See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/364830779 Steels for rail axles - an overview Article  in  Critical Reviews in Solid State and Material Sciences · April 2024 DOI: 10.1080/10408436.2022.2137462 CITATIONS 7 READS 9,201 4 authors: Desmond Klenam University of the Witwatersrand 50 PUBLICATIONS   212 CITATIONS    SEE PROFILE Lesley Heath Chown University of the Witwatersrand 72 PUBLICATIONS   1,347 CITATIONS    SEE PROFILE Jones Papo Mintek 31 PUBLICATIONS   87 CITATIONS    SEE PROFILE Lesley A. Cornish University of the Witwatersrand 301 PUBLICATIONS   2,693 CITATIONS    SEE PROFILE All content following this page was uploaded by Desmond Klenam on 11 April 2024. 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E. P. Klenam, L. H. Chown, M. J. Papo & L. A. Cornish To cite this article: D. E. P. Klenam, L. H. Chown, M. J. Papo & L. A. Cornish (2024) Steels for rail axles - an overview, Critical Reviews in Solid State and Materials Sciences, 49:2, 163-193, DOI: 10.1080/10408436.2022.2137462 To link to this article: https://doi.org/10.1080/10408436.2022.2137462 Published online: 29 Oct 2022. 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E. P. Klenama,b,c , L. H. Chowna,b , M. J. Papob,d, and L. A. Cornisha,b aSchool of Chemical and Metallurgical Engineering, University of the Witwatersrand, WITS, Johannesburg, South Africa; bDSI-NRF Centre of Excellence in Strong Materials, hosted by the University of the Witwatersrand, WITS, Johannesburg, South Africa; cAcademic Development Unit, Faculty of Engineering and the Built Environment, University of the Witwatersrand, WITS, Johannesburg, South Africa; dAdvanced Materials Division, Mintek, Randburg, South Africa ABSTRACT The comparative assessments and an overview of mechanical, chemical, and physical prop- erties of rail axles are presented. This review focused on the effects of compositions and microstructure on fatigue, fretting and corrosion fatigue of rail axles. The two main steel grades: low carbon steels for commuter trains and high strength low alloy steels for high- speed trains have ferrite-pearlite microstructures with 20–40mm ferrite grain sizes. Minimum allowable yield and ultimate tensile strengths are 330MPa and 600MPa, and the minimum longitudinal and transverse toughnesses are 35 J and 22 J. Most axle failures are associated with surface and sub-surface defects with micro-cracks nucleating from ballast (small-sized pebbles in rail tracks), minute oscillatory movement leading to fretting, microstructural inclu- sions, and corrosion pits. The implications for structural integrity are highlighted and areas for future research directions are highlighted. KEYWORDS Rail axle; endurance or fatigue limits; microstructure; plain carbon and alloyed steels; mechanical properties Table of contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .164 2. Overview of railway axles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165 2.1. Rail bogie – an overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 2.2. Rail axles – an overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 2.3. Overview of design approaches of railway axles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 2.4. Effect of service stresses on railway axles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 3. Overview of failure mechanisms in railway axles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .168 3.1. Failure by over-heating of roller bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 3.2. Failure by fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 3.2.1. Corrosion fatigue and stress corrosion fatigue of rail axles – overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 3.2.1.1. Effect of corrosive environment on fatigue strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 3.2.1.2. Pit-to-crack nucleation and propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 3.2.1.3. Corrosion-fatigue crack growth models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 3.2.2. Fretting fatigue failure overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 3.2.2.1. Fretting maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 3.2.2.2. Fretting-fatigue predictive models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 3.2.2.3. Fretting-fatigue remedial actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 4. Steels for rail axles – chemistry and mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178 4.1. European grades (EN 13103 and EN13104) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 4.2. High-strength low-alloyed (HSLA) steels for high-speed trains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 4.3. Australian grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 4.4. Association of American railroads (AAR) grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.5. Chinese steel grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.6. Japanese grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4.7. Indian standard grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 CONTACT: D. E. P. Klenam desprimus@gmail.com � 2022 Taylor & Francis Group, LLC CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 2024, VOL. 49, NO. 2, 163–193 https://doi.org/10.1080/10408436.2022.2137462 http://crossmark.crossref.org/dialog/?doi=10.1080/10408436.2022.2137462&domain=pdf&date_stamp=2024-03-29 http://orcid.org/0000-0003-1914-9633 http://orcid.org/0000-0001-9699-6065 http://orcid.org/0000-0002-9291-4584 https://doi.org/10.1080/10408436.2022.2137462 http://www.tandfonline.com 5. Areas for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .183 6. Summary and concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .184 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .184 1. Introduction An effective transportation system is a fundamental pillar of developed economies. In Europe, the trans- port sector contributes �10% to the economy.1 Globally, the transport sector is intermodal and com- plimentary, but rail transport is by far the safest, most reliable and cheapest (Table 1).2 Air transportation is expensive, and many cannot afford as nearly half of the world population are below the poverty line.3 Water transport is slow and expensive – not a viable option.2 The default is land, either by road or rail.2 Road is cheap and widely patronized due to increased availability of vehicles. However, its challenges are bad road networks, vehicular traffic in most cosmopolitan and metropolitan cities (due to increasing urbaniza- tion in developing countries), increased global warm- ing and environmental pollution and high vehicular accidents with associated increased deaths (Table 1).2 Rail transport is safe, although the mechanical components (e.g., rail axles and wheels) do fail. Rail axles are subjected to dynamic loading, with a work- ing life of �30 years (109 loading cycles).4–6 The axles are made from medium carbon and high strength low-alloyed steels. Derailment due to rail axle failure led to the pioneering work in fatigue studies in the late 18th Century.5–7 Many of the failures were pre- dominantly due to poor material choice and defects due to corrosion and ballast impact. Most of these broken axles were studied, in-service stresses analyzed, and failure modes assessed, leading to standards and ways to improve structural integrity. In the 1970s, the Union Internationale des Chemis de Fer (UIC) developed UIC 811-1 standards recom- mending the EA1N and EA4T steels for railway axles across Europe.8–14 These carbon steels had ferrite- pearlite microstructures with yield and UTS of 330MPa and 600MPa. The European Committee for Standardization (CEN) then approved the EN 13103, EN 13104 and EN 13261 standards in 2008 for design conformity.8–14 Similar standards were also reviewed in North America, Asia, and Australia, assessing shortcomings. One such shortcoming is poor corro- sion resistance. Generally, rail axles were not designed for corrosion since the emphasis was on strength. Alloy steels have been developed with higher yield strength, ultimate tensile strength, toughness and cor- rosion resistance than the UIC 811-1 standard steels.9,15–19 Rail axles produced from medium carbon steels had the solid-axle configuration and were about 70% of the weight of the bogie. Coaches with these axles had speed limits below 120 km/h, partly due to the dead weight of the chassis and the high payloads, resulting more fuel consumption. The weight