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• We propose a novel approach to numerically simulate speckle noise. • Surface roughness and other affecting factors are thoroughly considered. • The numerically simulated and experimentally acquired speckle noise are identical. • Some strategies for mitigating speckle noise are supported by characterization. Laser Doppler Vibrometer (LDV) is extensively applied in remote and precise vibration measurements for structural monitoring. Speckle noise is a severe signal issue restricting LDV applications, mainly when an LDV scans from moving platforms. Realistic simulations and thorough characterizations of speckle noise can support the despeckle procedure. A novel approach to numerically simulate speckle noise is proposed based on the statistical properties of speckle patterns. Surface roughness and other affecting factors are thoroughly studied. The simulated distributions agree well with the literature when investigating speckle properties. Single-point and continuously scanning speckle noise are both numerically generated and experimentally acquired. Their corresponding time-series and fast Fourier spectra present good agreement. In addition, similar amplitude distributions, approximating a Gaussian distribution, are achieved. Speckle noise is different from Gaussian white noise because of the varying frequency distribution. The speckle noise grows with increasing surface roughness to a critical value. When simulating and acquiring the scanning speckle noise, the noise energy increases with the scanning speed, but the signal drop-outs decrease in intensity and density. These promising results demonstrate the simulation accuracy and can further support despeckle procedures. [ABSTRACT FROM AUTHOR]

Abstract: Longitudinal cross-sectioning of squats reveals characteristic features of internal crack front propagation. Leading crack planes propagate over longer lengths and greater depths as compared to more superficial trailing crack planes. A favourite depth of crack propagation occurs in the subsurface (2–3mm), is related to the residual longitudinal stress profile, and may lead to an internal crack ‘terrace’. Especially during deeper crack propagation and branching oxidation processes are found to be metallurgical drivers of crack growth. Contact surface modification during squat growth can be distinguished between phases of transient local stress redistribution and of dynamic wheel–rail contact. If the hypothesized shearing wedge in the failure mechanism loses its load bearing capacity, this gives rise to a redistribution of normal stresses within the actual contact ellipse and the formation of a hardness envelope along the crack pattern. This may partially explain why maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks. A second effect occurs for contact patches not matching the failure ‘envelope’: due to the Poisson effect the surface overlying the crack planes settles slightly, experiences reduced contact, and corrosive products, ‘pumped’ from inside the cracks, may accumulate on the surface (as confirmed by SEM-EDX analysis). During progressive growth of the defect the harder and decoloured envelope as well as the original wedge is pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. This may cause high-frequency impact, resulting into progressive internal crack growth affecting the global stress response and rail fracture. [Copyright &y& Elsevier]

Abstract: A phenomenological investigation of squat defects on rail grade R260Mn is performed. The surface-breaking crack pattern, which is either linear or branched (V-shaped), shows a typical position and orientation in the running band. Particular characteristics are asymmetry of this pattern, with the presence of a leading and a trailing branch, and crack reflection or deviation at the running band border. Bending tests reveal a 3D internal crack pattern, with a pair of crack planes or ‘wings’ enclosing a wedge at the surface. Microstructural analysis of the rail upper layer shows metallurgical principles of crack initiation: delamination and transverse fracture of white etching material at the surface. This analysis moreover reveals a 3D anisotropic texture of the upper layer under combined bi-directional tangential surface stresses. Mechanical interpretation of the crack morphology shows that the leading or single branch of the surface-breaking crack pattern is a shear-induced fatigue crack, following the anisotropic microstructure when growing into the rail. The trailing crack of a branched squat is explained as the result of a subsequent transverse, wedge-shaped brittle failure mechanism of the surface layer of the rail, developing within the actual elliptical Hertzian contact patch – or the envelope of potential contact ellipses at the leading crack position. It is driven by the transverse shear loading towards the rail gauge face. [Copyright &y& Elsevier]

Abstract: This paper presents evidences obtained by field monitoring, measurement and survey to show the validity of some numerical predictions about squat growth. The predictions concerns a postulated squat growth process, the relationship between the dynamic contact force and the corrugation-like wave pattern that often follows squats, the high frequency wheel–rail interaction related to squats, and the influence of tangential force on squat growth. The observations reveal signature tunes of squats which may be used for early detection of squats, show the necessity to include high frequency dynamic wheel–rail interaction in squat-related analyses, and provide evidence of relationship between rolling stock performance and squat initiation and growth. In validating the numerical results the model is also verified for its applicability to analyses of squat-related problem and other problems similar in nature. The model can be employed for the solution of three-dimensional frictional rolling contact problems. It can also be used for analyses of loading conditions of wheel–rail contact at short wave defects, and the associated damages such as wear, plastic deformation, fatigue and corrugation. [Copyright &y& Elsevier]

