Numerical study on two-point contact by an explicit integration finite element method: A contribution to the modeling of flange squeal

The precise mechanism which activates squeal, especially flange squeal has not been fully explained. The complex non-Hertzian contact and the broad-band high frequency feature bring great challenges to the modelling work of flange squeal. In this paper, an explicit integration finite element method is presented to simulate the dynamic curving behavior of the outer wheel, which is believed directly related to flange squeal generation. By fully considering the normal, tangential force and spin moment, the non-steady-state wheel-rail interaction from one-point to two-point contact is reproduced. The critical time step of the explicit integration scheme is determined by the Courant stability condition, which, together with the detailed modelling of the structural and continuum of the wheel/track system, effectively guarantees that the reproduced vibration frequency can reach up to 10 kHz with desired accuracy. The aim of the work is to contribute to the modelling and understanding of the generation mechanism of the flange squeal from the viewpoint of the wheel-rail interaction.Railway Engineerin

Determining the angles of squat cracks via CT scanning and metallographic observations

This study investigates the angles θ1, θ2, and θ3 that squat crack faces form with respect to three orthogonal planes: the rail top, the longitudinal-vertical cross-section and the lateral-vertical cross-section. Rail samples with squats of various severities are obtained from the field. Their three-dimensional crack networks are reconstructed using CT (computed tomography) scanning and serial cutting. A 3D visualization method, together with the necessary geometric definitions, is developed for enabling effective measurement and characterization of the squat cracks. It is found that the cracks can be characterized by four orientations (T1 – T4). The variation ranges of the crack angles are determined for each orientation that satisfies 132° ≤θ1 ≤ 150°, 6° ≤θ2 ≤ 36° and 67° ≤θ3 ≤ 81°. By investigating the occurrence frequency of the orientations, it is found that T4 and T1 together form the primary V-shaped cracks of the squats, and T2 and T3 together form the secondary V-shaped cracks. A finite element modelling of the wheel-track system, in combination with contact mechanics and multi-axial fatigue analysis, successfully relates the stress state to the RCF cracks.Railway Engineerin

Computation of stress intensity factors in an initiating RCF crack using a 3D modelling approach

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Nucleation of squat cracks in rail, calculation of crack initiation angles in three dimensions

A numerical model of wheel-track system is developed for nucleation of squat-type fatigue cracks in rail material. The model is used for estimating the angles of squat cracks in three dimensions. Contact mechanics and multi-axial fatigue analysis are combined to study the crack initiation mechanism in rails. Nonlinear material properties, actual wheel-rail geometries and realistic loading conditions are considered in the modelling process. Using a 3D explicit finite element analysis the transient rolling contact behaviour of wheel on rail is simulated. Employing the critical plane concept, the material points with the largest possibility of crack initiation are determined; based on which, the 3D orientations/angles of the possible squat cracks are estimated. Numerical estimations are compared with sample results of experimental observations on a rail specimen with squat from the site. The findings suggest a proper agreement between results of modelling and experiment. It is observed that squat cracks initiate at an in-plane angle around 13°-22° relative to the rail surface. The initiation angle seen on surface plane is calculated around 29°-48°, while the crack tend to initiate in angles around 25°-31° in the rail cross-section.Structural EngineeringCivil Engineering and Geoscience

An investigation into the relation between wheel/rail contact and bolt tightness of rail joints using a dynamic finite element model