of solid axles is very high, 1.8–3 T, and depends on the train and the bogie system. However, high-speed bullet trains with speeds up to 350 km/h use a hollow axle configuration, which are produced from high strength low-alloy (HSLA) steels with strengths above 800MPa and give about 20% weight reduction. The HSLA steels have better mechanical and corrosion resistance, making them better than medium carbon steels.9,15–19 Typical alloying elements in the HSLA steels are Cr, Ni, Mo, and V, together with P, Si, Cu and S con- tents of standard steels. HSLA steels have strong car- bide formers, such as Cr, V and Mo, which lower the cementite (Fe3C) proportions, since they form differ- ent carbides from iron carbide, thereby reducing the amount of free C available for Fe to form Fe3C. Recent reviews focused on rail axle design concepts, mechanical fatigue and periodic inspections on struc- tural integrity.6,20–26 However, an appraisal of coun- try-specific standards and variation in compositions of rail axle steels has not been done and is needed because of the effect of composition variation on cor- rosion and mechanical properties. Thus, the purpose of this review is to provide comparative assessment of rail axle steel grades highlighting the composition- structure-property relationship and its effect on struc- tural integrity of typical commuter and high-speed train axles. An overview of fatigue, corrosion-fatigue, and fretting fatigue failure mechanisms, which are main failure modes of rail axles are also reviewed con- sidering recent developments made to mitigate against them. Rail axle steels grades from Europe, Asia, North America, Australia, and Africa were grouped into two classes based on their phase proportions. The paper concludes by identifying future research directions. Table 1. Fatality risk of passengers using different mode of transport in the European Union from 2008–2010.2 Mode of transport Fatalities per billion passenger kilometers Airline passenger 0.1 Railway 0.16 Car occupant 4.45 Bus/coaches 0.43 Powered two-wheeler 52.59 164 D. E. P. KLENAM ET AL. 2. Overview of railway axles 2.1. Rail bogie – an overview A typical rail bogie system is shown in Figure 1 with key components being a bogie frame, suspension sys- tem and axle box suspension.27,28 The bogies experi- ence high stresses as they carry the dead-weight and payloads of the train. About 70% of the weight of the bogie is the rail axle.15,25,29–32 A coiled spring and axle box suspension absorbs shocks between axle bear- ings and bogie frame. The axle box suspension is placed between the axle bearings and bogie frame to ensure vertical and not lateral movements.31 The wheelset comprises bearings, wheels, axle box, brake disk, gear system and axles,6,24,30,31,33 and is a pressed fit of the assemblage of the wheels and axle, which is held together by friction.34,35 Rail axle bearings allow rotary motion and are sub- jected to impact loads from rail joint, switches, wheel flats, static and dynamic loads.36–39 They also experi- ence axial loads resulting from lateral movement of trains on curved rails or snaking motion.40,41 Roller bearings are used to reduce frictional torques, increase operating speed, and can accommodate temperatures as high as 200 �C of the bearing casings to prevent “hot box” failure (overheating of the roller bearing due to increased friction).40,41 2.2. Rail axles – an overview Rail axles transmit driving torque to the rail wheels and maintain the positions of the wheels relative to each other and the chassis of the train.7,19,42 Any damage to the rail axle can result in derailment, which can be catastrophic.7 Thus, safety measures and design protocols ensure continual performance in service. Periodic inspections based on the damage tolerance principle are used for early crack detection to prevent cracks reaching critical sizes.7 Rail axles are classified as freight wagon axles, coach axles and locomo- tive axles. In 2011, the European Railway Agency (ERA) reported 329 railway axles broken between 2006 and 2009.43 Most of these failures were attributed to fatigue due to the cyclic nature of the applied stresses.5,19,26,44,45 Figure 2 shows the areas susceptible to fatigue, which includes all press-fits (wheel seat, brake disk seat, bearing seat and labyrinth seal seat (areas labeled 1 on Figure 2)) and the axle fillet radii (point 2 on Figure 2).46 Notches and transitions around the plain parts of the axle are prone to fatigue failures.19 The ratio of the diameter of pressed fitted part (D) to the journal (d), affects the fatigue strength of the railway axles (Figure 3).19 For smaller D/d ratios (�1.0), the pressed fitted parts are susceptible to crack nucleation. Cracks nucleate beneath the press fits by fretting.19,42 As the D/d ratios increase, fatigue failures occur in the mid-span section of the axle and the fil- lets become critical regions for crack nucleation.47 For typical Shinkansen axles used in Japan, the pressed fit- ted parts are more susceptible to crack nucleation due to the smaller D/d ratios (�1.10) compared to much higher values for other standards (�1.15), Table 2.5,48 Thus, typical European axle journals are the areas prone to crack nucleation due to high D/d ratios.5,48 Based on the D/d criterion, axles produced in Europe have good fatigue strength at the press-fitted parts, whereas there is good fatigue strength in the journal for axles produced in Japan.47 Axles for high-speed trains in Europe are annealed, while the Shinkansen axles from Japan are induction-hardened for increased surface hardness, wear resistance, and longer fatigue life from a hardened outer layer around the tougher inner core.49 Induction-hardening for the Shinkansen axles induces high compressive residual stresses to a Figure 1. Typical bogie system of a freight car.27,28 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 165 depth of 10mm, allowing higher stresses (up to 147MPa) than the 80MPa for European axles.50 The effect of the final processing on fatigue strength is shown in Figure 3.47 Other processes to induce bene- ficial residual stresses include forging, rolling and the differential shrinking and cooling (quenching stresses) at the surface and core of railway axles during second- ary heat treatment processes.51 The structural durability of railway axles depends on fatigue strength and the service stresses.26,42,46 This is largely influenced by the material properties, design, manufacturing and environmental factors, such as localized atmospheric corrosion and ballast impacts.42 To benchmark the fatigue strength of railway axle materials, testing is done under constant amplitude stresses for 106 - 107 cycles, which is below the endur- ance limit where no fatigue failure is expected,26,46 to determine the endurance limit. In service, the cumula- tive effect of stresses (constant and/or fluctuating) on the railway axle can cause premature failure by exceeding the predicted loads. Corrosion,15,46,52,53 fretting at the press-fits 48,52,54 and ballast impact 6,46,53,55 causes dents, micro-voids, scratches and gouges on the axles, providing stress raisers which can initiate cracks that could lead to catastrophic failures at stresses below the fatigue limit of the axle once these cracks exceed their critical sizes.15,46,52 A surface dent of 3mm could increase localized stress by at least 3%, which reduces the endurance or fatigue limit of the axle.15 The Rail Sciences Laboratory 15 identified the cause of prema- ture axle failure, especially below the predicted endur- ance limit, as either surface damage or journal fretting. 