• Gaussian process regression is used to directly infer railpad and ballast stiffness from FRF. • A careful selection of FRF features and a prediction fusion strategy ensure robustness. • The method outperforms a model updating method using particle swarm optimization. • The method enables fast predictions for large data sets. • The method only requires to measure a single FRF with a simple test setup. 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. [ABSTRACT FROM AUTHOR]

• The digital twin combines physics-based and data-driven models. • Track stiffness is correlated with local wavelet power spectrum of ABA. • Railpad and ballast stiffness are evaluated simultaneously at sleeper spacing resolution. • Track stiffness is evaluated in near real time. • Evaluated track stiffness is robust against varying track irregularities and vehicle speeds. While various train-borne techniques have been developed for measuring railway track stiffness, differentiating stiffness at different track layers remains a challenge. This study proposes a digital twin framework for the vehicle–track interaction system, which enables track stiffness evaluations based on axle box accelerations (ABA). The digital twin consists of a physics-based model, a model library and data-driven models. Compared to existing techniques, the proposed method simultaneously evaluates the stiffness of the railpad, sleeper and ballast layers at a sleeper spacing resolution, while being robust to varying track conditions, such as track irregularities and vehicle speeds. This is accomplished by employing a localized frequency-domain ABA feature capable of distinguishing between the characteristics of different track layers. Furthermore, track stiffness is evaluated in near real-time. This is achieved using a model library derived from physics-based simulations of a range of track conditions. Two data-driven models that can quickly select or interpolate model instances contained in the library are developed. During operation, the data-driven models use the measured ABA features as input and then infer the stiffness for the different track layers. The proposed method is applied to evaluate the track stiffness of a downscale test rig in a case study. The track stiffness evaluated by the proposed method is compared with that obtained through hammer tests and with the observations of the track component conditions. These comparisons show that the proposed method can capture the stiffness variations due to periodically fastened clamps and substructure misalignments at different speeds. In addition, the proposed method is demonstrated to be superior to the commonly used hammer test method for evaluating track stiffness under loaded conditions. [ABSTRACT FROM AUTHOR]

Operational modal analysis (OMA) enables the identification of modal characteristics under operational loads and conditions. Traditional frequency-domain methods cannot directly capture modal changes over time, while existing time-frequency representations are not sufficiently interpretable due to spurious modes and implicit parameter design. This paper develops a new OMA method in time-frequency representation based on frequency-domain decomposition (FDD). Short-time FDD and a convolution-based strategy are proposed to obtain singular values and local mode shape similarity, respectively, which are further fused into mode indicators by a fuzzy-based strategy mimicking the modal assurance criterion. The method provides not only a global view of the modal characteristics over time and frequency but also estimates of the modal parameters. It is applicable to strongly nonstationary responses under time-varying loads and conditions. All the parameters explicitly affect the time-frequency representation, and the interpretability is enhanced by including physical information from the user's prior knowledge in selecting parameters and peak bands. The proposed method is validated based on a study of railway sleepers under train passage. The rigid-body motions and bending modes are identified at frequencies up to 6,500 Hz in laboratory tests and 4,500 Hz in field tests at speeds up to 200 km/h. The identified natural frequencies and mode shapes agree with experimental modal analysis (EMA). The proposed method outperforms EMA in terms of broad frequency range and low measurement cost and can be potentially applied to structural health monitoring under operational conditions. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the transient contact solutions of wheel-rail frictional rolling impacts calculated by an explicit finite element model of the wheel-insulated rail joint (IRJ) dynamic interaction. The ability of the model to simulate the dynamic behavior of an IRJ has been validated against a comprehensive field measurement in a recent paper (Yang et al., 2018). In addition to the measured railhead geometry and bi-linear elastoplastic material model used in Yang et al. (2018), this study adopts a nominal railhead geometry and an elastic material model for the simulations to provide an overall understanding of the transient contact behavior of wheel-IRJ impacts. Each simulation calculates the evolution of the contact patch area, stress magnitude and direction, micro-slip distribution, and railhead nodal vibration velocity in the vicinity of the joint during the wheel-IRJ impacts. The simulations apply small computational and output time steps to capture the high-frequency dynamic effects at the wheel-IRJ impact contact. Regular wave patterns that indicate wave generation, propagation and reflection are produced by the simulations; this has rarely been reported in previous research. The simulated waves reflect continuum vibrations excited by wheel-rail frictional rolling and indicate that the simulated impact contact solutions are reliable. [ABSTRACT FROM AUTHOR]

This paper presents a sensitivity analysis of railpad parameters on vertical railway track dynamics, incorporating the nonlinear behavior of the fastening (i.e., downward forces compress the railpad whereas upward forces are resisted by the clamps). For this purpose, solid railpads, rail-railpad contact and preloaded springs are defined in a three-dimensional (3D) finite-element track model. In addition, railpads are modeled as Prony series so that measured frequency-dependent railpad properties for different clamp toe load, temperature, aging, and railpad type are considered. Through sensitivity analysis using time-domain and frequency-domain responses, the influence of the railpad parameters on the global track response is investigated. Railpad type, toe load and aging are identified as the most relevant parameters in the frequency range of 300-3,000 Hz. Furthermore, the information obtained over track response changes due to fastening loosening and railpad wear could be used for monitoring the condition of fastenings. [ABSTRACT FROM AUTHOR]

