Rolf Dollevoet

Differential wear and plastic deformation as causes of squat at track local stiffness change combined with other track short defects

Squats have become a major problem in the track of many railways. In the quest for the root causes of squats, it is observed that they are occasionally found at locations of track stiffness changes such as at fish-plated insulated joints and at switches and crossings. Obviously, there should be other factors in the track, which, together with the stiffness change, have played important roles otherwise there will be squats at all such locations. A validated hybrid multibody-finite element model of vehicle–track vertical interaction is extended to simulate the frictional dynamic rolling contact at a fish-plated insulated joint in order to identify such factors. Elastic–plastic rail material property is taken into account. It is found that it is track short defect in the preload condition of the bolts and the contact between the fishplates and the rail head, which together with the stiffness change, causes large normal and longitudinal contact force variation at the fishplate end so that differential wear and differential plastic deformation may accumulate at a fixed location. With proper wavelength, the accumulated rail top geometry deviation may grow into a squat. The significance of the present work lies in that other track short defects such as damaged and improper railpads and fastening, and ballast voids may also have such effects, which may be responsible for a large portion of the many squats in the tracks. This gives the direction for further work.

Improvements in Axle Box Acceleration Measurements for the Detection of Light Squats in Railway Infrastructure

Squats are a type of short-wave rolling contact fatigue defect whose early detection can contribute to cost reductions in the railway industry. This paper demonstrates how the early detection of squats is possible via enhanced instrumentation based on axle box acceleration (ABA) and adequate postprocessing. Three improvements are discussed. The first corresponds to the use of longitudinal ABA to enhance measurement sensitivity to light squats. Compared to vertical ABA, longitudinal ABA does not contain the vibrations of the rail, fastening, sleepers, and ballast, and thus, the impact-related vibration is considerably stronger in the signal. The second improvement considers the use of multiple sensors, noise-reduction techniques, and repeated measurements. Due to hunting, the wheels of a measuring train do not always pass over small squats; thus, light squats are more likely to be detected using multiple sensors and multiple measurement runs. The third improvement concerns the signal-processing solution for the reduction of disturbances from wheel defects. Extensive field measurements show that these improvements make the characteristics of squats more visible in signals and allow the squats to be distinguished from vibrations of other origins.

A Method for the Prediction of Head Checking Initiation Location and Orientation under Operational Loading Conditions

Head checking (HC) as a surface-initiated rolling contact fatigue damage in rails has been a problem for many railways. Countermeasures such as optimal design of wheel and rail profiles, optimal design of the vehicle-track interaction system, HC-resistant rail materials, etc. are being developed. Such development requires a better understanding of the relationship between the operational loading conditions of the rails and the initiation of HC. This article presents an effective non-Hertzian method for evaluating the stress state on the rail and wheel surfaces under operational conditions, so that HC initiation location and orientation can be directly related to the stress. It is shown that the HC initiation location and orientation derived from the computed stress state are in agreement with those observed in the field.

Squat Treatment by Way of Minimum Action Based on Early Detection to Reduce Life Cycle Costs

Squats are a type of rolling contact fatigue on the top of rails of straight tracks or shallow curves. In some countries they have become a major type of rail damage. Although it is widely recognized that preventive and early corrective actions are cheaper than late remedies such as rail replacement, timely treatment is hindered by the lack of automatic detection of early squats. So far, the treatment of squats has mainly been dependent on ultrasonic detection, which detects deep cracks and is applicable only to severe squats. The life cycle costs (LCC) of squat-infected track are therefore high. The operational safety and availability of the networks are also adversely affected. Based on a new method which detects initiating squats, this paper proposes a methodology for squat related LCC reduction. First a squat classification is introduced that facilitates early identification of squats. Then the growth phases of squats are discussed; typical growth rate of squats is analyzed, and thereupon based threshold values for minimum action by rail grinding are proposed. After a review of the currently available detection methods, the new detection method is presented, with results of field test demonstrating the capability of the method to detect light squats. Finally, a case study of the LCC of a monitored squat-infected track section shows that the proposed treatment can significantly reduce the LCC. In addition, operational safety and network availability can also be improved.© 2010 ASME

An investigation into the vertical dynamics of tracks with monoblock sleepers with a 3D finite-element model

This paper investigates the vertical dynamic behavior of railway tracks with monoblock sleepers. Whole- and half-track finite element (FE) models are presented in which the rails and sleepers are represented with their nominal 3D geometry using solid elements. The railpad encompasses the rail seat area modeled as multiple spring–damper pairs. The 3D FE models are employed for three purposes. First, the stiffness and the damping of the railpad and ballast are derived by fitting the simulations to a set of field hammer test measurements. Second, the origins of six of the seven main characteristics are identified. Third, the influence of the railpad representation on the track’s dynamic response is studied. The results show that, in contrast to the 3D FE half-track model, the 3D FE whole-track model reproduces six of the seven main vertical track characteristics with a maximum deviation of 10% from the measured frequencies. The seventh characteristic is reproduced at approximately the measured frequency when the frequency-dependent stiffness of the railpad is considered. This model can be used to derive track parameters that will aid in the study of track degradation.