2.3. Overview of design approaches of railway axles Although railway axles are designed for a �30 year life, they sometimes fail in service prematurely,6,9 so a combination of safe life design and routine inspec- tions based on damage tolerance principles is used.6 Standard design of axles is the ‘safe-life’ approach, where stresses induced on the axle are kept lower than the fatigue limit (endurance limit), to prevent premature in-service failures. In Europe, EN 13103 standard was designed for non-powered (trailing) axles,8 and EN 13104 governed powered (driving) axle Figure 2. Rail axle regions susceptible to fatigue failure.46 Figure 3. Effect of diameter ratio of press fitted parts to the journal on the fatigue strength of rail axles in Japan and Europe.20 Table 2. Fillet radii and diameter ratios of high- speed trains.47 Measurement Shinkansen (Japan) TGV (France) ICE (Germany) Diameter (D) 209 212 190 Diameter (d) 190 184 160 D/d 1.10 1.15 1.19 Radius (mm) 100 15 & 75 15 & 75 166 D. E. P. KLENAM ET AL. designs.8 There are also Japanese 18,22 and North American 17 standards. Limitations in current design methods for rail com- ponents and standards have been criticized.6,26 For example, maximum allowable loads are estimated based on constant amplitude loads, which may be suitable for worst-case scenarios and maximum allow- able stresses. However, these estimates do not adequately cater for in service dynamic amplitude loads (payloads), variation in track quality, mechanical and surface damage such as corrosion, fretting and ballast impacts.6,9 Railway axles are exposed to high number of loading cycles, �10,9 which exceed the endurance limits of axle steels, 106 - 107 cycles, and premature failures have occurred.20–23 Periodic inspection and maintenance protocols are used to ensure structural integrity. In Europe, the EN 13103 and 13104 standards specify the durability val- idation criteria for railway axles.8 Fail-safe and dam- age tolerance principles are used to improve working lives of axles and structural components.6,29,30 Fail- safe is characterized by redundant design and design- ing for crack arrest (so the cracks do not grow). Micro-cracks at the surface/sub-surface are detected before they reach critical defect sizes in damage toler- ance, by periodic inspections.30,56 The inspection intervals are determined using the probability of detection (PoD) approach.24,57,58 Nondestructive tech- niques, such as ultrasonic testing, magnetic particle inspection and eddy currents, are used for determin- ation of cracks in axles in service.6,20–26 Damage toler- ance is also used to correlate crack depth and the number of loading cycles.6,24,25,59 Modifications in design approaches for rail components, especially axles and wheels, were proposed.5,20–23,59 2.4. Effect of service stresses on railway axles Stresses on railway axles could be static, dynamic, tor- sional (due to braking and traction, press-fit loading), residual (induced by manufacturing process e.g., heat treatment, forging, rolling), higher frequency loading (stick-slip behavior), bending and axial tensile (stresses at crossovers, curved sections of tracks and switches on rail lines). The impact of these and the possible failures and accidents caused has been inves- tigated and documented.20,25,41,48,60–62 Axial compressive and tensile loads are generated beneath the press seat and the fillets, which affect the mean stresses, the stress ratios and hence the fatigue strength of the axles. Micro-slip at the contacting sur- faces of the axle and the wheels could be sites for crack nucleation. Defects from the asperities (surface roughness) of the contacting surfaces (axle-wheel interface) could result in slip displacement causing shear stresses and these can nucleate micro-cracks on the surface of the axle, which lower fatigue strength, and could lead to premature failure of the axle, if the micro-cracks were not detected early. Multiple crack nucleation and the cumulative effects of shear stresses give high scatter in the stress - number of cycles to failure (S-N) data. For HSLA steel grade 34CrNiMo6, reduced fatigue limit under rotating bending occurs in the press fitted regions.48 Significant reduction in fatigue strength is attributed to fretting fatigue at the press fits.26 During braking and traction, the effect of torsional stresses induced within the axles is detrimental. Although torsional stresses are negligible compared to other stresses acting on the axles, their cumulative effect can be detrimental as they can nucleate micro- cracks on the surface and sub-surface of the axles. The magnitude of torsional shear stresses varied depending on weather conditions,61 especially wet and dry. At curved track sections, crossovers and rail line switches, stresses are induced in the rail axles. At curved sections, most of the load is transferred unto the outer wheel becoming bending stresses in the axle with the disk of the wheel acting as the lever arm. There is direct correlation between the bending moment of the axle and the wheelsets: the outer wheels contribute to the maximum bending stresses experienced by the axle. A slight change in alignment can contribute to the axle failure and increased axle loading occurs in small-radius curves during curving of tilting trains.61 The dynamic and static loads induced in axles can result in failure. The weight of the chassis of passen- ger or freight trains induces bending stresses on the axles between the wheels. In service, the bending stresses are increased significantly by the cumulative effect of the dead weight (structural weight of the train) and the increased payload (weight of cargo in freight trains or weight of passengers with its associ- ated loads in commuter trains) conveyed. The impact of the dynamic and static loads result in irregularities such as out-of-round wheels which increase bending stresses on the axle by 20%.61 High frequency loading can lead to stick-slip and out-of-phase oscillations of the wheelsets and the rails.62 Stick-slip is a friction phenomenon, caused by vibration of the wheels and rails 63,64 and occurs in dynamic systems where friction is attributed to the CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 167 velocity difference of the two contacting surfaces in sliding motion. The kinetic energy due to the relative motion of the contacting surfaces dissipates by form- ing microscopic particles or asperities, which become debris that can cause micro-cracks between the sur- face of the axle and wheel. It occurs by both elastic deformation of the asperities of the contacting surfa- ces when sticking together, and plastic deformation of the contacting surface during the sliding motion. In high-speed trains, the loading frequency from the wheel rotation does not exceed 30Hz, thus not allow- ing sufficient vibrations in the wheel-rail interactions to cause fatigue.65 High frequency oscillations in wheel-rail interactions cause bending stresses in the rail axles,61 and increase torsional moments in the axles during driving and sudden braking of the train. Torsional shear stresses increased from 7MPa to �100MPa in extreme stick-slip cases,62 initiating cracks at the axle surfaces, which if not detected early, could lead to complete rupture.62 Stick-slip produces the squeaking sound when a train is stopped. Residual stresses are process-induced stresses, and occur without external stresses [48]. The effect of residual stresses on a component is similar to exter- nally applied stresses, and could lead to distortion and warpage in mechanical components when service stresses are applied, which could affect structural integrity.66 Residual stresses can be beneficial (com- pressive) or detrimental (tensile), by raising or lower- ing the mean stress experienced over a fatigue cycle.67 The manufacturing processes of railway axles induce residual stresses and can influence the fatigue strength, breaking strength, and corrosion resistance. Tensile residual stresses can nucleate micro-cracks during fatigue, so having compressive stresses at the surfaces helps prevent this. Techniques used to pro- mote compressive stress include shot peening, laser peening, autofrettage, cold hold expansion and case hardening.67 Surface defects (which could initiate crack nucleation and lead to premature failure at low applied stresses) are inhibited by in-plane compres- sion, due to compressive residual stresses at the sur- face. When a surface is in compression, the core experiences counterbalancing tensile stresses, making it defect-free with sufficient inherent strength to pre- vent failure.67 Undesirable tensile residual stresses tend to nucleate new cracks and/or propagate existing cracks, which could lead to premature failure. The ori- gin of residual stresses includes inelastic or plastic deformation, temperature gradients during thermal cycles and microstructural changes or phase transformations. Relieving residual stresses is achieved by normaliz- ing with controlled cooling, resulting in finer micro- structures in axles produced from EA1N steel.8,9 For axles produced from quenched and tempered EA4T steel, the residual stresses are induced from rapid cooling (quenching stresses) of the hardening proc- esses. Tempering between 260 �C and 760 �C has neg- ligible effect on relieving residual stresses. Final machining and cold working can also relieve detri- mental tensile residual stresses. 