This paper explores the use of pantograph–catenary contact force (PCCF) for monitoring of the current collection quality and detection of anomalies in the interaction between pantograph and catenary. The concept of catenary structure wavelength (CSW) is proposed as the dominant component of PCCF. It describes the signal components caused by the cyclical catenary structure in span and interdropper distance. To obtain the CSWs and non-CSW residual of PCCF, an automatic extraction approach based on the ensemble empirical mode decomposition (EEMD) is proposed. In the approach, the instantaneous frequency of each intrinsic mode function generated by the EEMD is employed for the extraction of CSWs. Some selected trials on the PCCF data from simulation and measurement are performed and indicate that the extraction approach is adaptive to the PCCF under various circumstances, including different operation speeds, pantograph type, and catenary structure. Analyses on the extracted CSWs and non-CSW residual show that, with certain tolerance against measurement noise, the approach can preserve intact the characterizations of current collection quality and make anomalies easier to detect. [ABSTRACT FROM PUBLISHER]

By considering real wheel and track geometry and structures, as well as their coupled interaction, an explicit finite element method is applied to simulate the transition of wheel–rail rolling from single-point tread contact to two-point tread-flange contact. The evolutions of the contact position, stress magnitude and direction, adhesion–slip distribution, and wheel–rail relative velocities in the two contact patches are considered to investigate the transient dynamic effects during the contact transition. Important findings include that the transition to two-point contact can result in friction saturation and excite waves in the contact, causing local intensification and relaxation of compression, as well as turbulence of the micro-slip; the local relative velocity in the contact patch is a good measure of the dynamic effects. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Polygonal wear is a type of damage commonly observed on the railway wheel tread. It induces wheel-rail impacts and consequent train/track components failure. This study presents a finite element (FE) thermomechanical wheel-rail contact model, which is able to cope with the three possible generation and development mechanisms of polygonal wear: initial defects, thermal effect, and structural dynamics. The polygonal wear-induced impact contact and further development of wear are simulated. The simulated elastic contact solutions are verified against the program CONTACT. Different material properties (elastic, elasto-plastic and elasto-plastic-thermo, i.e. with thermal softening) and initial polygonal profiles are then applied to the FE model to investigate the influence of wheel/rail material and wear amplitude on wheel-rail contact stress and wear development. The simulations indicate that the wheel-rail impact-induced temperature may reach up to 362 ℃ at the contact interface, and the high temperature at the contact area influences wheel-rail contact stress and wear depth. • An innovative thermomechanical FE model is presented to study polygonal wear growth. • The simulated contact solutions are verified against the program CONTACT. • Using elastic material models causes overestimation of contact stress and wear depth. • Incorporating thermal effects is crucial for wear predication with high slip ratio. • The simulated contact temperature reaches up to 362 ℃ at the wheel-rail interface. [ABSTRACT FROM AUTHOR]

This paper proposes a novel framework for simulating the dynamics of beams on elastic foundations. Specifically, partial differential equations modeling Euler–Bernoulli and Timoshenko beams on the Winkler foundation are simulated using a causal physics-informed neural network (PINN) coupled with transfer learning. Conventional PINNs encounter challenges in handling large space–time domains, even for problems with closed-form analytical solutions. A causality-respecting PINN loss function is employed to overcome this limitation, effectively capturing the underlying physics. However, it is observed that the causality-respecting PINN lacks generalizability. We propose using solutions to similar problems instead of training from scratch by employing transfer learning while adhering to causality to accelerate convergence and ensure accurate results across diverse scenarios. The primary contribution of this paper lies in introducing a causality-respecting PINN loss function in the context of structural engineering and coupling it with transfer learning to enhance the generalizability of PINNs in simulating the dynamics of beams on elastic foundations. Numerical experiments on the Euler–Bernoulli beam highlight the efficacy of the proposed approach for various initial conditions, including those with noise in the initial data. Furthermore, the potential of the proposed method is demonstrated for the Timoshenko beam in an extended spatial and temporal domain. Several comparisons suggest that the proposed method accurately captures the inherent dynamics, outperforming the state-of-the-art physics-informed methods under standard L 2 -norm metric and accelerating convergence. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Short track defects cause large dynamic contact forces at the wheel–rail interface, leading to fast deterioration of the track. Early detection of such defects is very important for timely maintenance. The track technical state can be assessed with the aid of axle box acceleration measurements, which can indicate short track defects like squats, welds with poor finishing quality, insulated joints, corrugation, etc. As a first attempt to determine a quantitative relationship between the characteristics of the accelerations and the track defects, axle box acceleration at a squat and a thermite weld were simulated through finite-element modeling. The magnitude and frequency contents of axle box acceleration at squats agreed with measurements. The results for welds had some discrepancies, which were mainly attributed to the fact that the modeled geometry of the weld deviated from the real one. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

