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.     

Investigation of a failed axle of a reduction gearbox

After 11,300 h of operation, one of the axles of the reduction gearbox of the hoist mechanism of a dockside crane failed due to the propagation of a fatigue crack. The axle contained two gears, the largest of which was mounted swith a key, while the smallest gear was an integral part of the axle. On both ends of the axle a bearing was mounted: one cylindrical bearing and one spherical roller bearing (locating bearing).The axle was made from 17CrNiMo6 steel, and the machined gear teeth were case carburised. The failure of the axle occurred in the middle between both bearings of the axle, on a non-contacting part of the gear machined on the axle.During visual inspection striations were observed on the fracture surface and three separate crack initiation zones could be observed on three neighbouring gear teeth. The ratio of final fracture versus fatigue fracture was low, indicating a low nominal stress on the axle.The three initiation sites were investigated with optical microscopy and SEM, and clear indications of a ductile overload fracture were found. In an etched longitudinal section of one of the gear teeth where initiation took place several cracks could be observed in the hardened case of the tooth.The hardness at the initiation site was found to be 777 HV, while the hardness at the final fracture (diametric position relative to the initiation of the crack) was found to be lower, namely 722 HV.It was concluded that at the initiation site during the case carburisation quenching cracks were formed, which then propagated through the whole section of the axle under relatively low operation load.     

Investigation into the causes of fracture in railway freight car axle

Railway axles are vital parts of railway. Their failure in the form of dynamic fracture is commonly of disastrous outcomes for railway vehicles. Accordingly, railway axles are designed to be highly reliable, while the maintenance system requires regular inspection in terms of crack initiation. However, due to complex exploitation conditions, complex stress state and multiple stress concentration, railway axles often experience fatigue failures. This occurrence has been studied in a large number of papers. This paper too sheds light on the causes of fracture occurrence in the axle of railway freight car for coal transport in a thermal power plant. Detailed analyses were conducted on the axle fracture surface and mechanical properties. Also, microstructure of the axle material, as well as on exploitation conditions and stress state was examined. Calculations indicated that, apart from working load impact, the influence of press fit joints, especially of the one between the labyrinth seal and the axle is of crucial importance for the analysis of railway axle stress state. The entire numerical–experimental analysis has shown that the considered axle failure was caused by inadequate maintenance, insufficient axle strength and adverse stress state in the railway axle critical cross-sections.     

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.     

40 Cr 重载车轴断裂失效分析

目的研究车轴在热校直过程中的断裂原因和机制。方法通过对该车轴进行化学分析、金相检测、力学性能检测,对车轴断裂的性质和产生原因进行了分析和讨论。结果准确得出了车轴断裂的性质和产生原因。结论车轴制造工艺不合理,导致车轴轴头产生了严重的过热组织,这是车轴断裂的主要原因;原材料中较多的非金属夹杂物等材质缺陷,造成了材料的综合力学性能降低,尤其是材料的屈服强度降低,是车轴断裂产生的重要原因。建议改进车轴的制造工艺,过热严重的车轴应报废处理。     

Fracture measurements on cement paste

This paper describes an investigation into the fracture behaviour of hardened cement paste. Notched specimens of the material were tested to failure in flexure and tension. In the initial flexural tests on beams of fixed overall depth, the stress intensity factor at failure as calculated from linear-elastic fracture mechanics appeared to be a material constant. However, further investigation showed that this factor varied with specimen size, and suggested that linear-elastic fracture mechanics and the concept of fracture toughness are not readily applicable to hardened cement paste, which would appear to be a relatively notch insensitive material whose strength is not greatly reduced by the presence of flaws. A “tied crack” model explains semi-quantiatively the observed behaviour.     

LZ50钢断裂韧度的合理统计模型

试验研究了LZ50车轴钢的断裂韧度的合理统计模型。结果表明,LZ50钢是一种偏脆性的材料,应当严格控制铁道车辆车轴的制造表面质量。同时证实了现有正态分布模型不能合理描述试验数据。为此,通过比较三参数Weibull、两参数Weibull、正态、对数正态、极小值和极大值6种常用统计分布对试验数据的拟合优度、数理一致性和尾部安全性,说明了极小值分布是良好统计模型。建立了断裂韧度的极小值概率测定方法,测定了典型存活概率和置信度下的材料断裂韧度值。     

A load–deformation formulation with fracture representation based on the J–R curve for tubular joints

