Fatigue Lifetime Of Railway Axles Research Articles

Quantitative dependence of oxide-induced crack closure on air humidity for railway axle steel

The paper analyses fatigue crack closure in the near-threshold regime of the EA4T railway axle steel. The dependence of fatigue crack growth threshold on air humidity was experimentally obtained. Varying air humidity led to changes of fatigue threshold values Kmax,th in the range from 4 to 7 MPa m0.5, which was explained by the effect of oxide-induced crack closure (OICC). Such a wide range of threshold values results in significant changes of residual fatigue lifetime of railway axles (by one or two orders of magnitude) operated under various humidity conditions. The Newman's model of plasticity-induced crack closure is not sufficient to explain high portions of crack closure near threshold and their variation, since roughness-induced crack closure (RICC) and OICC has also be taken into account. Detailed explanations and interpretations of the parameters quantifying crack closure are presented. A quantitative model for separation of the crack closure components was applied to threshold data from literature for dry and humid atmosphere as well as for new experimental data of the EA4T steel tested at various levels of humidity. The levels of OICC determined from threshold values were compared to those calculated from thicknesses of oxide debris measured on fracture surfaces. The oxide layers were measured after the experiments in scanning electron microscope using focused-ion-beam cuts. The results were plotted as dependences on absolute air humidity and were in a good agreement. Very low threshold values measured in dry air can occur in operation when the temperature is around 0 °C which is not exceptional in many countries.

Methodology for estimation of residual stresses in hardened railway axle

Residual stresses play an important role in propagation of potential fatigue cracks and they influence residual fatigue lifetime of railway axles. Essential step for assessment of residual fatigue lifetimes of railway axles is determination of magnitudes and distribution of residual stress in axle. This paper deals with determination of tangential residual stress in hardened railway axle made of EA4T steel. The tangential stress is obtained from difference between uncut and cut slit ring specimens, which are manufactured from considered railway axle. Then, based on theory of curved beams, stress distribution in the cylindrical specimen is calculated. Finally, results from slit ring method are compared with results of neutron and x-ray diffraction.

Crack closure in near-threshold fatigue crack propagation in railway axle steel EA4T

The paper is focused on fatigue crack propagation in the threshold area of the EA4T railway axle steel. Railway axles represent one of the most critical components for safety of trains. The main aim of this paper is to describe fatigue crack closure mechanisms in EA4T, which significantly influence the threshold value – a crucial parameter for determination of residual fatigue lifetime of railway axles. Investigated fatigue crack closure mechanisms are: plasticity induced crack closure, roughness-induced crack closure and oxide-induced crack closure. One part of this paper is focused on comparison of crack closure determined by NASGRO relationship (including plasticity induced crack closure mechanism only) with the experimentally obtained one. Difference between these two approaches is explained by presence of roughness and oxide induced crack closure. Based on theoretical models different sources of fatigue crack closure mechanisms was separated and evaluated. The significance of oxide-induced crack closure was confirmed based on experimental measurement of fatigue crack propagation in low humidity air. Finally, the effect of a reduced threshold value in low-humidity air on residual fatigue lifetime of railway axles is demonstrated.

Effect of residual stresses on the fatigue lifetime of railway axle

The operation of railway axles should fulfill at least two main demands: safety and low operation costs. A significant part of operation costs is given by the length of regular inspection intervals which should reveal potential fatigue cracks in railway axle. The detection of cracks is of a probabilistic nature, therefore their detection is not ensured in all cases. For the safe operation of trains, an existence of potential initial crack should be considered on the axle surface and residual fatigue lifetime should be conservatively determined for this case. Reliable procedure of residual fatigue lifetime estimation should take into account real axle geometry, material characteristics and loading of the railway axle. This paper shows methodology for determination of residual fatigue lifetime (RFL) based on the fracture mechanics approach, taking into account real spectrum of the loading cycles, existence of press-fitted wheels and surface residual stresses given by the thermo-mechanical surface treatment of the railway axle. It is demonstrated that the effect of the residual stresses is significant and should not be neglected in the numerical estimation of residual fatigue lifetime of the axle.

Fatigue lifetime estimation of railway axles

Abstract The railway axles are subjected to cyclic loading during their operation. Their load is of long-term nature, therefore a real risk of fatigue failure exists. This failure could lead to derailment of the whole train with serious consequences. To prevent such scenario, the railway axles have to be safely removed from operation before their final failure occurs. This paper presents methodology for the residual fatigue lifetime prediction of the railway axle based on the linear elastic fracture mechanics concept. The methodology contains estimation of the critical position of initial crack, prediction of the fatigue crack front shape development during crack propagation, separation of the bending and press-fitting contributions to the axle load, experimental measurement of the crack growth kinetics of EA4T steel and subsequent estimation of the residual fatigue lifetime of railway axle. Part of the presented study is also devoted to the probability aspects of determination of material characteristics describing fatigue crack propagation and retardation effects caused by existence of plastic zone ahead of propagating fatigue crack. Described methodology is already applied in the design process of new railway axles in Bonatrans company.

Influence of Extension of Load Spectrum on Estimation of Residual Fatigue Lifetime of Railway Axle

Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect in railway axle is often required. Because of different operation regimes of trains (fast/slow ride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not constant. The variability of load is defined by a load spectrum, which is determined experimentally by measuring of load in service conditions. Even though the load spectrum is measured on several hundreds or thousands of operation kilometres, the railway axles are in operation much longer time (often tens of years). Therefore, some load amplitudes higher than ones measured in the test can occur during a long-term axle service. The contribution presented deals with the effect of extension of load spectrum by rare high load amplitudes, which can occur during long-term operation, on residual fatigue lifetime of railway axles.

