Characterisation of residual principal stresses and their implications on failure of railway rails

The residual principal stresses of an unused and four used railway rails of differing service times and loading conditions have been determined with a new magnetic stress measuring system (known as MAPS).The distributions of residual principal stresses on the transverse cross-sections of the rails have been mapped out. Results of the present work have complemented other recent research efforts which have established the residual principal stress component in the longitudinal direction.Due to wheel-rail contact loads, a hardened layer of about 10 mm in thickness developed in each of the used rails, irrespective of loading conditions and times. The contact loads also induced changes in the patterns of residual stresses after an unused rail is put to service.Besides residual stresses, the microstructures and ways of crack propagation in the used rails have been characterised. The severity of grain deformation in the rails has been qualitatively studied using optical microscopy and quantitative measured with image analysis. Contribution of residual stresses to crack growth is discussed.     

Introduction to the damage tolerance behaviour of railway rails - a review

Despite substantial advantages in material development and in periodic non-destructive inspection together with periodic grinding and other measures in order to guarantee safe service, fatigue crack propagation and fracture is still in great demand as emphasised by the present special issue. Rails, as the heart of the railway system, are subjected to very high service loads and harsh environmental conditions. Since any potential rail breakage includes the risk of catastrophic derailment of vehicles, it is of paramount interest to avoid such a scenario. The aim of the present paper is to introduce the most important questions regarding crack propagation and fracture of rails. These include the loading conditions: contact forces from the wheel and thermal stresses due to restrained elongation of continuously welded rails together with residual stresses from manufacturing and welding in the field, which is discussed in Section 2. Section 3 provides an overview of crack-type rail defects and potential failure scenarios. Finally the stages of crack propagation from initiation up to final breakage are discussed.     

A comprehensive approach for modeling fatigue and fracture of rails

A comprehensive approach is developed for studying the fatigue phenomena (crack initiation and propagation) induced by repeated rolling or rolling-sliding contacts between wheel and rail. Cracks initiate and propagate in the rail head in a complex varying multiaxial stress regime due to Hertzian or non-Hertzian contacts generating 3D residual stress pattern. This paper presents the main steps of such an approach devoted to the modeling of defects induced in the rails by the traffic. Special attention is paid to some of the principal difficulties met as well as to the proposed solutions. Examples of applications for the prediction of initiation as well as propagation of some defects are presented. It is shown that numerical simulations predict very well the locus of crack initiation as well as its propagation in the rail. Our approach presents at least three main originalities: first, it is a global approach starting from the evaluation of the initial state of the rail to the simulation of the crack propagation under complex loading including multiaxial residual stresses. Second, special and original numerical methods for the evaluation of the initial states, the overloads and the elastoplastic state under service loading have been developed. Third, a new concept based on a “structural Paris law” has been developed and used in the crack propagation simulations.     

Fatigue characteristics of continuous welded rails and the effect of residual stress on fatigue-ratchetting interaction

ABSTRACT

The importance of ratchetting-fatigue interaction is garnering interest due to complex failure mechanism of rail welds under cyclic loading. The objective of this paper is to investigate the fatigue characteristics of continuous welded rails (CWRs) and the effect of residual stress on fatigue-ratchetting interaction. For this purpose, UIC60 rails have been modeled using a three-dimensional finite element model, including a combination of nonlinear kinematic and isotropic hardening. In addition, the interaction between cyclic loading and the effect of residual stress on fatigue is taken into consideration. Finite element model is validated against representative experimental findings. Smith–Watson–Topper (SWT) method is utilized in order to estimate the fatigue life of rail welds under static and cyclic loading. Lower fatigue life is predicted with increasing load due to the contact between rails and wheels. Simulation results also show that failure in the form of ratchetting occurs during the 10,236th cycle, while failure corresponds to the 15,290th cycle and the 145,161st cycle based on the SWT and Coffin-Manson fatigue models, respectively. These findings suggest that investigations on ratchetting and fatigue should be carried out simultaneously to estimate the failure of the CWRs.     

