Corrugation
by Dr. Shaoguang Li
Rail corrugation, characterized by periodic (quasi-) sinusoidal defects on rail surfaces, brings significant challenges in the railway industry due to its detrimental effects. It can exacerbate the wheel-rail vibrations, accelerate the degradation of vehicle-track components, generate noise, cause rolling contact fatigue (RCF), reduce the ride comfort, and pose a threat to the train operation safety. Over the past century, extensive research has been conducted to understand and address this issue.
Understanding Rail Corrugation Mechanisms
According to Grassie, Corrugation can be understood by two mechanisms: the wavelength-fixing mechanism and the damage mechanism. The wavelength-fixing mechanism is associated with the root causes of corrugation, characterized by the eigenfrequency of the coupled structure. This mechanism can also be referred to as the „frequency-fixing“ mechanism for most types of corrugation. The damage mechanism explains how corrugation develops under the excitation of the eigenfrequency. The excitation generates dynamic wheel-rail contact forces in either vertical or tangential directions, leading to differential wear or plastic deformation.
Challenges of Short Pitch Corrugation
While most types of corrugation can be understood with these two mechanisms, short pitch corrugation remains an enigma. In the case of short-pitch corrugation, its wavelength varies non-linearly with speed or changes minor with speed, which means the eigenfrequency determining the corrugation wavelength differs at different speeds. Much research assumes the pin-pin resonance as the root cause of short pitch corrugation.
There is also a theory that explains corrugation from the perspective of wheel-rail stick-slip vibration. This theory is based on the assumption that at high creepage, the adhesion coefficient exhibits a negative slope, which can trigger wheel-rail vibrations. Another explanation of short pitch corrugation is from the perspective of contact filtering effect, together with the governing structural eigenfrequency.
New Insights into Short Pitch Corrugation
Recently, an alternative explanation to the short pitch corrugation was proposed, which attributes it to longitudinal track dynamics. It depends on the effective track distance where the longitudinal track vibrations cannot be effectively suppressed or constrained. This effective distance can vary depending on the actual situation. The fundamental frequency can be very low, which means the frequency between neighbouring harmonics can be much lower.
As speed increases, the dominant frequency responsible for short pitch corrugation also tends to shift to a different one. Consequently, the corrugation wavelength remains consistent. This elucidates the consistent corrugation wavelength as field observation and measurement. Similar to pin-pin resonance, the track with this underlying issue has a memory effect. If the issue remains unresolved, short pitch corrugation will inevitably return, for instance after rail grinding.
Solutions for Rail Corrugation
It should be noted that the term „short pitch“ might not be always precise and sometimes can be misleading, as corrugations with short wavelengths in the above studies may have different root causes. Therefore, it might be more appropriate to expand the classification of corrugations in this context.
Once the specific root cause and damage mechanism are identified, proactive or passive solutions tailored to the situation can be applied.
These solutions encompass:
- optimizing vehicle-track components, such as optimization of the stiffness and damping of fastening systems,
- improving rail material, for instance, by utilizing head-hardened and anti-wear steel,
- implementing rail friction modifier,
- mounting vibrations absorbing dampers, and
- employing rail grinding for corrugation removal. In theory, optimizing vehicle-track components represents the best solution.
Challenges in Implementing Rail Solutions
However, it is also the most challenging, as it requires a thorough understanding of the exact root cause. While introducing head-hardened rail material can delay corrugation, it may also give rise to the risk of RCF problems. A balance between wear rate and RCF development should be considered. The same applies to the rail friction modification. The installation of rail dampers to reduce the noise level is increasingly prevalent in many countries especially when the rail track passes through districts densely populated by residents.
The effective working frequency range of the rail dampers should be tailored to the specific situation. Among them, rail grinding is the most effective way so far to address short pitch corrugation. Depending on the intervention stage, rail grinding can be corrective, maintenance, or preventive. It is widely adopted by most countries as the primary method to address corrugation. These approaches have yielded positive results in addressing various issues associated with rail wear, corrugation formation, and other damage mechanisms. By employing a customized combination of these solutions suited to the specific conditions and demands of the rail network, railway operators can effectively mitigate corrugation’s impact, ensure safety, and reduce noise emissions.
Importance of Rail Grinding and Maintenance
It should be emphasized that although rail grinding can effectively remove corrugation, the maintenance cost is considerable. Timely and early-stage rail grinding can maximize the rail lifespan and effectively control noise levels. To this end, corrugation should be measured, monitored, and evaluated.
According to the measurement principle, corrugation measurement is divided into two categories: inertial-based and chord-based measurements.
Two notable applications of the inertial principle are from RailTechnology GmbH:
- hand-push corrugation analysis trolley (CAT), which measures at walking speed, and
- the rail corrugation analysis (RCA) system, which operates at higher axle loads and can reach speeds of up to 20 km/h.
Chord-Based & Inertial-Based Corrugation Measurement
In contrast, chord-based measurement utilizes multi-point asymmetric chord and an inverted transfer function to calculate the corrugation parameters such as the corrugation space curve, root mean square (RMS), dominant wavelengths, and peak-to-peak values. Theoretically, a transfer function without zero values would yield a reasonable corrugation profile derivation. However, a challenge of the traditional 1-D laser-camera systems is that there should be a compensation system with motor-driven actuators to dynamically adjust for lateral position of the sensor box due to vehicle movement during the high speed measurement.
Alternatively, 2D laser-camera systems which covers a wider lateral measurement range can get rid of the compensation. Moreover, the lateral position of the export corrugation on the rail is adjustable. In comparison to the low-speed inertial-based system, which offers measurement accuracy of 0.01 µm and 0.1 µm, respectively, the chord-based system can achieve measurement accuracy in the range of several micrometers. With the highly accurate measurement data, long term monitoring of corrugation development for predictive grinding can be possible.
Summary of Corrugation Solutions
In summary, significant progress has been made in understanding the mechanisms behind various types of corrugation, including short pitch corrugation. Theoretically, proactive measures such as optimizing the design of vehicle-track components represent the best solution to corrugation, but it requires a good understanding of the underlying mechanisms. Rail grinding, particularly predictive grinding, supported by accurate corrugation measurement data, offers an effective and economical approach to addressing this issue.