This study presents a new fracture formulation to describe the ductile tearing and unstable fracture failure for circular hollow section (CHS) joints under monotonically increasing brace tension. The initiation of the ductile tearing occurs when the crack driving force in an assumed initial shallow crack reaches the material fracture toughness determined from a standard fracture toughness test. The joint behavior prior to the ductile crack initiation follows a previously proposed nonlinear formulation based on the latest strength equations recommended by the International Institute of Welding. The load–deformation characteristics beyond the crack initiation assume that the energy release rate and the amount of crack extension adhere to the experimentally measured J–R curve, prior to the unstable fracture failure. Unstable fracture, which leads to the total loss of the joint capacity, occurs when the crack driving force reaches the maximum fracture resistance determined from the J–R curve test. The proposed load–deformation representation for tubular joints, when implemented in the large-scale K-frame pushover analysis with a material fracture toughness test, predicts successfully the global frame response governed by the joint fracture failure, as observed in the frame test.     

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.     

Investigation on the mechanical properties and fracture toughness of graphite

In the present study, mechanical properties and fracture toughness of graphite as a brittle material were investigated. At first, some specimens were examined in two perpendicular directions to derive Young's modulus and ultimate tensile strength. Then, graphite fracture toughness tests were conducted using some three‐point bending specimens with a sharp machined V‐notch by two different methods. The first method is based on the applied force at the moment of fracture, and the second one uses energy released during the test. Moreover, a technique was adopted to reduce differences between the two methods. It was observed that considering the effect of dehydration of the specimens, the fracture toughness was reduced by about 8%. Finally, crack growth simulation of the experiment was performed and indicated that finite element analysis predicts about 25% lower crack length values when critical energy release rate is utilized as a crack growth criterion instead of fracture toughness. In other words, the required input displacement for crack growth would be overestimated by using the critical energy release rate criterion.     

A local approach model for cleavage fracture and crack extension direction of functionally graded materials

A local approach model has been developed for structural assessment of functionally graded materials in which the yield strength and the fracture toughness vary spatially. While the yield strength of the material at any point is taken to be deterministic, the local cleavage toughness is statistically distributed following a two-parameter Weibull model. The model is intended to determine the crack extension direction and failure probabilities of cleavage failure for a stationary pre-crack in a functionally graded material. The effect of independent variation in yield strength and toughness is discussed as a precursor to validating the model using a temperature gradient problem in which the yield strength and toughness are coupled through the temperature. The model is shown to closely reproduce experimental observations from cleavage fracture tests on mild steel subject to a controlled temperature gradient normal to the crack.     

Variable amplitude fatigue crack growth in a mild steel for railway axles: Experiments and predictive models

Railway axles, though designed for infinite life, are subject to failure due to various types of surface damage (ballast hits, corrosion) that might occur during their very long service life (30 years or 10⁷ km). This problem is dealt with by regular axle examinations in the form of nondestructive testing inspections, whose periodicity is calculated on the basis of the propagation lifetime of a given initial defect. The key point is therefore a reliable estimation of propagation lifetime under service loads.This paper discusses the application of predictive crack growth algorithms to the propagations of cracks in A1N steel axles. In particular, constant amplitude crack propagation tests on small-scale and “companion specimens” were carried out together with experiments on full-scale axles. Variable amplitude tests on companion specimens were performed and the results were then analysed using a simple “no-interaction” algorithm. Then, variable amplitude tests on companion specimens and the analysis of results with a simple “no-interaction” algorithm and the Strip-Yield model were dealt with.The results showed a negligible load-interaction effect on “companion specimens” and a significant retardation on full-scale axles. The consequences resulting from the application of predictive models to a full-scale axle under service spectrum were then analysed.     

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 limit of induction hardened railway axles

A new surface induction hardening technology was designed for the purpose of increasing the resistance of railway wheelsets to fatigue damage. This paper gives a detailed presentation of the technological aspects of induction hardening of axles. The increased fatigue resistance in hardened surfaces compared with standard heat treatment of EA4T steel was verified using tensile test specimens, press‐fitted wheel seat/axle joints at 1:3 scale and press‐fitted wheel/axle joints at actual size. The 70% increase in the fatigue limit of induction hardened EA4T steel specimens compared with material subjected to standard heat treatment clearly demonstrates the effectiveness of this technology.     

Determination of dynamic fracture parameters using a semi-circular bend technique in split Hopkinson pressure bar testing

Fracture initiation toughness, fracture energy, fracture propagation toughness, and fracture velocity are key dynamic fracture parameters. We propose a method to simultaneously measure these parameters for mode-I fractures in split Hopkinson pressure bar (SHPB) testing with a notched semi-circular bend (SCB) specimen. The initiation toughness is obtained from the peak load given dynamic force equilibrium. A laser gap gauge (LGG) is developed to monitor the crack surface opening displacement (CSOD) of the specimen, from which the fracture velocity and the fracture energy can be calculated. The feasibility of this methodology for coarse-grained solids is demonstrated with the SHPB-SCB experiments on Laurentian granite.     