Train derailment at a broken switch rail

Two train rail elements at a switch location broke as part of a locomotive derailment. Preexisting cracks present in the elements most likely had developed as a result of wear in the switch system. The cracks were not the principal reason for the system failure, but they did define a plane of weakness. No other material defects were noted. The steel elements were severely deformed, but the final breaks included some brittle character, most likely a result of a high rate of loading during the incident. The rolling wheels were already outside of the constraints provided by the wheel flange and track configuration in order to cause the twisting and rolling deformation of the leading ends of the thin switch rails, which then led to breaking and separation of the pieces. Nondestructive examination was performed on an additional 199 switch rails. Cracks and/or linear indications were found on 27 switch points or switch point protectors. The flawed or damaged rails were removed, and spare rails that had passed the examination were substituted.     

Rail integrity management by means of ultrasonic testing

This paper discusses the monitoring of the condition of the OREX ore export line rails in South Africa. Ultrasonic testing is widely used to monitor defect development and growth as well as to plan and manage programs to ensure a high level of rail integrity. An overview of the performance and history of the line is given in terms of rail defect levels and derailments due to rail failures in relation to traffic volumes. Some of the inherent characteristics of the rails are discussed and related to the defect development. From the available data it is clear that the rails are degrading in a ‘wear‐out mode’, with increasing defect rates. The use of high technology ultrasonic testing equipment has enhanced the level of knowledge of the condition of the rails tremendously and has led to programs that actually improved the overall condition of the line and eliminated derailments due to rail defects.     

Rolling contact fatigue cracking in rails subjected to in‐service loading

Rolling contact fatigue (RCF) is one of the most important failure mechanisms in rails with significant cost‐ and safety‐related implications on the operation of railway systems. In this work, a metallurgical analysis of RCF crack initiation and propagation, including geometrical characteristics of RCF cracks – length, depth from surface, angle of propagation and spacing between cracks, is presented. The role of proeutectoid ferrite in crack initiation has been studied. Analysis of the fracture surface of an RCF crack revealed a ductile initiation zone followed by a quasi‐cleavage crack propagation. Iron oxide formed in the interior of all cracks in rails exposed to stagnant water with implications to crack propagation rate because of crack closure effects. Sequential sectioning parallel to the rolling surface revealed that RCF cracks possess convoluted surfaces. The crack trace expands with depth from the rolling surface. Subsurface crack initiation has also been documented.     

Analysis of the failure of a cast iron pipe during its pressure test

This paper presents the failure analysis of a cast iron pipe during the pressure test performed once it was already installed and before entering in service. The pipe had previously passed an in-plant pressure test. In order to determine the causes of the failure, visual inspection together with mechanical, chemical and microstructural analysis of the material has been performed.It has been derived that the pipe section that failed during the pressure test had previously suffered mechanical damage during its transportation or installation. Moreover, it has been demonstrated that the material has a very brittle behaviour, with excessive hardness and poor tensile strength and elongation to rupture, and also presents a deficient microstructure in the outer layer of the pipe thickness. This, together with the presence of pre-existing defects caused by the mechanical damage, has produced the final failure of the pipe. Finally, a fracture assessment has been performed in order to estimate the size of the initial pre-existing defects.     

Failure of a valve rod caused by improper heat treatment and metallurgy

Fractography analysis was used to investigate a structural component which failed during its normal service life. It has been found that imperfections in the metallurgy and heat treatment were the major causes of failure. Some detrimental impurities were found in the material. A number of cracks were initiated at the interface between the impurities and the base material. These cracks were the main reason for the failure. Also several kinds of metallography defects contributing to crack initiation have been found. The metallographic structure of the material is bainite+ferrite; the amount of ferrite increases gradually from the outside to the inside of the part. This was the wrong metallographic structure for this kind of material; it should be sorbite. Cracks initiated and propagated under a relatively low stress level by stress corrosion cracking.     