Influence of Initial Inclined Surface Crack on Estimated Residual Fatigue Lifetime of Railway Axle

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

Influence of variable stress ratio during train operation on residual fatigue lifetime of railway axles

Railway axles are subjected to cyclic loading, which could lead to fatigue failure. Therefore, it is desired to know residual fatigue lifetime of railway axles to ensure safe operation of trains. Because of detection of relatively small fatigue cracks is not guaranteed an estimation of residual fatigue lifetime is based on damage tolerance approach. The acting stress ratio is variable due to variable amplitude loading and load caused by existence of press-fitted wheel in the vicinity of assumed crack. The contribution is focused on influence of variable stress ratio in EA4T steel on residual fatigue lifetime of railway axles. The influence of stress intensity factor on fatigue crack propagation rate was experimentally evaluated for three different stress ratios, which correspond to operation conditions. Two different expressions of fatigue crack propagation rate were used and mutually compared to show influence of the stress ratio on residual fatigue lifetime of structure made of EA4T steel. The first expression considers stress intensity factor range (respecting stress ratio R) and the second one uses maximal value of the stress intensity factor. The paper shows ability of both expressions to describe experimental data obtained under different stress ratios and their influence on estimated residual fatigue lifetime values. The results obtained contribute to the better estimation of residual fatigue lifetime of railway axles and generally to the safer rail transportation.

Residual fatigue lifetime estimation of railway axles for various loading spectra

The knowledge of the residual fatigue lifetime of railway axles is very important for the safe operation of trains. The railway axles could include some defects like cracks, scratches or inhomogeneities which could contribute to the fatigue crack initiation and in the final stage to axle failure. The non-destructive testing methods are not reliable for detection of relatively short (circa 1–2mm), but even dangerous cracks. For conservative estimation of residual fatigue lifetime the railway axle containing initial crack should be considered. The residual fatigue lifetime is given by the number of load cycles for fatigue crack growth from the initial size of the crack up to the critical size with consequent fatigue failure of railway axle. Railway axles are used in different types of trains which are used under different conditions (regional trains, urban trains, high speed trains, freight trains, etc.). This work is focused on numerical estimation of residual fatigue lifetime of railway axle while several different load spectra taken from the literature are considered in the performed estimations. This paper also shows the influence of the discretization level of the continuous load spectrum and effect of magnification or diminution of load spectrum on the calculated residual fatigue lifetime of the railway axle.

Influence of Threshold Values on Residual Fatigue Lifetime of Railway Axles under Variable Amplitude Loading

The paper deals with an estimation of residual fatigue lifetime of railway axles under real loading spectrum. The residual fatigue lifetime is given by magnitude of fatigue crack propagation rate. This rate depends predominantly on load, geometry and material of the axle. Standard steel EA4T for manufacturing of railway axles is considered in this paper. The scatter of data in v-K curve could be caused by inaccuracy of experimental measurement or by local change of material properties. The paper shows important differences between obtained residual fatigue lifetime estimations considering scatter in measured material data, especially near the threshold region.

Influence of Different Crack Propagation Rate Descriptions on the Residual Fatigue Lifetime of Railway Axles

The paper deals with an estimation of the residual fatigue lifetime of the railway axles. The railway axles can include some cracks either from manufacturing process or from previous loading operation. Because of cyclic loading of the railway axles there is a risk of fatigue failure of the railway axles with unacceptable consequences. Based on this fact, for conservative establishment of the residual fatigue lifetime of the railway axle is necessary to consider an existing crack in the railway axle during design process. The fatigue lifetime estimation of railway axles is very sensitive to used crack propagation rate description (e.g. v-K curve). Typical bending of this curve (knee) can be found in the vicinity of the threshold value in fatigue crack propagation rate dependence (typically v-K curve expressed in log-log coordinates). For accurate estimation of residual fatigue lifetime of the railway axle is necessary to use approximation of v-K curve that takes into account existence of the knee close to the threshold value of the stress intensity factor. The paper shows important differences between different crack propagation rate descriptions on the residual fatigue lifetime estimation of the railway axles. Results obtained can be used for safer design and operation of the railway axles.

Comparison of Different Load Spectra on Residual Fatigue Lifetime of Railway Axle

The railway axles are subjected to variable amplitude loading. The variability is caused by many different regimes of train operation. The dominant load is caused by the weight of the vehicle, which generates rotary bending during train movement. Nevertheless, there are additional forces, which are generated when train goes through curved track, over crossovers, switches, rail joints etc. Present non-destructive defectoscopy can reliably detect only relatively long cracks (approximately 2mm long or longer). Hence there is a risk that the existing crack is not detected. Therefore, for conservative estimation of the residual fatigue lifetime the railway axle with a crack should be considered. The behaviour of the crack depends on load spectrum of railway axle. Hence for accurate determination of residual fatigue lifetime of the railway axle is necessary to know representative load spectrum. This paper compares several measured load spectra of railway axles, which are available in the literature. The effect of different load spectra is shown on two widely used materials for railway axles: EA1N and EA4T steels. Obtained results could be used for safer operation of railway axles.