3. Overview of failure mechanisms in railway axles Most axle failures are either environmentally induced or due to operating conditions which induce stresses that could lead to catastrophic damage. Most failures and derailment resulted in much property damage and life loss.68,69 Resistance from the wind (drag force) is a load and the cumulative load effects can contribute to rail derailment. Wear and corrosion also increase stresses, resulting in railway axle failures. Other load effects due to operating circumstances include train misalignment.54,70 The two main failures of railway axles 41 are from overheating of the bearing and fatigue.19 3.1. Failure by over-heating of roller bearings Roller bearings are made from copper alloys, and fail- ure by over-heated roller bearings (‘hot box’) is from increased friction in the bearing, due to irregular- ities,41 copper penetration from the roller bearings into the grain boundaries of the railway axle steel at high temperatures, or inadequate lubrication of the parts in contact. When regions around the axle and the locking ring of the bearing box are stressed at high temperatures, copper precipitates at the steel’s grain boundaries, causing embrittlement, promoting sub-surface cracks, damaging the structural integrity of the axle.71 Excessive heating of the bearing results in complete failure of the axle, known as “burnt-off journal” (Figure 4).41,72 3.2. Failure by fatigue Fatigue is predominant in railway axle failures, and rail axles were among the first mechanical compo- nents identified to have failed from fatigue.73 Fatigue is a surface and sub-surface-related mechanism, result- ing in a progressive localized permanent structural damage in components subjected to conditions of 168 D. E. P. KLENAM ET AL. fluctuating, repeating or alternating stresses locally, which culminates in micro-cracks and finally complete rupture after the fatigue life of the component is exceeded.73,74 The acceptable mechanism of fatigue in ductile materials is effect of cyclic plasticity involving the movement of dislocations which results in alter- nate blunting and resharpening of any preexisting crack tip while advancing.75–78 The fatigue life ‘Nf’ is the number of cyclic loads a component can sustain before it ruptures. The S-N or Wohler curves, which are plots of the stresses to the number of cycles to failure, are used to determine the fatigue strength of a material. Potential mechanisms causing surface defects (Figure 5) at the wheel seat, axle shaft and transitions include fretting fatigue dimples,52,80–83 hard nonmetal- lic inclusions at or beneath the surface of the axle,23,38,54,84–86 local ballast impacts indenting the wheel seat or axle surface,24,50,80,87 corrosion pits/cor- rosion fatigue, turning marks (grooves, offsets) ,23,38,54,84–86 stress concentrators and surface asperities,23,38,52,54,80–86 which increase the surface roughness of the axle or wheel seats. If not detected and remedied, these can result in fatigue failures in the presence of increased fluctuating stresses. Axle fatigue failures have flat and smooth cylindrical surfa- ces, with or without shear lips, and characteristic beach marks. The areas of the axles susceptible to fatigue are shown in Figure 6. Failure by fatigue occurs in three main stages. Stage 1 is associated with crack initiation and is pro- moted by defects such as pits, precipitates, and inclu- sions. These are mainly sub-surface and or surface defects which create geometrical discontinuities. Typical nucleated cracks on rail axles have depths of at least 250mm. Most of these cracks are parallel and at �60� to the contact surfaces (Figure 7a). The crack initiation paths are determined by the ferrite-pearlite morphologies and inclusions. Stage 2 is where cracks start propagating while new cracks nucleate, making it a continuous process. During this stage, initially some of the cracks propagate between the pearlite lamellae (Figure 7b), and later the same cracks can be inde- pendent of any microstructural feature (Figure 7b and pass transversally through the pearlite lamellae. Stage 2 is characterized by striations, due to the fluctuating load and hence incremental growth of the fatigue cracks. Once the cracks exceed the critical size, the final stage is reached, where the component fractures in a brittle manner as the residual area is too small to support the applied load on the structure. For rail axles, most surface defects cause decreased fatigue resistance,88–95 because they are potential sites for crack initiation as they are stress concentration points. Thus, fatigue behavior of rail axles is con- trolled by the nature of surface and sub-surface cracks: thus depth, width and orientation.96 Increased defect size and depths greatly decrease the fatigue limit and fatigue life of rail axles.97 Material defects include: nonmetallic inclusions, minute precipitates, micro-shrinkage, corrosion pits and pores, geometric defects: minute notches from manufacturing, surface roughness and scratches.85,98 The latter defects can cause abrupt changes in the geometry of the compo- nent, which induce high stress concentrations at these defects.96,99,100 Regions around the stress concentra- tion points can nucleate new cracks, while there is continuous propagation of existing cracks.98 Typical sites for crack nucleation from ballast impacts is shown in Figure 8. The relationship between Ni, stress concentration factor, Kt and defects is described in Equation 1.96 n ¼ KtDr 1� r� q � � � Dre � � Na i (1) (1)where: n ¼ coefficient of fatigue life; Kt ¼ stress concentration point; Dr ¼ stress range; r� ¼ distance ahead of main defect causing the crack initiation; q ¼ curvature radius of the crack tip; Dre ¼ fatigue limit; Ni ¼ fatigue crack initiation cycles and a ¼ exponent of fatigue life. 3.2.1. Corrosion fatigue and stress corrosion fatigue of rail axles – overview Corrosion fatigue is the progressive degradation of usually uncoated rail axles or damaged areas of coated rail axles due to the cumulative effect of cyclic stresses in an aggressive environment.79,97,101,102 The aggres- sive environments include atmospheric conditions such as fog, rainwater, marine and effluent Figure 4. Rail axle failure due to over-heating of the roller bearing.41,72 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 169 contamination in heavy industrialized areas,70,103,104 which allow electrochemical reactions and are a major cause of fatigue failure in railway axles. Thus, corro- sion fatigue is the interaction of irreversible cyclic plastic deformation and localized electrochemical reac- tions, a precursor for continuous nucleation and acceleration of minute cracks.97,101,103 Corrosion pits and localized or general corrosion on the axle surfaces are major causes of failures.89,97,104 The reactions of the axle to corrosive media create surface defects such as micro-cracks, scratches, pits and discontinuities at the sub-surface (often aided by stress concentration points) which propagate under cyclic loads, leading to brittle failure, if not detected early.54,58,70,89,104 An example of combined effect of corrosion and fatigue damage at the axle seat surface is shown in Figure 9.79 The pressing of the wheels onto the axle caused scratches (shown by arrows) which acted as corrosion initiation sites and later intensified to crevice corro- sion (Figure 9b). Generally, the corrosion occurs due to the semi-enclosed gap (crevice) between the axle and the wheels, where the conditions are different. The semi-enclosed gaps are also sites for the initiation of fatigue cracks as shown in Figure 6. High corrosion rates were observed at the pressed surface due to localized pits and at the edge of the wheel seat, result- ing from crevice corrosion from the semi-enclosed gaps. In the harsh environments where most trains are used, corrosion is exacerbated by the synergy between mechanical (fretting, wear and fatigue) and electrochemical actions (pitting and crevice corro- sion).79 Thus, damage due to corrosion increases the fatigue crack nucleation rates and the coalescence of these minute cracks can occur due to the corrosive action and the mechanical movement. Fatigue failure is increased due to the corrosion providing surface defects that cause failure to take place at lower stresses