Rail joints have a shorter service life than most other railway track components. The discontinuity between rail ends turns the rail joint into a weak spot, and consequently, into a track component demanding more frequent maintenance measures, which result in high maintenance costs. Moreover, difficulties are often found when assessing the damage condition of rail joints since damage conditions like cracks in the rail web or loose bolts cannot be detected by visual inspection. A better understanding of the damage mechanisms and degradation process of rail joints may help to develop adapted maintenance measures and to improve rail joint design. In this paper, a 3D Finite Element model is presented as base for rail joint study. The model represents accurately the main components (rail, sleeper, joint bars and wheel) and the interaction between them (contact). The model is validated between 150 and 800 Hz with measured axle box accelerations of resilient wheels. Higher frequencies may be reached with an improved model of the rubber. In the paper, the influence of the bolt tightness is studied. The results showed that contact force, specially its variation, is strongly affected by the bolt tightness; loose bolts cause higher contact forces. The effect of vehicle speed on wheel/rail contact is also significant mainly due to the interaction between rail and sleepers in the vicinity of the rail joint. Apart from bolt tightness conditions and vehicle speed, the validated model has the potential to study the influence of other track parameters and damage conditionsStructural EngineeringCivil Engineering and Geoscience

Scaling strategy of a new experimental rig for wheel-rail contact

A new small–scale test rig developed for rolling contact fatigue (RCF) investigations in wheel–rail material. This paper presents the scaling strategy of the rig based on dimensional analysis and mechanical modelling. The new experimental rig is indeed a spinning frame structure with multiple wheel components over a fixed rail-track ring, capable of simulating continuous wheelrail contact in a laboratory scale. This paper describes the dimensional design of the rig, to derive its overall scaling strategy and to determine the key elements’ specifications. Finite element (FE) modelling is used to simulate the mechanical behavior of the rig with two sample scale factors of 1/5 and 1/7. The results of FE models are compared with the actual railway system to observe the effectiveness of the chosen scales. The mechanical properties of the components and variables of the system are finally determined through the design process.Structural EngineeringCivil Engineering and Geoscience

A new small-scale test rig for the wheel-rail contact studies

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Robust optimisation of railway crossing geometry

This paper presents a methodology for improving the crossing (frog) geometry through the robust optimisation approach, wherein the variability of the design parameters within a prescribed tolerance is included in the optimisation problem. Here, the crossing geometry is defined by parameterising the B-spline represented cross-sectional shape and the longitudinal height profile of the nose rail. The dynamic performance of the crossing is evaluated considering the variation of wheel profiles and track alignment. A multipoint approximation method (MAM) is applied in solving the optimisation problem of minimising the contact pressure during the wheel–rail contact and constraining the location of wheel transition at the crossing. To clarify the difference between the robust optimisation and the normal deterministic optimisation approaches, the optimisation problems are solved in both approaches. The results show that the deterministic optimum fails under slight change of the design variables; the robust optimum, however, has improved and robust performance. © 2016 Taylor & FrancisRailway Engineerin

Fast and robust identification of railway track stiffness from simple field measurement

We propose to combine a physics-based finite element (FE) track model and a data-driven Gaussian process regression (GPR) model to directly infer railpad and ballast stiffness from measured frequency response functions (FRF) by field hammer tests. Conventionally, only the rail resonance and full track resonance are used as the FRF features to identify track stiffness. In this paper, eleven features, including sleeper resonances, from a single FRF curve are selected as the predictors of the GPR. To deal with incomplete measurements and uncertainties in the FRF features, we train multiple candidate GPR models with different features, kernels and training sets. Predictions by the candidate models are fused using a weighted Product of Experts method that automatically filters out unreliable predictions. We compare the performance of the proposed method with a model updating method using the particle swam optimization (PSO) on two synthesis datasets in a wide range of scenarios. The results show that the enriched features and the proposed fusion strategy can effectively reduce prediction errors. In the worst-case scenario with only three features and 5% injected noise, the average prediction errors for the railpad and ballast stiffness are approximately 12% and 6%, outperforming the PSO by about 6% and 3%, respectively. Moreover, the method enables fast predictions for large datasets. The predictions for 400 samples takes only approximately 10 s compared with 40 min using the PSO. Finally, a field application example shows that the proposed method is capable of extracting the stiffness values using a simple setup, i.e., with only one accelerometer and one impact location.Railway Engineerin

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

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.Railway EngineeringOLD Road and Railway Engineerin