Fracture mechanics and scale effects in the fatigue of railway axles

Fatigue of railway axles is one of the basic problems of fatigue. However, in spite of the criticality of this component, modern approaches have not been used for addressing a critical revision of traditional design. The scope of this paper is to study the scale effects in fatigue limit and in crack growth rate for a high strength steel used for high speed railway axles.Fatigue limit tests on micro-notched specimens led to the determination of fatigue thresholds for small cracks of the examined steel. This allowed us to successfully analyse the `scale effect' and the fatigue strength of full-scale axles in terms of threshold stress for short cracks emanating from small non-metallic inclusions.A series of crack propagation tests on small scale specimens lead to the definition of an EPFM crack propagation model which has been successfully compared with propagation data on full-scale components. These results support the application of the crack propagation model for the determination of axle inspection intervals.     

An experimental approach to determining the residual lifetimes of wheelset axles on a full-scale wheel-rail roller test rig

The paper describes an experimental method for determining the residual lifetime of wheelset axles which was developed and proved. The procedure includes all necessary steps: crack initiation from an artificially generated surface defect, monitoring of crack growth, and specification of the end-of-test criterion. The crack propagation tests described in this paper were carried out on a complete wheelset that was installed on a full-scale wheel-rail roller test rig using a measured load spectrum. During both the test planning and test implementation phases, considerable attention was paid to the complex processes involved in crack propagation in wheelset axles. In addition to axle material and design issues, important factors that have to be taken into account include sequence effects, the reliability of load cycle omission strategies to reduce the overall duration of testing, static stresses introduced by press-fitting procedures and residual stresses caused by manufacturing processes, and crack closure effects. The results obtained indicate that the method produces reliable results that are of practical relevance. Examples were also presented that indicated how far experimentally determined residual axle lifetimes could still differ from lifetimes calculated using current fracture mechanics modelling techniques.     

Fatigue crack growth in railway axles: Assessment concept and validation tests

Railway axles are safety relevant components which are usually designed for up to 30 years of service. Besides the experience based definition of inspection intervals, the application of fracture mechanics tools is currently being introduced as an appropriate method. Basic fatigue crack growth data both in the range of stable crack propagation and near the threshold have been experimentally determined for the heat-treated railway axle steels 25CrMo4 (EA4T) and 34CrNiMo6+QT under constant and variable amplitude loading at relevant stress ratios (predominantly fully reversed load cycles, R = −1). For the computational modelling of fatigue crack propagation, a generally applicable stress intensity factor solution has been derived by finite-element analyses. The results are employed for predicting fatigue crack growth in a reference railway axle within the shaft and in the fillet zone near a press fit. Additionally, the influence of press fitting on the crack propagation behaviour in a fillet is discussed. Finally, fatigue crack growth curves experimentally determined on 1:3 and 1:1 scaled axles at constant and variable amplitude loading are compared to the test results for standard M(T) specimens, as well as to respective analytical predictions.     

Failure analysis of fretting fatigue initiation and growth on railway axle press-fits

Fretting fatigue failure of press fitted railway axle-wheel assembly was presented. Size, distribution and propagation profiles of the circumferential fretting cracks on the full-scale axles were determined by magnetic particle and metallographic slicing methods. The distribution of multiaxial stress cycle along the press-fit seat was obtained by finite element analysis (FEA). The obtained stress path was used for interpretation of fractographic evidence collected at crack initiation sites and the crack propagation plane. Metallurgical and mechanical characterization of the axle material (34CrMo4) was made on the specimens sampled from the broken axles. The threshold conditions for propagation of small fretting cracks were determined by Kitagawa analysis and El-Haddad correction method. The results of the experimental study were compared with EA1N grade steel which is given as the reference axle material in the EN standards. The metallurgical factors affecting the fretting fatigue crack initiation and propagation were investigated. The causes of the examined axle failures were associated with the deteriorated mechanical properties of the axle material.     