and after lower numbers of cycles (N), i.e. below the fatigue limit of the railway axle material.89,104 Corrosion products concentrate Figure 5. Multiple cracks from different nucleation sites and corroded surface at the axle seat edge.79 Figure 6. Sites on rail axle susceptible to fatigue failure: (1) press fit cracks beneath the wheel and gears, (2) crack at T notch transition (3) cracks at U and/or V notch transition and (4) cracks initiated largely by corrosion pits at the shaft.6 170 D. E. P. KLENAM ET AL. around the micro-notches, concentrating stresses at the notch pit.103 This shifts the electrochemical poten- tial to lower values, increasing the dissolution of the rail axle steel and causing micro-notch growth. As the local stresses on the axle increase, the fatigue strength decreases due to the corrosion pits. As the number of cycles increases, the cracks grow, culminating in axle failure when the cracks exceed the critical length.104 Aside corrosion pits, minute microstructural features, e.g., inclusions and precipitates have been used to pre- dict the fatigue strength of rail axles which agrees with experimental data.20,48,56,79 Figure 7. SEM images showing fatigue crack propagation between the pearlite lamellae initially, and some after the crack propa- gates transversally across the pearlite lamellae.79 Figure 8. Crack initiation sites on typical rail axle due to ballast impact.98 Figure 9. Initial corrosion damage (a) and intensive crevice corrosion (b) of the press-fit of the axle and the wheel seat.79 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 171 High cyclic stresses of the journal and corrosive liquids on the surface of the axle could lead to stress corrosion cracking (SCC), which has a similar mech- anism to corrosion fatigue, leading to railway axle fail- ure.103 Reviews on various aspects of SCC are available,103,105,106 and the SCC mechanisms have been summarized.106 Failure by stress corrosion crack- ing is from brittle cracking at fairly low stresses.54 Derailments due to railway axle failure by corrosion and stress corrosion cracking in the United Kingdom,5,15,70,104 Canada,45,107,108 United States 15 and Europe 15,70,104 have been studied (Table 3). In North America, 52 failures of axles were documented between 1998 and 2002, most being attributed to either corrosion fatigue or stress corrosion cracking.15 Thus, corrosion resistance of railway axles is import- ant, and could reduce failures in rail transportation.54 3.2.1.1. Effect of corrosive environment on fatigue strength. In corrosive environments, crack propaga- tion and pit-to-crack growth rates are faster than in air.101,103 The combined effects of fatigue and corro- sion result in no fatigue limit as it is suppressed with the disappearance of the “knee” in the S-N curve (Figure 10).54,89,97,101,103 At high stress levels above the air fatigue limit of the rail axle steel, the S-N curves and the fatigue limit in air and corrosive medium are comparable.54 Below the air fatigue limit and at relatively low stress levels, the fatigue strengths of the steel in the corrosive medium were very low. After fatigue experiments, the samples showed cracks of varying sizes around �1mm (Figure 10).54 The stress amplitude design limit for axles based on European Standard EN 13103/13014 is �332MPa and that of Germany, BASS is �220MPa.8 The fatigue limit was reduced by at least 50% due to the effect of rain water,54,101 than in air.58 Similar results were also observed for AISI 1018 steel with �120MPa fatigue strength at 2� 106 cycles in air, which further reduced to �90MPa in tap water at �107 cycles.93 When there was insufficient stress for the nucleation and propaga- tion of cracks, especially in air, the presence of a cor- rosive medium promotes the initiation and propagation of cracks from the geometrical disconti- nuities or defects associated with corrosion.95 These minute cracks, resulting mostly from corrosion pits, then grow, leading to total failure, i.e. decreasing over- all fatigue strength.58,107,108,113 3.2.1.2. Pit-to-crack nucleation and propagation. The main stages for corrosion-fatigue to occur are: � initiation of minute corrosion pits, � transition from pits to minute cracks, � coalescence of minor cracks to form macro-cracks, � final rupture of component.54,58,101,107,108,113,114 This is shown by the fracture surface analyses of a typical EA1N axle steel in Figure 11.101 Under the action of cyclic stresses in the presence of a corrosive medium (rainwater), localized corrosion is initiated at susceptible sites, resulting in pits (white areas in Figure 11a).54,95,101 The corrosive medium allows the transfer of electrons, resulting in corrosion and hence metal loss, creating the small pits (micro-cavities). Fewer pits were nucleated at the interface between the matrix and inclusions, whereas many more pits were nucleated within the ferrite grains and the ferrite-fer- rite grain boundaries.101 Corrosion pits were seldom found in the pearlite grains and within the ferrite- pearlite grain boundaries (Figure 12). Under continu- ous fluctuating stresses, these pits coalesced, resulting in the development of micro cracks, with some coa- lescing to form macro-cracks (Figure 11b). Thus, the typical pit-to-crack transition is driven by mechanical stresses, loading time and electrochemical reactions, and can be complex.114 The simultaneous action of the electrochemical reaction and coalesced micro-cracks causes growth (Figure 11c) generating primary (pits) and secondary Table 3. Summary of failures of rail axles. Place of failure Causes of failure Rickerscote, Stafford (1996) Corrosion pits5 Shields Junction, Scotland (1998) Stress corrosion fatigue104 Trudel, Quebec, Canada (2001) Pitting corrosion45 Bennerley Junction, Nottingham (2002) Corrosion pits104 Luton Station (2002) Electric arcing104 Basingstoke (2002) Electric arcing104 Simplon tunnel, Switzerland (2006) Corrosion pits109 New South Wales, Australia (2006) Ballast impact110 S Bahn failure in Germany (2006) Fretting Corrosion at press fits68,111 Tichborne, Ontario, Canada (2007) Corrosion pits108 Cologne, Germany (2008) Stress corrosion cracking/Ballast impact68 Viareggio, Tuscany, Italy (2009) Corrosion pits at U-Transition Benevento Station, Italy (2019) Fatigue at press fits112 172 D. E. P. KLENAM ET AL. (coalesced micro-cracks) cracks. The primary and sec- ondary cracks grow into each other under cyclic stresses, and the corrosion medium nucleates more minute pits. With growth, the coalesced micro-cracks became macro-cracks as shown in Figure 11d, demon- strating the pit-to-crack nucleation and propagation. The macro-cracks from the coalesced primary and secondary running in different directions can form “zig-zag” patterns.54,101 Corrosion fatigue does not usually exhibit the “knee” (Figure 10) in a typical fatigue limit in air, due to the disappearance of the non-propagating crack which is critical for the crack closure mechanism.115 The four transition stages asso- ciated with corrosion fatigue of EA4T rail axle steels are pit initiation, pit to crack transition, crack coales- cence and final macro-crack rupture.114 3.2.1.3. Corrosion-fatigue crack growth models. The prediction of corrosion fatigue of typical rail axle steels from corrosion pits or micro-cavities is by the Hobson-Brown model (Equation 2).116 This is the same relationship used for fatigue testing in air.91 da dN ¼ BðDrÞban (2) (2)where: B¼materials constant, Dr ¼ stress range, b ¼ materials constant, a¼ crack length and Figure 10. Comparison of S-N curves of typical rail axle steel tested in air and in corrosive environment.54 Figure 11. Corrosion fatigue pits-to-crack growth mechanism showing (a) initiation of pits from electrochemical action, (b) transi- tion of pits to micro-cracks, (c) growth and coalescence of minute cracks and (d) macro-cracks on A1N rail axle steel.101 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 173 n¼materials constant. The material constants were extracted from crack propagation data. The distribution of surface cracks during fatigue damage follows a typical Weibull statistical distribu- tion function.54,97 For typical small pits (less than the grain size), the S-N curve is estimated in terms of the propagation of cracks. As cracks nucleate from corro- sion pits at different positions in the microstructure and propagate, they begin to grow and/or coalesce to form long cracks (few mm). This is shown