns) can be reduced using harder rail contact areas .

All the mitigation measures proposed here above can be applied for new systems in order to avoid excessive corrugation.

For existing systems, most of them can no more be applied. The unique solutions are then top of rail lubrication or rail grinding, which is definitely very effective in reducing noise and controlling corrugation. But rail grinding treats corrugation symptom, it does not eliminate its cause. It is very important to remove all corrugation patterns completely from the railhead during grinding. A quality control of the grinding process is absolutely necessary: this can be done with a real surface analyser. Read more.

Why is it important to measure rail corrugation and how?

Research indicated that corrugation growth is exponentially and existing corrugation is one of the main contributors to the development of more corrugation.

One of the conclusions of the CORRUGATION project stressed the importance of the quality of the grinding process. It is important that all corrugation is completely removed after grinding.

Corrugation measurements usually required a dedicated vehicle to be brought on the tracks, what is not always easy for small networks. In the scope of this project a manually operated measurement device was developed for the easy measurement and quantification of corrugation. This device is now also commercially available on the market. Read more

What are the consequences of corrugation?

Rail corrugation is one of the most serious and one of the most expensive problems experienced by transit systems:

  • The wear results in reduced rail and wheel lifetime and it can lead to urgent safety measures (rail replacement). Most of the European urban transport policies tend to increase the operation period, then leading to smaller time intervals in the night for maintenance works. This problem is going to increase in the coming years.
  • The emitted noise is irritating, and can be painful and possibly harmful to passengers in subways. At the wayside, wheel/rail noise due to corrugated rail contains a distinctive tonal character, and the noise may be heard and identified at large distances from the track. Noise from corrugated rail is particularly severe and may be a source of significant community reaction. Most of the European cities are adopting more and more severe environmental noise policies, constraining the public transportation networks to reduce their noise emission, even on existing lines. This general trend is also going to increase in the coming years.

The extra yearly maintenance costs caused by corrugation curing are in the average 3 million € for a small metro network as Brussels. The yearly expenses related to corrugation curing in Europe alone are estimated to be above 60 million €. Beside that cost, rail renewal works are also a major cause for traffic disturbance.



 
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