Load interaction effects in propagation lifetime and inspections of railway axles

As well known, an interaction effect arises, on crack propagation, when a specimen or a component is subjected to variable amplitude fatigue loading. Depending on the applied load sequence, a certain amount of retardation or acceleration can then be observed, on the fatigue crack growth rate, with respect to the constant amplitude case. In the case of structural ductile materials, the interaction phenomenon is mainly addressed by the local plasticity at the crack tip and can be explained, from a global point of view, by adopting the crack closure concept. In the present research, load interaction effects in a medium strength steel for railway axles are experimentally analyzed by companion and full-scale specimens. The experimental outcomes show a significant retardation with respect to a simple no-interaction approach and the Strip-Yield model offers good, yet conservative, estimates of crack advance. The consequences of crack growth retardation on the inspection periodicity of railway axles are then discussed.     

Fracture mechanics assessment of railway axles: Experimental characterization and computation

The results of a joint research project aiming at developing validated fracture mechanics assessment procedures for railway axles are presented. Experimentally determined fatigue crack growth parameters for the commonly used axle steel 25CrMo4 (A4T) and the high strength steel 34CrNiMo6 are included in the range of stable crack propagation and near threshold. The results are employed for predicting fatigue crack growth for cracks initiating at the axle shaft. For the computational modelling of fatigue crack propagation a generally applicable solution for stress intensity factors has been derived. Furthermore, the influence of variable amplitude loading (block loading) on the crack propagation behaviour has been studied and is discussed. The computational results are in good agreement with experimental data determined on standard fracture mechanics specimens as well as down-scaled and geometrically similar axle specimens.     

Fatigue lifetime estimation of railway axles

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.     

Experimental and numerical study on crack initiation under fretting fatigue loading

Press-fitted railway axles and wheels are subjected to fretting fatigue loading with a potential hazard of crack initiation in press fits. Typically, the resistance against crack initiation and propagation in press fits is investigated in full-scale tests, which procedure is both costly and time consuming. In this context, combined experimental and numerical approaches are of increasing practical importance, as these may reduce the experimental effort and, moreover, provide a basis for the transferability of experimental results to different axle geometries and materials. This study aims at evaluating stress–strain conditions under which fretting fatigue crack initiation is likely to occur. Experiments on small-scale specimens under varying fretting fatigue load parameters and their finite-element modelling to characterize the resulting stress–strain fields are performed. Subsequently, different multiaxial fatigue parameters are applied to predict crack initiation under fretting fatigue conditions.     

Specific Features of In-Service Failures of Parts of the Landing Gear of AN-124 “Ruslan” Aircrafts

We present the results of investigation of the axles of wheels of AN-124 Ruslan aircrafts after failures occurring either in service or in the process of resource testing. It is shown that, in service, the axles fail according to the intergranular mechanism. At the same time, in the course of stand tests, we observe fatigue fracture. On the basis of the analysis of the structure of landing gear and the results of investigation of external factors, we propose a model of formation and propagation of cracks in the axle of a wheel. We also propose several procedures aimed at the prevention of failures.     

Parameters affecting the damage tolerance behaviour of railway axles

The paper provides a discussion on damage tolerance options applied to railway axles and factors influencing the residual lifetime as well as the required inspection interval. These comprise material properties such as the scatter of the da/dN–ΔK curve, the fatigue crack propagation threshold ΔK_th and the toughness of the material. Parameters affecting axle loading such as the press fit, rotating bending, load history and mixed crack opening modes are discussed. Finally the influence of the initial crack geometry on residual lifetime is simulated.     

Finite element approach toward an advanced understanding of deep rolling induced residual stresses,and an application to railway axles