in Equation 3 where the initial and final crack lengths are key parameters used to estimate the fatigue limit of the rail axle. Nprop ¼ 1 BðDrÞb ðaf ao da an (3) (3)where: ao ¼ average size of pit (initial length of crack), which are few microns, and af ¼ final length of crack after micro-cracks coalesced. Although Equation 3 has been used for corrosion fatigue life prediction of mechanical components including rail axles, it does not account for initial pit formation and the pit-to-crack transition.52,91 However, it is at best expression of a median S-N curve for corrosion fatigue phenomenon.52,54,88,97,99 To account for pit initiation and the pit-to-crack tran- sitions, two models are used: modified Hobson-Brown 116 and the superposition models.91 There are three main stages associated with the modified Hobson- Brown model (MHM): pit development, short fatigue crack growth rate and long fatigue crack growth rate. At the pit development stage, the relationship is given by Equation 4. dap dN � � p ¼ Cp at � apð Þ (4) (4)where: Cp ¼ pitting constant, at ¼ pit to crack transition (mm) and ap ¼ pit length (mm). The pitting constant expressed in (Equation 5) is dependent on the corrosion environment and the specific alloy: Cp ¼ ApðDctÞap (5) (5)where: Dct ¼ total shear strain range and Ap ¼ material constant and ap ¼ constant that depends on the environment. Once the corrosion pits nucleate, they act as stress concentration sites which can grow into cracks,54,85,98 similar to a typical crack nucleated during fatigue tests in air. Corrosion fatigue for the short crack growth rate is estimated using Equation 6, and by integrating it with the upper and lower limits of dm and at, the corrosion fatigue lifetime for short crack Nsc (sub- script “sc” is short crack) can be estimated. da dN � � sc ¼ Csc di � að Þ (6) (6)where: (da/dN)sc ¼ corrosion fatigue short crack growth rate (mm/cycles), Csc ¼ GiðDctÞb, Gi and b¼material constants, Dct ¼ total shear strain range, di ¼ grain size (varying from 60–120 mm for rail axle steel grades) and a¼ average crack length (mm). Assessment of the long crack corrosion fatigue life- time (Nlc) during corrosion fatigue is done by Equation 7, with the upper and lower limits being the grain sizes and the total crack length. Generally, the Cp, Csc and Clc constants are used to determine the growth rate and total corrosion fatigue life and are determined empirically based on pits/cracks at specific stress levels. da dN � � lc ¼ Clca � D (7) (7)where: Clc ¼ Blc Dctð Þblc and D ¼ Blc Dcfl � �b lc dm, (da/ dN)lc ¼ corrosion fatigue long crack growth rate (mm/ cycles), D¼ threshold crack growth rate (mm/cycle), Dcfl ¼ fatigue limit total shear strain range (%), dm ¼ deceleration barrier length (mm), Blc and blc ¼ mater- ial-dependent constants. Evaluating the overall corro- sion fatigue number of cycles required for Figure 12. Ferrite-pearlite microstructure of EA1N rail axle steel showing: (a) corrosion pits initiated at the ferrite-ferrite grain boundaries and (b) pearlite colonies and grain boundaries surrounding corrosion pits initiated within ferrite grains and ferrite grain boundaries.101 174 D. E. P. KLENAM ET AL. propagation of pit/crack is by the cumulative of life- times for the three stages as shown in Equation 8. Nt¼ NpþNscþNlc (8) (8)where: Np ¼ pit development, Nsc ¼ short crack growth and Nlc ¼ long crack growth. The transition is from a0 ! at ! dm ! af. Using the superposition model, corrosion fatigue can be estimated by the cumulative effect of fatigue crack growth in air and the effects of environment inducing the corrosion [EAC] behavior.91 The driving force for fatigue crack growth rate in air is strain,92,93,101,117 whereas for the EAC regime the driving force is the synergy between rail axle compos- ition and the corrosive environment, hence is chem- ically driven.103,106,114 Thus, environmentally assisted cracks are propagated beyond microstructural barriers such as grain boundaries, inclusions, and precipitates.91 3.2.2. Fretting fatigue failure overview Fretting is the deterioration of surfaces subjected to repetitive and small relative motion.118–121 It is the cold welding of asperities on the contacting surfaces and is enhanced by slip,121–123 which breaks them fur- ther. The freshly exposed surfaces act as sites for crack nucleation, and these surfaces also undergo oxidation, leading to fretting corrosion. Fretting is complex and includes fretting fatigue, fretting wear and fretting corrosion.124 Fretting fatigue failure is due to the sim- ultaneous action of repetitive stresses on rail axle and localized stresses resulting from relative slip at the axle and wheel interface.124,125 The depth of critical fretting damage for smooth and solid axles ranges between 200–250 mm in most European coun- tries.4,79,126,127 The loading process in fretting-fatigue is generally characterized by superposition of heter- ogenous fluctuating stress field (fretting loading) at the surfaces of the contacting components (i.e., axle and wheel assemblage) and the induced stress field resulting from cyclic stresses (Figure 13).125 The evo- lution of crack damage is shown in Figure 13b as a function of the stress distribution. No cracks initiate below the fretting-fatigue threshold and the system is under the safe crack initiation condition (A in Figure 13b). For B in Figure 13b, initiation of crack is pos- sible, although the cracks do not propagate due to reduced contact stresses at the contact interfaces. This is a typical safe crack arrest regime. For relatively high bulk fatigue loading and contact stresses, the nucleated cracks continue to propagate until final fail- ure (C in Figure 13b), defining total failure or rup- ture regime. The combined actions of stresses and slip contrib- ute to reduce the fatigue strength.125 Three main fretting fatigue modes are tension-tension (or tension- compression), bending and torsional fretting fatigue, Figure 14.124 Fretting causes wear, formation of deb- ris, and then crack propagation with reduced fatigue strength.48,118,128–130 Fretting occurs at the pressed fits or interference fits on axles and can lead to cata- strophic failure if not detected. Fretting fatigue is a main cause of failure of rail axles under rotation bending loads.34,35 3.2.2.1. Fretting maps. A typical fretting map is given in Figure 15,131 showing partial slip regime (PSR), mixed fretting regime (MFR) and gross slip regime (GSR) (also called slip regime (SR) ).125 The gross slip regime is also known as just slip regime.124 The PSR, MFR and GSR are characteristic and dependent on material properties and the fretting conditions.124,131 Thus, fatigue limits of rail axles due to fretting are much shorter than for typical “plain” fatigue under the same nominal bending stress amplitude.124,132 In PSR, there is simultaneous abrasive and moderate oxi- dative wear occurring due to wear debris generated at Figure 13. Schematic diagrams showing: (a) fretting and fatigue loading with heterogenous and homogenous stress fields and (b) the three main regimes and characteristics fretting-fatigue behavior.125 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 175 the interference fit of the wheelset (region between the axle and wheel, which has different materials with different mechanical properties and microstruc- tures).132 For most alloys, the shortest fatigue lifetime with accompanying crack initiation and propagation occurs in the MFR. Within the GSR, the fatigue life- time is higher than the MFR, but is not safe as it is close to the yield strength. Typical fracture morphologies within the PSR, MFR and GSR for 7075 alloy are shown in Figure 16.124 Within the PSR, very slight fretting scars were observed, whereas a very long crack at the boundary of the contact zone near the loaded area was observed in the MFR. In the GSR, severe wear zones were observed at the contact zones. 