• A new 3D visualization method is presented. • The necessary geometric definitions and conventions are provided. • A combined CT - metallography method is used to reconstruct the crack networks. • The angles and orientations of squat cracks are visually demonstrated and measured. This study investigates the angles θ 1 , θ 2 , and θ 3 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° ≤θ 1 ≤ 150°, 6° ≤θ 2 ≤ 36° and 67° ≤θ 3 ≤ 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. [ABSTRACT FROM AUTHOR]

• Weld-induced rail squats are studied through a five-year continual field monitoring and numerical simulations. • Different stages in the development of squats were observed in the field monitoring. • The development process is presented, from inhomogeneous material in HAZs to two-lung-shaped squats associated with cracks. • The formation mechanism in terms of dynamic contact force and differential plastic deformation is proposed. • Numerical simulations are performed to verify the pre-cracking process. Weld-induced squats are a major damage type in high-speed railways as well as in conventional railways. They incur high maintenance costs and endanger operational safety. This paper first presents and analyzes five-year continual field monitoring observations and measurements of squats at rail welds. A hypothesis of the formation and development process of the squats is proposed, which includes three steps. Steps 1 and 2 are pre-cracking, and Step 3 is post-cracking. To verify the pre-cracking process, a three-dimensional (3D) finite element (FE) model is then built up to simulate the vehicle-track interaction with detailed consideration of the local wheel-rail frictional rolling contact. Not only dynamic contact forces but also plastic deformation and wear are calculated. Starting from a smooth rail surface with varying yield stress derived from field-measured hardness, the numerical analysis confirms the hypothesis that the varying hardness at heat-affected zones (HAZs) leads to initial V-shaped irregularities due to differential plastic deformation. Afterward, the surface irregularities excite the dynamic longitudinal contact force, which in turn produces a W-shaped surface pattern through further differential plastic deformation. The growth of the W-shaped pattern leads to the formation of squats. This work provides insight into the squat formation process at rail welds and suggests that welding quality control in terms of hardness variation in the HAZs could reduce or even avoid squats. Early detection of squats with dynamics-based methods is possible. [ABSTRACT FROM AUTHOR]

The condition-based maintenance of high-speed railway catenary is an important task to ensure the continuous availability of train power supply. To improve the condition monitoring of catenary, this paper presents a novel scheme to detect catenary local irregularities using pantograph head acceleration measurements. First, a series of experimental inspections is carried out in a section of the Beijing–Guangzhou high-speed line in China. The time intervals between the inspections are shortened from the traditional six months to about 40 days, which enables monitoring the short-term degradation of local irregularities. Then, based on the wavelet packet entropy, an approach is proposed to detect local irregularities with different scales in length. Criteria for identifying and verifying the local irregularities are established based on the gradient and repeatability of entropy from multiple measurements. Results from the experimental inspections show that different scales of local irregularities can be detected by the proposed scheme. By using frequent inspections, local irregularities can be effectively verified after about seven inspections. The spatial distribution of local irregularities is found to be closely related to the catenary structure. These findings provide valuable information to deploy the scheme for a railway network. [ABSTRACT FROM AUTHOR]

Highlights • A decision support approach is proposed for condition-based maintenance of rails. • The methodology uses big data analysis and expert systems. • ABA measurements, video images and prior knowledge of the track are employed. • Integrated estimation of the rail health condition supports maintenance decisions. • Measurements from the track Amersfoort-Weert in the Dutch railway network are used. Abstract In this paper, a decision support approach is proposed for condition-based maintenance of rails relying on expert-based systems. The methodology takes into account both the actual conditions of the rails (using axle box acceleration measurements and rail video images) and the prior knowledge of the railway track. The approach provides an integrated estimation of the rail health conditions to support the maintenance decisions for a given time period. An expert-based system is defined to analyse interdependency between the prior knowledge of the track (defined by influential factors) and the surface defect measurements over the rail. When the rail health conditions is computed, the different track segments are prioritized, in order to facilitate grinding planning of those segments of rail that are prone to critical conditions. In this paper, real-life rail conditions measurements from the track Amersfoort-Weert in the Dutch railway network are used to show the benefits of the proposed methodology. The results support infrastructure managers to analyse the problems in their rail infrastructure and to efficiently perform a condition-based maintenance decision making. [ABSTRACT FROM AUTHOR]