混凝土断裂及亚临界扩展的细观机制

通过模型和三点弯曲断裂SEM试验,详细研究了混凝土断裂全过程及亚临界扩展的细观机理。结果表明:混凝土断裂是一个复杂的不规则过程,存在明显的亚临界扩展现象。混凝土亚临界扩展路径是曲折的,并非经典断裂力学假定的平直路径,混凝土亚临界扩展和临界失稳扩展呈现分形特征。用起裂断裂韧性iICK和分形等效断裂韧性feICK,来描述混凝土抵抗初裂和临界失稳扩展的能力。给出了考虑亚临界扩展弯折效应的混凝土亚临界扩展长度、混凝土起裂断裂韧性iICK和分形等效断裂韧性feICK,的计算表达式。计算表明:混凝土失稳断裂时的分形等效断裂韧性feICK ,与混凝土亚临界扩展的分维数D成正比。     

Magnetically actuated peel test for thin films

Delamination along thin film interfaces is a prevalent failure mechanism in microelectronic, photonic, microelectromechanical systems, and other engineering applications. Current interfacial fracture test techniques specific to thin films are limited by either sophisticated mechanical fixturing, physical contact near the crack tip, or complicated stress fields. Moreover, these techniques are generally not suitable for investigating fatigue crack propagation under cyclical loading. Thus, a fixtureless and noncontact experimental test technique with potential for fatigue loading is proposed and implemented to study interfacial fracture toughness for thin film systems. The proposed test incorporates permanent magnets surface mounted onto micro-fabricated released thin film structures. An applied external magnetic field induces noncontact loading to initiate delamination along the interface between the thin film and underlying substrate. Characterization of the critical peel force and peel angle is accomplished through in situ deflection measurements, from which the fracture toughness can be inferred. The test method was used to obtain interfacial fracture strength of 0.8-1.9 J/m² for 1.5-1.7 μm electroplated copper on natively oxidized silicon substrates.     

Insights into the effects of tensile and compressive loadings on microstructure dependent fracture of trabecular bone

The micro-scale finite element models used in the past to understand yielding failure of trabecular bone have not addressed the microcrack formation and its effect on microstructure dependent fracture. An understanding of microcrack based failure mechanisms can be important to develop insights into response of trabecular bone to external loading before final failure. With this goal, we analyze tensile and compressive fracture failure at two different ages in two trabecular bone micrographs obtained from an ovine femur using a recently developed cohesive finite element method (CFEM) framework. The results and analyses indicate that examined trabecular microstructures are optimally designed for resisting compressive loading. Under tensile loading, initial damage in a microstructure is localized in a single random trabecula. Final microstructure failure occurs immediately after the failure of the trabecula. However, under compressive loading, failure of the first trabecula does not precede immediate complete failure of microstructure. Under compression the propagation fracture toughness (characterized by change in energy release rate as a function of crack density) increases with increase in crack density. However, under tension the propagation fracture toughness decreases with increasing crack density. The fracture mechanism remains unaffected by age variation. Effect of tissue property random variation on the variation in fracture strength diminishes under tension and increases under compression with increase in the age. Overall, results indicate that structural arrangement of the trabecular bone (besides the hierarchical chemical composition) can be an important contributor to its unique fracture resistance properties.     

Fracture toughness correlation with microstructure and other mechanical properties in near-eutectoid steel

The variation of yield strength and fracture toughness was investigated for four different heat treatments attempted on specimens of a near-eutectoid steel. The aim of this study was to optimize the microstructure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensile and charpy impact tests was compared with those measured directly according to ASTM Designation: E 399. Among the four different heat treatments attempted in this study, the plane strain condition was valid in the fracture toughness tests for (i) normalized and (ii) hardened and tempered (500°C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength and plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength. The pearlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nature of variation of empirically determined toughness values from tensile tests for different heat treatments is similar to that measured directly through fracture toughness tests, although the two sets of values do not match quantitatively. On the other hand, the toughness data deduced from charpy impact test is in close agreement with that evaluated directly from fracture toughness tests.     

自卸车前桥断裂分析

某自卸车前桥在使用过程中断裂。采用宏、微观检验、力学性能测试以及化学成分分析等方法对断裂前桥进行了分析。检验结果表明，前桥的断裂属于脆性断裂，断裂起源于前桥工字梁前侧焊接处。由于在前桥上焊接钢带时，在焊接热影响区产生了高硬度的马氏体和焊接裂纹，从而导致前桥受力时产生脆性断裂。