A full-scale railway axle, made of medium strength steel EA4T and adopted for high-speed applications, is deep rolled. The induced residual stresses were experimentally characterized by X-ray diffraction and hole drilling. A realistic finite element model is proposed to overcome some of the existing shortcomings in simulation of deep rolling. Deep rolling coverage is defined, formulated and incorporated into the simulation. The model is validated by the experimental measurements. A parametric study is performed to investigate the effect of rolling force (4–19 kN), rolling feed (0.1–0.7 mm/rev) and roll geometry (1.5–10 mm roll tip radius) on the distribution of residual stresses and the induced hardening. A fatigue crack propagation algorithm is used to analyze the influence of the technological parameters on the lifetime of railway axles. Lower feeds, higher loads and thicker rolls, all resulting in higher coverage, can result in higher improvement against fatigue crack propagation. However, extremely high coverage can deteriorate the performance of deep rolled components. Coverage can effectively serve as a master parameter in deep rolling. As a general rule of thumb, adopting deep rolling feed to get a coverage level of 500–900%, while avoiding too high rolling loads and too thin rolls, can induce a suitable compressive residual stress distribution; and effectively prevent/retard fatigue crack propagation.     

Experiments and stochastic model for propagation lifetime of railway axles

Inspection intervals of critical safety components should be calculated on the basis of sophisticated crack growth algorithms able to describe crack propagation under service loads. The present paper deals with a probabilistic application of the NASGRO crack growth algorithm to estimate the propagation lifetime of railway axles. The analysis of experimental crack growth and threshold data of an A1N steel has enabled us to propose two random variable models for the estimation of propagation lifetimes, discussing their statistical properties and their implications for lifetime estimates.     

Effects of thermal exposure on cyclic deformation and fracture behavior of Ti600 titanium alloy

Effects of thermal exposure on cyclic deformation and fracture behavior of Ti600 alloy were investigated by laser scanning confocal microscope (LSCM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results demonstrated that both the nonthermal exposure (NTE) specimens and the thermal exposure (TE) specimens showed the cyclic softening, within a total strain amplitude range from ±0.45% to ±1.00%. During thermal exposure, since the harder α₂ (Ti₃Al) phase precipitated in the α_p (primary α) phase, the resistance of crack propagation of α_p phase could be increased by the precipitation of α₂ phase. Therefore, the fracture behavior of TE specimens is different with that of NTE specimens. For the NTE and TE specimens, the crack mainly passes through the α_p phase with “cutting” and “bypass”, respectively.     

Bayesian analysis for the results of fatigue test using full-scale models to obtain the accurate failure probabilities of the Shinkansen vehicle axle

Bayesian analysis was performed to estimate an appropriate value of the uncertain propagation rate of cracks that can be initiated at the wheelseat of a Shinkansen vehicle axle. In the analysis, fatigue life distribution obtained by numerical simulation that employed the crack propagation rate obtained from small specimens was used as the prior distribution. Then it was modified by the results of the fatigue test of full-scale models as additional information to obtain the posterior distribution. It was indicated that the variances of fatigue life distribution reduced through the analysis. By using the crack propagation rate obtained from the posterior fatigue life distribution, the failure probabilities of the Shinkansen vehicle axle in operation, that were calculated previously by using the crack propagation rate due to the experiment of small specimens were recalculated. The resulting probabilities of failure were almost the same as those that were not modified, but were slightly lower. Although the difference was not so significant, it was thought that more confident values of the failure probability were obtained.     

A case study on the fracture of sinter machine pallet axles

A sinter machine produces sinter at 1200–1400 °C to supply raw materials for blast furnace ironmaking. The sinter machine is composed of a series of pallet cars which are constrained on rails. Each pallet car has four axles which connect the pallet car and four running rollers. In sinter production, the pallet axles experience periodic thermal expansion and periodic load changes under high temperature, high moisture, corrosive atmosphere and heavy dust. After serving for about seven years, pallet axles of one of ArcelorMittal sinter machines started breaking, causing unscheduled production delay. The fracture of the axles is in the orientation of top and bottom, and the cracks are deeper in upper portions than in lower portions of axle cross sections. The axles have been studied through chemical analysis, strength analysis, stress analysis, lifetime analysis via deterministic mechanics and lifetime distribution analysis. It has been found that the axles are breaking in fatigue-crack mechanism. The fatigue cracks are resulted from effect of thermal expansion on net tensile stress of the axles. Considering harsh working environment and dynamic loading, lifetime analysis, using deterministic mechanics, indicates that originally installed pallet axles are approaching the end of their lifetime. Weibull distribution function can very well describe lifetime distribution of pallet axles. Accordingly, about 50% of originally installed axles will be gone before next two and half years, and more than 90% of them will break before next six and half years. An intelligent replacement strategy is recommended. In a scheduled maintenance time, carefully inspect all axles using ultrasonic crack detectors, and replace those axles whose expected lifetime is shorter than next scheduled maintenance date. To replace steel SAE 1045 with steel SAE 4040 for fabricating new pallet axles may significantly increase lifetime of the new pallet axles.     