The vertical and the longitudinal dynamic responses of the vehicle–track system to squat-type short wavelength irregularity

The squat, a kind of rolling contact fatigue occurring on the rail top, can excite the high-frequency vehicle–track interaction effectively due to its geometric deviations with a typical wavelength of 20–40 mm, leading to the accelerated deterioration of a track. In this work, a validated 3D transient finite element model is employed to calculate in the time domain the vertical and the longitudinal dynamic contact forces between the wheel and the rail caused by squats. The vehicle–track structure and the wheel–rail continua are both considered in order to include all the important eigencharacteristics of the system related to squats. By introducing the rotational and translational movements of the wheel, the transient wheel–rail rolling contact is solved in detail by a 3D frictional contact model integrated. The contact filter effect is considered automatically in the simulations by the finite size of the contact patch. The present work focuses on the influences of the length, width and depth of a light squat on the resulted dynamic contact forces, for which idealised defect models are used. The growth of a squat is also modelled to a certain extent by a series of defects with different dimensions. The results show that the system is mainly excited at two frequencies separately in the vertical and the longitudinal dynamics. Their superposition explains the typical appearance of mature squats. As a squat grows up, the magnitude of the excited vibration at the lower frequency increases faster than the one at the higher frequency.

An approach to determine a critical size for rolling contact fatigue initiating from rail surface defects

ABSTRACT A methodology for the determination of a critical size of surface defects, above which RCF can initiate, has been developed and demonstrated with its application to the passive type of squats under typical Dutch railway loading conditions. Such a methodology is based on stress evaluation of transient rolling contact at the defects, for which a detailed 3D frictional rolling contact model is integrated in the vehicle–track interaction system. Through comparing the maximal von Mises stress at defects of different sizes with the tensile strength of the rail material, the critical size is derived for squats. Observations during a field monitoring test show a good validation of the determined critical size. In practice, the critical size can be used for distinguishing between light squats and trivial defects by visual inspection or by automatic image recognition, so that false statistics of squats can be reduced or prevented. With necessary modifications and improvements, the developed methodology may also be applied to RCF of other rolling contact pairs in general, such as bearings and gears.

3D FE Modelling and Validation of Frictional Contact with Partial Slip in Compression-Shift-Rolling Evolution

Abstract This paper presents a three-dimensional finite element approach capable of realistically simulating a wide range of contact types with partial slip. The approach is demonstrated by analysing the evolution of instationary contact from compression to shift till stationary rolling. It is systematically validated using classical solutions, i.e. Hertz theory for the frictionless normal problem, and Spence solution for frictional compression, Cattaneo solution for tangential shift and Kalker’s CONTACT for frictional rolling. The effect of plastic deformation on the distributions of pressure, adhesion–slip regions, surface shear traction and micro-slip is investigated. It is shown that plastic deformation makes the normal contact solution dependent on the tangential contact solution. Plastic deformation partly damps out the structure vibrations and significantly diminishes the magnitude of micro-slip. The presented model can be extended to simulate the start-off and braking of railway vehicles, to optimize the traction and braking control systems of trains, as well as to reduce the related damages.

Facilitating maintenance decisions on the Dutch railways using big data: The ABA case study

This paper discusses the applicability of Big Data techniques to facilitate maintenance decisions regarding railway tracks. Currently, in different countries, a huge amount of railway track condition-monitoring data is being collected from different sources. However, the data are not yet fully used because of the lack of suitable techniques to extract the relevant events and crucial historical information. Thus, valuable information is hidden behind a huge amount of terabytes from different sensors. In this paper, the conditions of the 5Vs of Big Data (Volume, Velocity, Variety, Veracity and Value) in railway monitoring systems are discussed. Then, general methods that can be applied to facilitate the decision of efficient railway track maintenance are proposed for railway track condition monitoring. As a benchmark, axle box acceleration (ABA) measurements in the Dutch tracks are used, and generic reduction formulations to address new relevant information and handle failures are proposed.

A big data analysis approach for rail failure risk assessment

Railway infrastructure monitoring is a vital task to ensure rail transportation safety. A rail failure could result in not only a considerable impact on train delays and maintenance costs, but also on safety of passengers. In this article, the aim is to assess the risk of a rail failure by analyzing a type of rail surface defect called squats that are detected automatically among the huge number of records from video cameras. We propose an image processing approach for automatic detection of squats, especially severe types that are prone to rail breaks. We measure the visual length of the squats and use them to model the failure risk. For the assessment of the rail failure risk, we estimate the probability of rail failure based on the growth of squats. Moreover, we perform severity and crack growth analyses to consider the impact of rail traffic loads on defects in three different growth scenarios. The failure risk estimations are provided for several samples of squats with different crack growth lengths on a busy rail track of the Dutch railway network. The results illustrate the practicality and efficiency of the proposed approach.