Narrow Results

Attention is drawn to the fact that the recent increase in axle loads, speed and traffic volume in railway tracks, as well as concerns over passengers' riding comfort and safety have resulted in fresh challenges that are needed to be addressed. These challenges can only be successfully tackled with a more accurate modeling of the dynamic behavior of railway tracks. Although a significant amount of research involving mathematical modeling of railway track dynamics has been conducted in the last two decades, the nonlinearity of track support systems has not been given sufficient attention. This paper is concerned with the effect of nonlinearity of the support sub-layers on the dynamic responses of the railway track. To this end, a railway track model that considers the nonlinear properties of the track sub-layers is developed. Then, a field investigation into the dynamic responses of the railway track system under moving trains is conducted. The effect of the nonlinearity properties of the track support system on the track responses is investigated by comparing the results obtained by the numerical model, with or without consideration of track support nonlinearity, with those from the field tests. It is illustrated that consideration of the nonlinear properties of the track support system improves the accuracy of the calculated responses by a factor of three. It is also shown that the train axle loads and track accumulative loading have a significant effect on the nonlinearity of the track support system and, as a result, on the modeling of track responses.

Application of the piezoelectric patch to mitigate the train wheel squeal is presented in this paper. A complete model for the wheel, rail and foundation is employed in the frequency domain. The wheel squeal model with the shunted piezoelectric element is presented and the consequent added damping is calculated for the critical frequencies. Two different shunt circuits including the resistor ($R)$ and resistor–inductor (RL) are applied and the consequent performances in the noise mitigation are evaluated for different frequencies. The effect of contact point lateral displacement on occurrence of the wheel squeal is investigated. In the parametric study, the effects of shunting circuit and its parameters, size, position and number of piezoelectric elements on the control performance are evaluated. It is found that the resistance shunt circuit has optimal performance when the added damping over a broadband frequency range is implemented. In addition, both the added damping and added damping per piezoelectric volume become saturated by increasing the number of piezoelectric patches further over a specific limiting value.

Noise and vibration from the railway tracks with fractional railpads are studied in this paper. The track system is modeled by a beam on a viscoelastic foundation including the fractional railpads and nonlinear ballast. The nonlinear fractional governing equation is obtained in the time domain by use of the fractional finite element formulation. The Newmark’s method is combined with the Adomian decomposition method (ADM) to solve the nonlinear fractional equation. The acoustical pressure field around the vibrating beam is obtained by the Rayleigh integral approach. The effects of the fractional parameters, load speed and condition on the track responses are investigated. It is found that the dynamic deflection and stress alongside the railways are more influenced by the loading condition, while the acoustical pressure is remarkably influenced by the load speed.

The vehicle scanning method (VSM), an indirect approach for bridge measurement, has attracted intensive attention since it was proposed. By this method, a moving test vehicle is employed to detect the “mechanical” properties of the bridge, e.g. frequencies, mode shapes, damages, etc., utilizing the interaction between the two substructures. Compared with the conventional direct approach that requires quite a few sensors and data loggers to be fitted on the bridge, the advantage of the VSM is obvious: mobility, economy, and efficiency. As for railways, the broader vehicle-based techniques have long been used to detect the “geometrical” properties of the track, such as track profiles and rail conditions. Relatively little use has been made of the interaction between the moving vehicle/train and the track/bridge. This paper is a state-of-the-art report of the VSM’s applications to highway bridges and the vehicle-based techniques to railway tracks. It starts with a summary of the pioneering works by Yang and co-workers on the VSM. Then, the applications of the techniques to highway bridges and railway tracks will be separately reviewed. Conclusions will be made, along with future research directions, at the end of the paper.

In this study, the dynamic responses of a cracked beam layed on a visco-elastic foundation subjected to moving loads are calculated. An Euler–Bernoulli beam model is used to describe the beam behaviors. In addition, the beam has several open cracks one-sided with different depths. By using the Fourier transform, the dynamic responses of the beam are determined analytically in the frequency domain with the help of Green’s function. By coupling with the periodic supported beam model, an analytical model of the railway sleeper is developed. This model allows the fast calculation of the dynamic responses of a damaged sleeper. A dynamic computational model using the FEM method was also developed and compared to the analytical model. The results from the two methods are relatively comparable for three cases of beams without cracks, beams with one crack, and beams with two cracks.

Rail fasteners are resilient components that connect the rails to sleepers or track slabs, ensuring the integrity of the track structure. Their dynamic characteristics play a crucial role in the noise and vibration performance of the track. The rail fastening system, as a resilient element (RE), can be described by a $12\times 12$ dynamic stiffness matrix based on the interaction between two rigid bodies and the RE. By applying the principles of momentum and angular momentum to the rigid bodies, an algebraic relationship between the dynamic stiffness of the resilient component and the frequency response functions (FRFs) of the assembly is established. Consequently, the dynamic stiffness of the resilient component can be determined based on the experimentally measured FRFs. Two methods are presented for calculating the dynamic stiffness from the FRFs, called the Complete Method and the Partial Method. The two methods are subsequently applied to the structurally complex DT-III type fastener. To do so, a “rail-like” block and a “sleeper-like” block were purposely designed so that they not only can be fastened by the fastener in the same way as in reality, but can also be treated as rigid bodies for frequencies up to 2000Hz. FRFs of the rail-like and sleeper-like blocks were obtained through impact tests, and the dynamic stiffness of the fastener in the frequency range 50–2000Hz was obtained using both methods. It turns out that, although the two methods have similarly good repeatability, the Partial Method gives more reliable result; in general the dynamic stiffness of the fastener increases with frequency, but exhibits multiple peaks and dips, indicating distinct modal characteristics of the fastener system. The results also show that there is a significant difference between dynamic stiffness seen by the rail and that by the sleeper.

The track loading vehicle (TLV) is an important equipment for evaluating track stiffness, vibration characteristics and detection of common faults on the railway. The frequency of TLV fixed-point loading directly affects the accuracy and reliability of the test results. Taking into consideration the interaction between TLV and track structure, a vibration analysis model of ballastless track/ballast track based on finite element theory is established. The model is then used to study the influence of vertical load and load frequency on the dynamic responses of track structure. The results show that the sensitive frequency of rail is 10-15Hz, and that the sensitive frequency of the ballastless track bed and sleeper-bed system is about 15Hz. These sensitive frequencies of track structure should be avoided because they can cause sudden changes and distortions to the track displacement data.