This paper describes an approach for characterizing the dynamic behavior of the vehicle/track interaction at railway crossings. In the approach, we integrate in situ axle box acceleration (ABA) measurements with roving-accelerometer hammer tests to evaluate the influence of train speed, train moving direction (facing and trailing directions), sensor position (leading and rear wheels of a bogie), and the natural response of track structure on ABA signals. The analysis of data from multiple sensors contributes to the following findings: the major frequency bands of the vertical ABA are related to the natural frequencies of the crossing; thus, these ABA frequency bands are not greatly affected by variations in train speed, moving direction, and sensor position. The vibration energy concentrated at the major ABA frequency bands increases at higher train speeds, along the facing moving direction and from the leading wheel. The crossing rails vibrate as a combination of bending and torsion rather than solely bending at the major ABA frequency bands, since the vibrations of the wing rails are not synchronized. These results help enhance our understanding of the vehicle/track interaction at crossings and can be used to improve the dynamic response-based system for monitoring the condition of crossings. [ABSTRACT FROM AUTHOR]

This paper studies the evolvement of the wear irregularity of contact wire using wire thickness data measured yearly from a section of railway catenary. The power spectral density and time-frequency representation based on the wavelet transform are employed for data analysis, with an emphasis on local wear irregularities that are crucial for contact wire condition assessment. To investigate the cause of wear irregularity evolvement and the mutual influence with the pantograph-catenary dynamic interaction, simulations considering the influence of wear irregularity are carried out based on the finite element method. Analyses of the actual wear irregularities and the dynamic contact force under singular and complex wear irregularities are performed. Although the wear irregularity has limited impact on the pantograph-catenary interaction, it can induce the vibration of pantograph and contact wire that will lead to a notable increase of contact force standard deviation. The evolvement of wear irregularity is closely associated with the span length and dropper distribution of catenary structure and the running direction of pantograph. In addition, it is found feasible to detect the wear irregularity based on contact force, on condition that the sampling frequency is high enough to reflect the indicative frequencies. [ABSTRACT FROM AUTHOR]

In this paper, we present a method for evaluating the performance of railway crossing rails after long-term service. The method includes 1) 3D profile and hardness measurements; 2) finite element simulation of wheel/rail interaction; and 3) numerical prediction of rail degradation. We conducted a case study on a crossing that had been in service for several years. The results indicate that the crossing experienced a run-in process in the major traffic direction, manifested as a widening of the running band, an enlargement of the contact patch size, a decrease in contact stress and eventually a reduction in plastic deformation and wear. However, the wheel/rail interaction was exacerbated in the minor traffic direction which induced more severe plastic deformation and wear. [ABSTRACT FROM AUTHOR]

Rail squats originate from a number of sources, such as corrugations, indentations and welds. A five-year continual field monitoring study was performed on squats induced by corrugations. This study indicated that a small black depression formed at the corrugation under wheel-rail dynamic forces, and then, a primary crack typically initiated on the gauge side edge of the depression. Subsequently, the crack began to propagate in the rail surface in a U shape toward the gauge side in both the traffic direction and the opposite-traffic direction and into the rail toward the field side at an angle of approximately 20°. Rail inclination could influence the crack initiation location and propagation path. The geometry of the black squat depression was initially elliptical, and then, its edge followed the U-shaped cracking path as it grew. The squats turned into a kidney-like shape, typically with a U-shaped crack. Tensile stress likely led to the squat crack initiation and propagation. This cracking phenomenon and mechanism are analogous to the ring/cone crack formation of brittle materials under sphere-sliding contact. As the squats grew further, a ridge formed in the middle part of the depression, and an I-shaped crack appeared at this ridge due to the impact of the wheels. This process eventually led to two-lung-shaped mature squats, typically with a Y-shaped crack. The findings of this paper provide insight into the formation of rail squats. [ABSTRACT FROM AUTHOR]

As essential track components for rail connections and signal transmission and control, insulated rail joints (IRJs) have been widely used in traditional and high-speed railways. However, the IRJ is considered as one of the weakest parts of railway track structures due to the significant discontinuities in stiffness and geometry. When a train runs over an IRJ, a wheel-rail impact occurs and it increases with train speed. The impact consequently leads to vibration and noise and accelerates track deterioration in the vicinity of the IRJ. This paper establishes an explicit finite element wheel-IRJ dynamic interaction model to simulate high-frequency impact vibration and noise generated at a typical IRJ in the Dutch railway network, and validates the model against a comprehensive hammer test and a pass-by measurement. Good agreements between the simulation and measurements indicate that the proposed model can effectively reproduce high-frequency impact vibration and noise up to 10 kHz. This paper also connects the dominant frequencies of wheel-IRJ impact vibration and noise with the dynamic behaviour of the target IRJ, which may contribute to the mitigation of impact vibration and noise at IRJs as well as to train-borne detection of deterioration types of IRJs. [ABSTRACT FROM AUTHOR]