Failure investigation and condition assessment using field metallography

In many cases it is not possible or practical to extract samples for a subsequent laboratory failure investigation. Examples of such cases are condition assessments of larger structures that have been exposed to fire and examination of cracks or defects observed in components such as pressure vessels which are still in operation. In other cases the first step of a failure investigation will be to gather a maximum of information on-site, prior to a laboratory examination. In all these situations field metallography represents a useful tool to obtain as much information as possible.

The most common field metallographic tools are briefly presented in the article.

Cases of failure investigations where field metallography has been an important tool will be presented. This includes:

• Assessment of defects in a pressure vessel in service at elevated temperature.

• Condition assessment of a ship hull damaged by fire during construction.

• Investigation of fractured wheel shaft on high speed train.

Rail defects: an overview

For about 150 years, the steel rail has been at the very heart of the world's railway systems. The rail works in a harsh environment and, as part of the track structure, it has little redundancy; thus, its failure may lead to catastrophic derailment of vehicles, the consequences of which can include death, injury, costs and loss of public confidence. These can have devastating and long‐lasting effects on the industry. Despite the advances being made in railway permanent way engineering, inspection and rail‐making technology, continually increasing service demands have resulted in rail failure continuing to be a substantial economic burden and a threat to the safe operation of virtually every railway in the world. This paper presents an overview of rail defects and their consequences from the earliest days of railways to the present day.     

Statistics of defects in one-dimensional components

Equations related to spatial statistics of defects and probability of detecting defects in one-dimensional components have been derived. The equations related to spatial statistics of defects allow to estimate the probability of existence of safe, defect-free zones between the defects in one-dimensional components. It is demonstrated that even for a moderate defect number densities, the probability of existence of clusters of two or more defects at a critically small distance is substantial and should not be neglected in calculations related to risks of failure. The formulae derived have also important application in reliability and risk assessment studies related to calculation of the probability of clustering of evens on a given time interval. It is demonstrated that while for large tested fractions from one-dimensional components, the failures are almost entirely caused by a small part of the largest defects, for small tested fractions almost all defects participate as initiators of failure. The problem of non-destructive defect inspection of one-dimensional components has also been addressed. A general equation has been derived regarding the probability of detecting at least a single defect when only a fraction of the component is examined.     

dynamics of armature acceleration in a railgun channel

We have analyzed the results of experiments on the acceleration of an aluminum armature with a mass of about 3 g in a railgun with steel rails. The experiment was aimed at studying processes in a high-velocity contact at a velocity close to the transition value related to the contact velocity skin effect. In the absence of high-current arcs, a velocity of 1.2 km/s has been reached with the aid of armature pressing to the rails. A retarding force that acts upon the armature that moves in the railgun channel has been determined.     

Characterisation and quantification of defects in rotors and casings of steam turbines

The loading situation of components in modern fossil-fired power plants is characterised by higher cyclic service in comparison to older plants. This leads in combination with larger units to increased stresses in the relevant components. To evaluate the risk of failure due to natural defects, life time approaches have been developed on the basis of advanced methods of fracture mechanics. The assessment of natural defects using fracture mechanics is, however, based on the following preconditions: ? knowledge and data on the behaviour of such defects in terms of damage mechanism taking into account crack initiation and crack propagation under loading situations comparable to those in service,

? data on the real size and geometry of defects,

? data on the exact location of the defects.

To create such a database an extensive joint research programme was started in the early eighties by German turbine manufacturers, steel makers and research institutes. The paper provides an overview of the results of these efforts. Special emphasis is placed on describing the improvements in ultrasonic testing techniques. The results of feature tests are reported. They have been performed to obtain information and data on the behaviour of natural defects under creep, creep fatigue and fatigue load. Subsequent metallographic investigations determine the real size and location of the defects but also the relevant damage mechanisms. Comparison of metallographic data and NDT data prove the reliability of ultrasonic testing. Thus an improved rating of the permissible ultrasonic indications in turbines and castings was obtained.     