Slip initiation on frictional fractures

Direct shear tests and biaxial compression tests are conducted to investigate the onset of slip along a non-homogeneous frictional surface and to determine the effect of specimen thickness and confining stress on slip initiation and propagation. The specimens are made of two and three acrylic blocks with the contact surfaces between blocks having on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. The specimens are then loaded in direct shear or biaxial compression with confining pressures ranging from 0.7 to 3.5 MPa. The onset of slip, slip propagation, and the stress field generated at the front and center of the blocks interfaces are monitored using a photoelastic technique where a thin photoelastic film is placed at the location where observations are made. The onset of slip at the weak-strong zone interface is treated as propagation of a frictional crack under Mode II loading. The critical stress intensity factor, K_IIC, at the onset of slip is obtained from photoelastic techniques. The results show a weak dependency of K_IIC on the normal stress applied and no influence of the specimen size for specimens thicker than 25.4 mm; for thinner specimens the K_IIC values are smaller because the boundaries of the specimen prevent the full development of the stress field ahead of the crack tip. The experiments show a linear increase of the critical energy release rate with normal stress which is explained with linear elastic fracture mechanics theories.     

Review of the fatigue damage tolerance of high-speed railway axles in Japan

Railway axles are one of the most important components in railway systems since a fail-safe design is not available. In the present paper, the fatigue tolerance of the high-speed railway axle in Japan is reviewed. To maintain the safety, the fatigue strength of the axle has been extensively studied. Theses case histories and consequent improvements in manufacturing process are presented. The crack propagation behavior of the induction hardened axle is studied based on the fracture mechanics. Concerning the powered railway axles, the fatigue design method in Japan is compared with that in Europe and the effect of the train velocity on the allowable load is discussed.     

Failure Analysis of Induction Hardened Automotive Axles

Rollover accidents in light trucks and cars involving an axle failure frequently raise the question of whether the axle broke causing the rollover or did the axle break as a result of the rollover. Axles in these vehicles are induction hardened medium carbon steel. Bearings ride directly on the axles. This article provides a fractography/fracture mechanic approach to making the determination of when the axle failed. Full scale tests on axle assemblies and suspensions provided data for fracture toughness in the induction hardened outer case on the axle. These tests also demonstrated that roller bearing indentions on the axle journal, cross pin indentation on the end of the axle, and axle bending can be accounted for by spring energy release following axle failure. Pre-existing cracks in the induction hardened axle are small and are often difficult to see without a microscope. The pre-existing crack morphology was intergranular fracture in the axles studied. An estimate of the force required to cause the axle fracture can be made using the measured crack size, fracture toughness determined from these tests, and linear elastic fracture mechanics. The axle can be reliably said to have failed prior to rollover if the estimated force for failure is equal to or less than forces imposed on the axle during events leading to the rollover.     

Effect of specimen geometry on fatigue crack growth rates for the railway axle material EA4T

This paper presents experimental results on fatigue crack growth propagation for the EA4T steel widely employed in the manufacturing of railway axles. Apart from standard M(T) and C(T) specimens, the investigations include bending tests on cylindrical bars and rectangular plates containing semi-elliptical surface cracks, geometries representative of surface flaws in components. The results give an evidence of the crack growth rate dependency upon the geometry and loading conditions of cracked specimens. The paper also concludes that fatigue crack growth rates correlate with crack tip plasticity, even within low and middle stress intensity factor ranges. Some remarks are provided with respect to the significance of the results for assessing residual lives of railway axles under in-service conditions.