A new downscale test rig is developed for investigating the contact between the wheel and rail under impact-like loading conditions. This paper presents the development process of the setup, including review and synthesis of the potential experimental techniques, followed by scalability, mechanical and operational analysis of the new setup. The new test rig intends to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by taking into account the factors that contribute to high-frequency dynamics of the wheel-track system. The paper first reviews the functionalities of the existing test techniques in the literature. Based on this survey, the category of the scaled wheel on the rail track ring is chosen. Afterwards, three potential alternatives are identified under the chosen category and the optimum mechanism is achieved through finite element modelling and analysis of the structures. A downscale test rig, consisting of multiple wheel components running over a horizontal rail track ring, effectively fulfilled the requirements needed for analogical testing of the wheel-rail contact behaviour. The new test rig is a unique experimental setup due to the involvement of high-frequency dynamic vibrations in the wheel-track system and analogy of the incorporated elements and loading to those of the real-life system. This paper further presents the results of some real experiments carried out using the newly-built setup to support substantial ideas behind its development. [ABSTRACT FROM AUTHOR]

This research presents a coupled thermomechanical modelling procedure for the wheel-rail contact problem and computes the flash-temperature and stress-strain responses when thermal effects are present. A three-dimensional elasto-plastic finite element model was built considering the wheel-track interaction. When the wheel is running on rail, frictional energy is generated and converted into heat. To evaluate the contribution of thermal effects and plasticity, five different material models were studied among them TEPS was nonlinear and temperature-dependent including thermal softening. Discussions were made on the effect of solution type and material type. The rail temperature, calculated for a critical creepage case, confirmed the potential of martensitic phase transformation. Thermal effects were also important at lower creepages, where a synchronization effect causes earlier damage. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue (RCF) defects are associated with complex crack networks at the subsurface. A computed tomographic (CT) scanning technique has been developed to reconstruct the 3D geometry of the RCF cracks in the railhead. Sample rails having squats of different severities were taken from the Dutch railway network. Four specimens of different sizes were prepared and investigated with the CT scanner. The detailed procedures of the CT experiment and post-processing work were described. A sequence of high-quality X-ray images was collected during each scan. These 2D images were combined to construct the 3D visualization of the specimen. Various image processing tools were applied to extract and rebuild the internal crack geometries, thus allowing the crack networks to be differentiated from the bulk steel. For validation, the CT results were compared with metallographic observations of the rail surface for all the defects and the vertical section when needed. Discussions were made regarding the proper size of the rail samples and severity of the squats. According to the results, CT allows for a 3D visualization of RCF defects, providing high-quality data on the geometry of the internal cracks. By choosing the appropriate settings and specimen size, CT could accurately reconstruct the squat cracks at different growth stages. This research shows the potential of the CT technique as an intermediate detection and characterization tool among the methods for detecting macro cracks and those for characterizing micro/nano cracks. Finally, a practical specimen design and a detailed scanning procedure are proposed, based on the CT experiments performed in this research. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the capability of an axle box acceleration (ABA) system to evaluate the degradation at railway crossings. For this purpose, information from multiple sensors, namely, ABA signals, 3D rail profiles, Global Positioning System (GPS) and tachometer recordings, was collected from both nominal and degraded crossings. By proper correlation of the gathered data, an algorithm was proposed to distinguish the characteristic ABA related to the degradation and then to evaluate the health condition of crossings. The algorithm was then demonstrated on a crossing with an unknown degradation status, and its capability was verified via a 3D profile measurement. The results indicate that the ABA system is effective at monitoring two types of degradations. The first type is uneven deformation between the wing rail and crossing nose, corresponding to characteristic ABA frequencies of 230-350 and 460-650 Hz. The second type is local irregularity in the longitudinal slope of the crossing nose, corresponding to characteristic ABA frequencies of 460-650 Hz. The types and severity of the degradation can be evaluated by the spatial distribution and energy concentration of the characteristic frequencies of the ABA signals. [ABSTRACT FROM AUTHOR]

The half-space assumption has been employed in many solution methods for non-conforming contact problems in elasticity such as the Hertz theory and the Kalker's variational theory. It is generally believed that to guarantee acceptable accuracy in these half-space-based methods, the characteristic size (twice length of one semi-axis) of the contact patch should be much smaller than the significant dimensions (i.e. the height, width, length and the principal radii of curvature) of each body in contact. In engineering practice, the 3x rule is often employed, which requires that the significant dimensions be at least three times as large as the characteristic size. However, this requirement has not been justified. In this paper, the applicability of half-space-based methods is investigated by comparing the solutions obtained using the Hertz theory and the Kalker's theory with those of the Finite Element (FE) method which is not limited to the half-space assumption. Different combinations of significant dimensions in terms of height, width and length are studied. Various contact patch eccentricities and contact body shapes are considered. It is found that the half-space-based methods yield high-accuracy calculation for non-conforming contact problems. Even when the significant dimensions are as small as 1.1x the characteristic size, the differences between the solutions of the half-space-based methods and the FE method are within 9%. The findings of this paper indicate that the typically assumed 3x restriction can be greatly relaxed. Since a clear estimation of the deviation of the results of half-space-based methods from those of the FE method is provided, the applicability of half-space-based methods in mechanical engineering can be much better understood. [ABSTRACT FROM AUTHOR]