Effect of Railcar Wheel Tire Profiles on the Contact Stress Level in Subway Rails

Strength of Materials - The statistical analysis performed on the defectiveness of rails in the Kiev subway railway shows that the main cause of the occurrence of most defects is the contact...     

Fatigue crack growth rates under sequential mixed-mode I and II loading cycles

One common mode of failure that occurs in rolling bodies such as gears, bearings and rails is due to the fatigue process. Several research workers suggest that rolling contact fatigue cracks are subjected to mixed mode I and II loading cycles. It is believed that the correct modelling of loading cycles can help us to study the mechanics of crack growth because fatigue comprises a major safety consideration in the design process. Experiments have been performed under nonproportional mixed-mode I and II loading cycles with fixed degrees of overlap, so that coplanar cracks were produced. Three empirical crack propagation laws have been established which are related to both mode I and mode II effective stress intensity factor ranges.     

Experimental study of the fatigue characteristics and reliability of continuous welded rails

In recent years, joint types in railway superstructure have shifted to continuous welded rails (CWRs), which can be constructed by various welding techniques to form uninterrupted rails several kilometres long. Because of the numerous advantages of this method, CWR systems are highly preferred today for the construction of new railway lines. The increase in the number of trains in operation is inducing fatigue damages, linking to life‐threatening risks in the rails as well as in trains' wheels and axles. In this study, CWR specimens formed by the flash butt‐welding process are investigated. Specimens extracted from rail sections are subjected to four‐point bending fatigue tests to establish S–N curves under various loading levels. The surfaces of those specimens which fail are then investigated in detail in order to determine the initiation points of the failures. The findings provide experimental data on the dynamic life cycle of CWR and identify the failure mechanism of the CWR system.     

MODELLING OF EMBEDDED FATIGUE CRACKS IN RAILROAD RAILS

Abstract— A first-generation fatigue crack growth model has been developed which realistically models the patterns of subsurface shells and detail fracture crack growth that are commonly observed in railroad rails. (These types of defects are further described in the Introduction.) The key assumptions upon which the model is based are reviewed and the input data on railroad rail residual stresses and wheel-induced stresses are described. Examples of the predicted growth patterns for shells and detail fractures are shown and compared to actually observed growth patterns. Apparent deficiencies in the model are discussed and probable avenues for improvement in the accuracy of the crack growth life estimates are identified.     

Failure Mode Changes with Successive Component Improvements

The relatively recent appearance of formerly rare vertical split rim wheel failures provides a good case study of how a shift in failure mode occurs when a material improvement is made to solve an existing problem. The mode of railroad wheel failures have changed three times over the past 40 years when the dominate failure mode has been identified and corrected. When the principal failure mode has been corrected the service life increases, but a new or previously minor failure mode assumes the role of principle failure cause. Because of the large population of railroad wheels in North American service, failure trends can be easily observed. This paper will discuss the evolution of failure mode with successive material and structural improvements.     

The role of manufacturing defects in munition component failures

The U.S. Army Research Laboratory, Weapons and Materials Research Directorate performs numerous failure analysis investigations on munition-related components. Many of the failures are attributable to defects that can be traced back to the manufacturing process. Typical manufacturing defects encountered include those associated with the material, forging, casting, welding, and heat treatment processes. Dimensional anomalies have also been noted. Munition component failures are very costly and may seriously affect the safety and readiness of the armed forces. Additionally, repeated failure may lead to the grounding (removal from service) of a system, depending on the severity of the problem. Specific examples of component defects/failures discussed in this report include bomb fin retaining bands, general purpose bomb suspension lugs, missile launcher attachment bolts, cluster bomb tailcones, general purpose bomb fins, and Gatling gun breech bolt assemblies. This paper focuses on the importance of proper manufacturing techniques to the munitions industry and, by inference, to numerous other industries.