Mechanisms and mitigation of short pitch rail corrugation

Short pitch corrugation is a (quasi-) periodic rail surface defect with shiny crests and dark valleys. It primarily occurs on tangent tracks or gentle curves with a typical wavelength in the range of 20-80 mm. Short pitch corrugation excites high-frequency wheel-rail dynamic contact forces and generates a high level of noise, which is a nuisance to both the passengers and the residents near the railway lines. The resulting large dynamic forces accelerate the degradation of the track components and may induce other rail defects (such as, squats), which increase the maintenance cost. The goal of this dissertation is to better understand the formation mechanism of short pitch corrugation and develop the root-cause solutions to mitigate it. Three steps are taken to achieve this goal: 1) identification and control of rail vibration modes which are crucial to short pitch corrugation formation; 2) design of a new rail constraint to mitigate short pitch corrugation; 3) experimental study of short pitch corrugation using an innovative V-Track test rig.
Step 1 focuses on the identification and control of rail vibration modes. First, the vibration modes and dispersive waves of a free rail are simulated employing a finite element (FE) approach. The modal behaviors, wavenumber-frequency dispersion relations, and phase and group velocities of six types of propagative waves are derived and discussed in detail in 0-5 kHz. The operating deflection shape (ODS) approach distinguishes different types of rail vibration modes experimentally. A synchronized multiple-acceleration wavelet (SMAW) approach is proposed to experimentally study the propagation and dispersion characteristics of these waves. Both the laboratory and in-situ experimental results demonstrate the effectiveness of the ODS measurement for coupled rail mode identification and the SMAW approach for wave dispersion analysis. Afterward, the ODS and SMAW approaches are further applied to investigate rail vibration modes and wave propagation under fastening constraint. A three-dimensional (3D) FE rail-fastening model is also developed and validated against the ODS and SMAW measurement results. Subsequently, a sensitivity analysis of fastening parameters using this FE model is performed to gain insights into the control of rail vibrations. The results indicate that under fastening constraint, ODS measurement identifies vertical bending modes, longitudinal compression modes and lateral bending modes with shifted frequencies and significantly reduced vibration amplitude compared to free rail. Fastenings constrain the rail longitudinal vibrations less strongly compared to the vertical and lateral directions. The variation of fastening parameters can control rail mode frequencies and their vibration amplitudes, and influence the wave propagation velocities and attenuation along the rail.
Step 2 proposes a methodology to design a new rail constraint to mitigate short pitch corrugation. First, a parametric investigation of fastenings is conducted to understand the corrugation development mechanism and gain insight for a new rail constraint design for corrugation mitigation. A 3D FE vehicle-track dynamic interaction model is employed, which considers the coupling between the structural dynamics and the contact mechanics, and the damage mechanism is assumed to be differential wear. Various fastening models with different configurations, boundary conditions, and dynamic parameters are built up and analyzed. The results indicate that the fastening longitudinal constraint to the rail is the major factor determining the corrugation development. The fastening vertical and lateral constraints influence corrugation features in terms of spatial distribution and wavelength components. The increase of fastening constraint in the longitudinal dimension helps to mitigate corrugation, and the inner fastening constraint in the lateral dimension is necessary for corrugation alleviation. Based on these insights, a methodology is proposed to mitigate short pitch corrugation by rail constraint design. First, short pitch corrugation is numerically reproduced employing a 3D FE vehicle-track interaction model. Then, the corrugation initiation mechanism is identified by examining the ODSs of rail longitudinal compression modes. Afterward, different rail constraints are designed, and their effects on longitudinal compression modes are analyzed. Models of these rail constraints are also built and validated. Finally, the rail constraint models are applied to the 3D FE vehicle-track interaction model, and their validity on short pitch corrugation mitigation is evaluated. It is found that a relative rigid constraint can completely suppress rail longitudinal compression modes and significantly reduce the fluctuation amplitude of the longitudinal contact force to mitigate corrugation. A direction is pointed out for corrugation mitigation in the field by strengthening the rail longitudinal constraint.
Step 3 performs an experimental investigation of short pitch corrugation using the downscale V-Track test rig. First, a force measurement system named dynamometer is developed in the V-Track to measure the wheel-rail contact forces for short pitch corrugation experiments. The dynamometer consists of four 3-component piezo-electric force sensors and is mounted between the wheel assembly and the steel frame, enabling it to measure the forces transmitted from the wheel-rail interface to the frame. Static tests are first carried out to calibrate the dynamometer in three directions. Then, several tests are performed in the V-Track to examine the reliability and validity of the dynamometer for measuring the wheel-rail contact forces under running conditions. Experimental results show that the dynamometer is capable of reliably and accurately measuring these forces. Utilizing the measurement results from the dynamometer, the control of the wheel-rail contact forces in V-Track has also been achieved. Afterward, the V-Track test rig is used to investigate the development mechanism of short pitch corrugation experimentally. The loading conditions of the V-Track are designed to simulate the vehicle-track interaction on tangent tracks where short pitch corrugation mainly occurs in the field. Short pitch corrugation is successfully reproduced in the V-Track, and its spatial distribution, wavelength components, and hardness variation are captured by the 3D HandyScan and the hardness tests. Based on the measurement results of wheel-rail contact forces and track dynamic behaviors and observations, the development mechanism of short pitch corrugation is identified. It is found that rail longitudinal and lateral vibration modes contribute to the consistent development of short pitch corrugation.
Overall, the major contribution of this dissertation is threefold: 1) a better understanding of vibration modes and wave propagation of the rail in free condition and under fastening constraint is obtained by ODS and SMAW measurement, which is essential to understand and mitigate short pitch corrugation; 2) a new rail constraint is designed which can effectively suppress rail longitudinal compression modes and mitigate short pitch corrugation; 3) experimental evidence is provided to demonstrate that initial excitation and longitudinal compression modes play a significant role in the consistent growth of short pitch corrugation.