White etching layer (WEL) is a frequently observed microstructural phenomenon in rail surface, formed during dynamic wheel/rail contact. It is considered as one of the main initiators for rolling contact fatigue cracks. There are several hypotheses for the formation mechanism of WEL. However, due to the complicated wheel/rail contact conditions, none is directly proven. Currently, the most popular hypotheses refer to either formation of martensitic WEL by phase transformations or formation of nanocrystalline ferritic WEL by severe plastic deformation. In this work, WEL formation by martensitic transformation in R260Mn grade pearlitic rail steel was simulated by fast heating and quenching experiments. Microstructural characteristics of the simulated WEL and WEL observed in a field rail specimen were characterized by microhardness, optical microscopy, scanning electron microscopy and electron backscatter diffraction. Microstructures of the two WELs were compared and similarities in morphology were identified. Numerical simulation shows the possible temperature rise up to austenitizing temperatures. Combining comparisons of experimental simulation with observation of WEL in the rail and the thermodynamic calculations, the hypothesis for WEL formation via martensitic transformation is supported. [ABSTRACT FROM AUTHOR]

This paper presents a health condition monitoring system for insulated rail joints (IRJs) based on axle box acceleration (ABA) measurements. The ABA signals from all the wheels of the measuring train are processed to extract those characteristics that better represent the quality of the IRJ. Then, different indicators are used for damage assessment, the most relevant being a set of frequency bands in the ABA power spectrum. A detection algorithm is proposed based on the derived frequency characteristics of the ABA signal. We compared the responses of IRJs in good condition with those in poor condition. Track inspections were performed to validate the health condition monitoring methodology in different IRJs of the Dutch railway network. The hit rate of IRJs detection was 84% for two validated tracks. The damage assessment procedure allowed to prioritize the IRJs that require maintenance. This information is useful for the railway inframanagers as it allows to predict where safety compromises will be faced. [ABSTRACT FROM AUTHOR]

A field sample of rail steel was metallurgically examined to characterize its rolling contact fatigue (RCF) damage. In addition to the well-known white etching layer (WEL), a possible different type of surface modification layer was identified in parallel. The layer has some similar features as the WEL but exhibits a significantly different etching response to 3 vol% Nital etchant. After etching, the new layer exhibits a brown color under the same light reflection. This layer was named as “brown etching layer” (BEL) to distinguish it from the WEL. Similar to the WEL, cracks are observed to be closely related to the BEL. The cracks are found to penetrate deeper than those initiated by the WEL reported in existing publications. Further, they are found to propagate downwards without branching, which may eventually cause rail fracture. Although its formation mechanism is not yet clear, WEL has been considered by some authors in the literature as a possible RCF initiation source. It is therefore of critical importance to understand the characteristics of the BEL and its formation mechanism. This may also lead to better understanding of the formation mechanism of the WEL. To this end, microstructural features of the BEL were studied using micro-hardness tests, optical microscopy and scanning electron microscopy. The BEL was found to be distinctly softer than the WEL and lamella-type features are found within the BEL. The microstructural features of the BEL were compared with the WEL reported in the literature. Finally, the formation mechanism of the fatigue damage was discussed based on the comparison, observations and material characterization. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of four models of rolling contact used for online contact force evaluation in rail vehicle dynamics. Until now only a few wheel–rail contact models have been used for online simulation in multibody software (MBS). Many more models exist and their behaviour has been studied offline, but a comparative study of the mutual influence between the calculation of the creep forces and the simulated vehicle dynamics seems to be missing. Such a comparison would help researchers with the assessment of accuracy and calculation time. The contact methods investigated in this paper are FASTSIM, Linder, Kik–Piotrowski and Stripes. They are compared through a coupling between an MBS for the vehicle simulation and Matlab for the contact models. This way the influence of the creep force calculation on the vehicle simulation is investigated. More specifically this study focuses on the influence of the contact model on the simulation of the hunting motion and on the curving behaviour. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Squats are associated with high frequency vibrations of the wheel-rail system and can be registered using axle box acceleration (ABA) measurements. In this paper a three-dimensional finite-element (FE) model is proposed to capture the dynamic features of the ABA related to squats in the high frequency range. The FE model is validated relying on a set of real-life ABA measurements at two characteristic rail surface defects (light and severe squats), located in a section of the railway network in Assen. The Netherlands. The FE model managed to properly capture the dynamic features in the range of frequencies up to 2000 Hz. [ABSTRACT FROM AUTHOR]

