This paper presents an innovative method for determining the fatigue life of railway bogie frames with track irregularities and wheel polygonal wear. This method was employed to predict wheel polygon limits to protect against bogie frame fatigue. Firstly, a comprehensive coupled vehicle-track system dynamic model was established. The developed model was validated by comparing the numerical results with field test data. Secondly, the dynamic stress characteristic of the bogie frame with different excitation forms were investigated. A methodology was proposed to compile the stress spectra of bogie frame by considering the influence of the wheel polygonal wear. This approach enabled the construction of comprehensive stress spectra by considering influences arising from both track irregularities and wheel polygonal wear. Subsequently, the stress spectra of the concerned positions on the bogie frame were developed and further used to study the influence of wheel polygonal wear amplitude on the fatigue life of the bogie frame. This facilitated the development of the maintenance limit of wheel polygonal wear from fatigue life perspective. The results suggest that the maintenance limit of wheel polygonal wear should not exceed 0.034 mm in the presence of a 24th-order wheel polygon for a typical high-speed train.

The S38C steel axles show excellent fatigue resistance due to large-depth compressive residual stress (CRS) along the axial direction. However, residual stress (RS) relaxation may occur in case of a fatigue crack, which results in a reduced damage tolerance and a shortened service lifetime. Destructive methods cannot retain the residual stress and associated retardation effect of cracks. To tackle this concern, time-of-flight (TOF) methods of Braggedge transmission (BET) imaging and neutron diffraction (ND) were introduced to quasi-in situ investigate the residual stress relaxation during fatigue crack propagation by single-edge notch bending samples. The BET imaging results indicate that lattice parameters change as the crack develops, which then leads to a decrease of residual strain epsilon(110). It was clearly found that the maximum epsilon(110) was released by similar to 31.4 % if a crack propagated to 10.0 mm. In addition, it was observed from ND results that all three RS components decrease with the crack growth. By contrast, the CRS in the axle surface was almost fully released when the crack propagated to the matrix material zone (from -566 to 41 MPa). Furthermore, RS relaxation was validated by the extended finite element method, and the effect of stress relaxation on stress field at the crack tip was also evaluated meticulously. These new insights confirm stress relaxation in railway S38C axles during crack propagation, which must be considered while carrying out the structural integrity assessment.

Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about -520 MPa (axial), -710 MPa (hoop), and -40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0-3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles.

Non-destructive and quantitative mapping of gradient residual strain distribution in surface-hardened railway S38C axles could provide a positive reference for determining service lifetime and maintenance strategy. To tackle this concern, time-of-flight neutron Bragg-edge transmission imaging was employed by real axle samples with and without impacted crater. A novel and simple procedure to formulate the residual strain field was also developed in this work, with the transmission batch code in Appendix A. By mapping the global two-dimensional residual strains, it can be verified that the residual strains into the axle are uniformly distributed in the hoop direction. Subsequently, it was revealed that the axial and hoop residual strains, respectively in the cylinder and the long strip samples prepared from a real S38C hollow axle, indicated a gradient evolution distribution with a depth of similar to 8 mm, covering a range of -5500 similar to 1000 mu epsilon for axial strains and -6500 similar to 1000 mu epsilon for hoop strains. More importantly, the maximum compressive lattice strain of the cylinder sample was increased by 15.61 %, and 22.35 % at the impacting speeds of 100, and 125 m/s, respectively; and that of the long strip sample increased by 29.17 %, and 43.70 %, respectively. It can thus be concluded that lattice strains have redistributed around the impact crater, demonstrating the local alteration of the residual strain field. These new findings suggest the localized variation in residual strains should be taken into account while evaluating the service damage evolution of railway axles, especially those affected by high-speed impacts during operation.

High-speed railway S38C axles undergo surface induction hardening for durability, but are susceptible to fatigue cracks from foreign object impact. The neutron diffraction method was employed to measure the residual strain in S38C axles, obtaining microscopic lattice distortion data, for the gradient layer at a depth of 8 mm under the surface. The results showed that after induction-hardening, the microscopic lattice distortion had a gradient distribution, decreasing with the distance from the surface. However, in the case of impacting speed of 600 km/m, the average microscopic lattice distortion increased with the distance from the surface, reaching a maximum augmentation of 55 pct. These findings indicate a strong experimental basis, and improve our understanding of the relationship between macroscopic residual stress and decision-making, in regard to operation and maintenance.

The fatigue life assessment of railway wheels is a critical issue to establish an effective maintenance strategy. Presently, the traditional nominal stress method is commonly adopted, assuming the wheels are defect-free. However, the wheels are always subjected to railway ballast, corrosion, and more, rendering the traditional design method inadequate. To address this, our study introduces a stepwise fatigue assessment approach. Initially, the traditional nominal stress method is employed as the first level, followed by the application of advanced damage tolerance analysis as the second level, which takes into account the impact of defects. The safe-critical position of the wheels is identified using the finite element model. Three fatigue crack growth rate models are compared to assess the residual life of the wheels with defects. The numerical simulation results illustrate that the iLAPS model is proved to be effective, demonstrating the effectiveness of the developed assessment method. This work is expected to provide a scientific guidance for establishing and optimizing of maintenance strategy for railway wheels.

High-speed railway axles are suffered from extremely complex variable amplitude (VA) loads during operation. However, in the design codes and operation maintenance of railway axles, most of them are based on the equivalent constant amplitude (CA) load spectrum to perform the fatigue resistance design by using the traditional nominal stress method, and carry out the routine inspection based on running experiments and data. In this paper, an innovative time-domain stepwise fatigue assessment (TSFA) approach has been introduced to include the influence of VA loads on the accumulated damage and fatigue crack propagation. Based on this procedure, the fatigue load spectrum from the multi-body system (MBS) dynamics simulation was integrated into the fatigue damage and remaining lifetime assessment of the axles. The calculation indicates that the safe lifetime of the powered and non-powered axles under the time-domain loading is considerably larger than the design value, which shows that the axle could safely operate until they are retired. However, for an original crack with the morphology ratio of 0.6 propagating to 45.0 mm, the lifetime of the powered and non-powered axle is sharply reduced to 1.32, and 2.07 years, respectively. Compared with the classical nominal stress method, the newly-developed TSFA procedure could produce a more conservative but practical result.

AbstractHigh‐speed maglev trains are subjected to severe dynamic loads, thus posing a failure hazard. It is necessary to account for the vehicle dynamics to improve the structural strength and fatigue life assessment approach under harsh routes and super high‐speed grades. As the most critical load‐carrying part between the vehicle body and levitation frames, the swing bar was taken as an example to demonstrate the significance of vehicle dynamics to integrate classical structural strength and fatigue life with the service conditions. A multiphysics‐coupled dynamic model of an alpha improvement scheme for an electromagnetic suspension maglev train capable of 600 km/h was established to investigate the complex dynamic loads and fatigue spectra. Using this model, the structural strength and fatigue life of the wrought swing bars were investigated. Results show only a slight effect on the structural strength and fatigue life of swing bars by the super high‐speed grades. The nonaxial bending moments caused by the uncompensated relative displacement between the vehicle body and bolsters are identified as the decisive factors. The minimum safety factor of the structural strength for wrought swing bars is 1.33, while the minimum fatigue life is 34 years. Both match the design requirements but are not conservative enough. Therefore, further verification and optimization are recommended to improve the design of swing bars.