• We compared the failure of railway thermite welds through real field-testing. • We simulated the dynamic impact force induced by the weld geometry. • We reported the progression of the surface hardness of welds in track condition. • We discussed the noticed failures; wear and cracks. This paper details the field-testing approach and results of thermite welds used in railway applications. Rail steels made from R260 grades are welded by two different processes: the standard process according to the European standards, and a recent technology. Welds are introduced in the same testing location to ensure comparable loading conditions in the aim of studying their degradation behaviours. The total testing-period is 6 months. During the in-service period, the surface hardness of the running band, in the welded area, is recurrently measured. For an accurate assessment of field results, a vehicle/track interaction (VTI) model evaluated the expected dynamic loads induced by the initial vertical irregularities. The simulations show that the highest dynamic load at the wheel/rail contact happens at the location of the maximum absolute gradient, in accordance with previous research. Particularly, dipped welds show relatively high dynamic forces inducing a high loss of the transversal profile. In respect of the field results, the comparison of initial and final surface hardness indicates a significant increase for 'ALFONS' welds over the welded areas. Additionally, all welds depicted a cyclic increase and decrease of the running band hardness. This result is discussed according to the ratcheting susceptibility of welds and eventual wear progression. For a testing-period of 10 weeks, the gauge corner of one 'ALFONS' weld developed a crack. The assessment of the longitudinal profiles revealed changes of the vertical irregularities that may modify the dynamic loads, and further the rolling contact fatigue mechanisms and degradation rates. [ABSTRACT FROM AUTHOR]

Abstract: Nowadays squats are a severe rolling contact problem on some railways. The root causes of it are not yet clear. From railway operation and maintenance points of view, root causes have to be known so that effective maintenance policies and plans can be made based on measurements and prediction. In the present work first a correlation analysis is performed based on measured data and field observations, and then a relation between squat occurrence and some parameters of the vehicle–track system is identified. Subsequently a transient finite element model of vehicle–track interaction is built up to analyze the contact forces and the resulting stresses and strains. The numerical results show good correspondence with observed phenomena at squats, and suggest certain relation between squat development and some eigen frequencies of the coupled vehicle–track system. A squat growth process is accordingly postulated. [Copyright &y& Elsevier]

• A time-frequency analysis reveals that the impact force consists of a peak force followed by oscillations with three dominant frequencies. • The beam model with a Hertzian contact spring overestimates the peak force. • The beam model is comparable to the continuum model up to about 800 Hz. • Comparisons with field observations suggest the continuum model is more accurate. • Effects of the stress wave propagation on the discrepancies are discussed. A singular rail or wheel surface irregularity, such as a squat, insulation joint or wheel flat, can cause large wheel-rail impact force. Both the magnitude and frequency content of the impact force need to be correctly modelled because they are closely related to the formation, deterioration and detection of such irregularities. In this paper, we compare two types of commonly used wheel-track interaction models for wheel-rail impact problems, i.e., a beam and a continuum finite element model. We first reveal the differences between the impact forces predicted by the two models due to a typical rail squat using a time-frequency analysis. Subsequently, we identify the causes for the differences by evaluating the effects of different model assumptions, as well as different model parameters. Results show that the impact force consists of a forced vibration peak M1 followed by free vibration related oscillations with three dominant frequencies: f 1 (340 Hz), f 2 (890 Hz) and f 3 (1120 Hz). Compared with the continuum model, the beam model with a Hertzian contact spring overestimates the M1 peak force. The discrepancy can be reduced by using a Winkler bedding contact model. For the track model, the beam model is comparable to the continuum model up to about 800 Hz, beyond which the track damping starts to deviate. As a result, above 500 Hz, the contact forces dominate at f 2 for the beam while at f 3 for the continuum model. Finally, we show that the continuum model is more accurate than the beam model by comparing to field observations. The effects of stress wave propagation on the differences are also discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

This study evaluates the degradation of wheels and rails at railway crossings. The evaluation method is composed of 1) finite element simulation of dynamic wheel/crossing interaction and 2) multi-criteria analysis of wheel/rail degradation in terms of yield behavior, rolling contact fatigue (RCF) and wear. With the aid of this method, we conducted a case study identifying the proper yield strength of rail steel material for a 54E1-1:9 crossing under a specified traffic condition. The case study indicates that the wear of contact bodies is more sensitive to train speed compared with yield and RCF; the increase of rail yield strength suppresses rail degradation while exacerbating wheel degradation; and rail yield strength in the range of 500–600 MPa is preferred to achieve a good trade-off between the wheel and rail degradations. • A method is proposed to evaluate the degradation of wheels and rails at railway crossings. • The method uses finite element simulation and multi-criteria analysis. • Wheel/rail degradation is evaluated in terms of yield behavior, rolling contact fatigue (RCF) and wear. • A case study is conducted to identify the proper yield strength of rail steel material for a 54E1-1:9 crossing. [ABSTRACT FROM AUTHOR]