3.2.2.2. Fretting-fatigue predictive models. The main fretting-fatigue predictive models used for assessing mechanical properties of low carbon and high- strength low alloys are classical, fretting maps, plain fatigue, multiaxial fatigue, and fracture mechan- ics.133–140 A typical empirical approach to estimating the fretting fatigue life of a mechanical component follows the process in Figure 17.137 This combines fracture and damage mechanics approaches using strain constitutive equations (finite element model- ling) with fracture mechanics crack propagation laws.137 Fretting fatigue is complex, with four main locations for crack initiation and an empirical model should account for steep stress gradients, multiaxial stress states and non-linear bifurcaton. To address these, especially at the fretting fatigue crack initiation stage, various empirical models have been developed and modified as summarized in Figure 18 [132]. The four main approaches are continuum damage approach, stress invariant method, critical plane and applying fretting specific parameters. The classical approach only uses empirical data such as nominal stresses, relative displacement of con- tacting surfaces and interfacial shear stresses to predict fretting fatigue life. A relationship between S-N curves and fatigue limit can be determined. The fatigue limit as a function of the reduction caused by fretting is given in Equation 9.150 For wheelset, transition fillets between seat and axle body result in �50% increase in overall fretting fatigue strength. rwf ¼ rwo � q 8lP 2pb (9) 9where: rwf ¼ fretting fatigue resistance, rwo ¼ fatigue limit for plain specimen, q¼ notch sensitivity factor, P¼ contact force, m ¼ coefficient of friction and 2 b¼width of the contact strip. The main drawback of using the classical model is the inability to correlate all fatigue and fretting parameters,52 which could lead to under- or overesti- mation of the fretting fatigue properties. This model is also difficult to use with other numerical models for fretting fatigue. The classical criteria and the fret- ting maps are useful for estimating the overall critical areas susceptible to fretting fatigue. Fretting fatigue prediction based on plain fatigue approaches have been studied.136–138,151 This is gener- ally based on applying uniaxial plain fatigue test data, assuming the contact stress at the axle and wheel assembly is generally quasi-uniaxial. Generally, the Coffin-Manson low-cycle fatigue criterion (Equation 10) 152 is used to estimate the maximum strain ampli- tude and fretting fatigue limit.52,151 The disadvantage is that the sensitivity of the parameters is materials dependent. emax,Re ¼ C1 Nfð ÞC2 þ C3 Nfð ÞC4 (10) 10where: emax, Re ¼ emax(1–Re) m, C¼ fitting constant, Nf ¼ fatigue life, emax ¼ maximum strain amplitude along the contact interface, Re ¼ strain ratio and m¼ fitting parameter. Equation 10 can be modified for multiaxial strain conditions.52 Generally, the Figure 14. Fretting fatigue modes: (a) tension-tension (b) bending and (c) torsional fretting.124 Figure 15. Typical fretting map showing fretting-fatigue wear mechanisms.131 176 D. E. P. KLENAM ET AL. fretting fatigue life is estimated at a material-depend- ent distance from the stress concentration point. These continuum approaches describe fretting fatigue explicitly but do not have enough parameters to model fretting fatigue cracks. 3.2.2.3. Fretting-fatigue remedial actions. Surface engineering has been used to improve fretting corro- sion resistance at ambient and higher tempera- tures.35,153–155 Thermally sprayed Fe-Ni coatings on axles increased fatigue strength to 156MPa from 103MPa for uncoated axles.35 The coating acts as a barrier, interrupting crack nucleation and propagation on the surface, improving the fretting resist- ance.35,153–155 However, under rotating bending condi- tions, not all hard coatings improved fatigue limits, as cracks initiated beneath the coatings. Coatings with fine grains increase fatigue strength, as long as they do not have porosity.154,156 Induction hardening of the rail axles and wheels has greatly increased the resistance to fretting fatigue,20 and has been done for rail axles used for the Figure 16. Fretting damage morphologies of Alloy 7075 showing the fretting running regime focusing on PSR, MFR and SR.124 Figure 17. Total fretting fatigue lifetime prediction process flow chart.137 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 177 Japanese commuter and bullet trains. The disadvan- tage is that thermal stresses can induce cracking. 4. Steels for rail axles – chemistry and mechanical properties The basic microstructures for rail axles were ferritic- pearlitic (Figure 19) ,158,159 although more advanced alloy design produced other microstructures, especially with better manufacturing and increased understand- ing of the kinetics and thermodynamics influencing microstructures.17,18,48,160 In ferrite-pearlite micro- structures, smaller pearlite interlamellar spacing increases yield strength and UTS [99]. For typical fer- rite-pearlite microstructure, the ferrite grain sizes ranges between 20–50 mm. Generally, allotriomorphic and Widmanst€atten ferrite are observed for rail axle steels. With an axle of diameter of 180mm, high hardness is observed at the surface which decreases to the core, because of the higher volume fraction of pearlite at the surface. Typical pearlite morphologies observed in rail axles that contribute to strength are given in Figure 20. These morphologies include granular, laminar (layered colonies that are oriented parallel to each other), tilted pearlite (differently ori- ented) and some secondary cementite.161 The cement- ite lamellae of the granular pearlite are generally kinked, bent and fragmented to allow plastic deform- ation coordination of ferrite. This is mainly driven by thermal stresses from heat treatment and mechanical stresses and strain fields. The different morphologies and orientations are useful for improving strength since they provide different pathways for diverting and arresting cracks. Many standards exist for the selection of steels for axles in Europe, Asia, North America, and Australia. The standards used in Europe are EN 131038 for non-pow- ered axles, EN 131048 for powered axles and EN 132619 for typical axle requirements including material type, nondestructive testing and surface roughness tolerances. These standards served as benchmarks for the develop- ment of other alloyed steels. The Association of American Railroads (AAR) developed M101 for carbon steels,17 India developed the ISR 16/95 standard, and Italy developed UN 16787-71 for steel axles. The American Society of Testing and Materials (ASTM) also recom- mended steel grade A730-99 for railway axles. 4.1. European grades (EN 13103 and EN13104) Annex A of the European Standard EN 13261 has two grades of steels for manufacturing railway axles: EA1N and EA4T grades.8,9,25,30,126,162,163 The EA1N grade (also known as C35) is a normalized low strength steel. The compositions of EA1N and EA4T grades are shown in Table 4. The pearlite colonies are distributed between ferrite grains of 20–40mm.170 Fatigue limit stresses were determined for ferrous alloys, and played a significant role in material selec- tion and design.20,170 The EA4T steel has better fatigue properties and lower notch sensitivity than EA1N steel, and hence is more widely used for axles. The EA1N steel grade is produced by vacuum degass- ing, then either forged or rolled into solid or hollow axles,9 giving the properties shown in Table 5. The Figure 18. Parametric models applied to fretting fatigue crack initiation. The letters denote the references associated with specific models: A -,133 B -,134 C -,135 D -,136,137 E -,138 F -,139 G -,140 H -,141 I -,142 J -,143 K -,144 L -,145 M -,146 N -,147 O -,148P -.149 178 D. E. P. KLENAM ET AL. fatigue limit stress of the EA1N is largely affected by grain size, texture, and phase distribution, which are influenced by alloy chemistry, mechanical working, and heat treatment. Fatigue properties and characteristics are deter- mined for correct dimensioning and designing to ensure structural integrity and optimum performance of the railway axle under in-service stresses.8,9 The fatigue limits are estimated with reduced (axle geom- etry is not considered) and full-scale test pieces. Full scale railway axle testing is done under simulated environmental and stress states to assess worst case scenarios (maximum stresses). The results are used to estimate safety factors and possible conditions that the axle can be subjected to practice. Full scale testing is also used as verification and validation of calculated and permissible stress ranges referred to in EN 13103 and EN 13104 standards.8,9 The fatigue limits are also estimated for smooth (fatigue limit RfL) and notched (fatigue limit RfE) test Figure 19. Microstructures of rail axle carbon steel showing ferrite (white) and pearlite (dark): (a) optical micrograph and (b) SEM image in secondary electron mode.157 Figure 20. SEM-SE images showing different pearlite morphologies which are of the granular, tilted (differently orientated) and laminar (lamellar) types with some secondary cementite phases.161 Table 4. Composition of various axle materials. Alloy type Composition (wt%) ReferenceC Si P S Mn Mo Ni Cr Ti V Nb Conventional European grade 0.37-0.38 0.15-0.46 0.04-0.05 0.04-0.05 1.12-1.20 0.05-0.06 0.05-0.06 0.3-0.4 – 0.05-0.06 – 8,9 MS3 0.29-0.30 0.63-0.70 0.02-0.03 0.02-0.03 1.54-1.60 – – – 0.02-0.03 0.10-0.30 – 164 MS6 0.19-0.20 0.34-0.35 0.02-0.03 0.01-0.02 1.32-1.40 – – – – 0.11-0.20 – AAR 0.45-0.59 0.15 0.045 0.05 0.6-0.9 – – – – – – 165 EA1N 0.40 0.50 0.02 0.02 1.20 0.08 0.30 0.30 – 0.06 – 166 EA4T 0.22-0.29 0.40 0.02 0.02 0.5-0.8 0.15-0.3 0.30 0.90-1.20 – 0.06 – 35NiCrMoV12 0.26-0.32 0.40 0.02 0.015 0.40-0.70 0.40-0.60 2.70-3.30 0.60-1.00 – 0.08-0.13 – 35CrMo 0.35 0.27 0.016 0.015 0.55 0.2 0.06 0.9 – – – 167,168 LZ50 0.47 0.26 0.014 0.01 0.78 – 0.028 0.02 – – – 169 Note: MS3 and MS6 alloys were designed in India for rail axles164. AAR grades are axle materials by Association of American Railroads and are mostly car- bon steels. Grades EA1N and EA4T are used extensively in Europe for rail axle according to EN 13103/13104. Grade 35NiCrMoV12 is a HSLA steel. CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 179 pieces, as well as the surface of a solid axle (F1) and the bore surface of hollow axle (F2). Fatigue limits determined from reduced test pieces are used to determine the notch effect of the material, in accord- ance with security coefficient ‘S’, which is defined in EN 13103 and EN 13104 8,9 and also to estimate the notch sensitivity coefficient (q) of the railway axle. These are used to determine maximum allowable stresses and safety factors, as damage tolerance.171,172 Standard fatigue limit values for European standard axle steels are shown in Table 6. A minimal safety coefficient of 1.2 is expected for European axles in accordance with EN 13103 and 13104 standards, but the influence of dynamic stresses, torsion and vibra- tions on a driven axle could result in increased safety coefficients.19 The fatigue limits of EA1N and EA4T axle steel are listed in Table 6. Although EA4T had better fatigue resistance, due to its high notch sensitiv- ity, EA1N is the preferred grade. 4.2. High-strength low-alloyed (HSLA) steels for high-speed trains High strength low-alloyed (HSLA) steels are used for manufacturing hollow axles for high-speed trains, especially 30NiCrMoV12 and 34CrNiMo6 in Europe.19 Grade 30NiCrMoV12 steel is quenched and tempered, to give a mixed ferrite-pearlite microstruc- ture with very high strength (Table 7). These grades were developed by Lucchini RS Company, Lovere, Italy, for rail applications according to standard UN 16787-71.173 The compositions are given in Table 4. They have high fatigue strengths and notch sensitivity values. Axles produced from alloyed steel grade 30NiCrMoV12 had at least 20% weight reduced than EA4T, and about 30% less than EA1N.166 This light- weight axle is achieved by hollow axle design, where the external diameter of the axles can be decreased. The mechanical properties of this grade of rail axle steel are shown in Table 7.19 Axles have been produced for high speed trains, e.g. the Italian Pendolino and the German Inter-City Express (ICE), using various grades of high strength low alloy steels such as 30NiCrMoV12 and 34CrNiMo6,174,175 giving relatively high yield and ultimate tensile strengths. The high strength and hard- ness are improved by precipitation hardening and high amounts of retained austenite,176 and allow hol- low axle design, which is critical for light weighting to reduce the inertia moments for high speed trains.166,170,175 These grades have no temper brittle- ness and retain useful machinability at relatively high hardnesses. Impact test results carried out at cryogenic temperatures (down to �125 �C) showed some ductil- ity, while carbon steels are fragile below 0 �C.177,178 The good cryogenic properties enabled the use of these steels in most Scandinavian countries. The fatigue characteristics of steel grade 30NiCrMoV12 are shown in Table 8, and are higher than most standard grades, e.g. EA4T steel, but they have similar notch sensitivity.4 4.3. Australian grades Forged and rolled axle bars are mostly produced from AS 1448/K5 and AS 1442/5FG steel grades in Australia. The AS 1448/K5 grades are normalized or tempered carbon-manganese steels, giving a good Table 5. Mechanical properties of rail axle steels.8,9 Steel Yield strength (MPa) Tensile strength (MPa) Elongation (%) longitudinal Charpy U-notch (J) Transverse Charpy U-notch (J) EA1N �320 550-650 �22 �30 �20 EA4T �420 650-800 �18 �40 �25 Table 6. Comparison of standard and typical rail axle steel fatigue limit values in EN 13261.8,9,19 Limit Fatigue strength (solid axle) F1 (MPa) Fatigue strength (notched surface) F2 (MPa) Fatigue limit (smooth specimen) RfL (MPa) Fatigue limit (notched specimen) RfE (MPa) Notch sensitivity coefficient q¼ RfL/RfE Standard �200 �80 �250 �170 1.47 EA1N �200 �80 �250 �170 1.47 EA4T �240 �96 �350 �215 1.63 30NiCrMoV12 �300 �120 �480 �320 1.5 Table 7. Mechanical properties of the HSLA steel grade.19 Steel Yield strength (Re) (MPa) Ultimate tensile strength (Rm) (MPa) Elongation (%) Impact toughness (longitudinal direction) (J) Impact toughness (transverse direction) (J) 30NiCrMoV12 �834 932-1079 �15 �47 �22 180 D. E. P. KLENAM ET AL. combination of hardness, tensile strength, and tougher core than plain carbon steels, and so balance the tradeoff between strength and ductility. They are heat treated by surface flame or induction hardening. Most of these carbon-manganese steels are fully pearlitic, and typical compositions are shown in Table 4. 4.4. Association of American railroads (AAR) grades Railway axles in North America are produced from carbon steels, according to the AAR M-101 standard specifications.165 The specified mechanical properties include minimum UTS of 607MPa, yield strength of 345MPa with an elongation of 22% and about 37% reduction in area. Axles produced based on the AAR standard are mostly eutectoid carbon steels, with dou- ble normalizing and tempering, and were most of the axle steels.15 The double normalizing and tempering heat treatment improved yield and tensile strengths,15 with improved ductility.15,182 The compositions according to AAR standard are listed in Table 4. For ASTM standard A730-99, rail axles are also manufactured from alloy steels such as AISI 4140.183 These axles are normalized, quenched, and tempered. Experimental AISI 4140 steels have a tensile strength of 860MPa, yield strength at 725MPa and with elong- ation and reduction in area at 18% and 50%.183 Axles produced from ASTM standard A730-99 had better properties than the conventional grade based on the European standard. 4.5. Chinese steel grades The Chinese have two main steel grades for rail axle production: LZ50 and 35CrMo rail axle steels with the compositions shown in Table 4.167–169,184 The LZ50 grade is an alloyed medium carbon steel, with a microstructures of coarse ferrite and pearlite.169,184 Axles are is normalized for 9000 seconds at 860 �C and then tempered for 5400 seconds for 575 �C.130,184 The 35CrMo steel grade has a bainitic microstruc- ture,167,168 comprising very fine aggregates of ferrite and cementite. Axles made from the 35CrMo steel grade are austenitised at �850 �C for 1800 seconds, quenched in oil, then tempered at 580 �C for 1800 seconds and finally air-cooled.167,168 4.6. Japanese grades The Japanese railway axle grades are guided by the standards JIS E4502-2-2001 179,180 for rolling stock and JRIS J0401-2007 185 for induction-hardened axles for high-speed vehicles. The JIS E502 and JRIS J0401 standards specify axle classes, chemical compositions, mechanical properties, and manufacturing require- ments. The JIS S38C (AISI 10038) is a medium car- bon steel, which is quenched, tempered, and used for producing induction-hardened axles.13 The improved chemical and mechanical properties are in the 2001 edition of JIS 179,180 and the 2007 edition of JRIS 185 standards. After forging and machining, axles are induction hardened, which induces compressive residual stresses, increasing fatigue strength.13,97,186 Carbon steels grades that are also used for axles are SFA55, SFA60, SFA65 and SFAQA. The induction heat treatment of railway axles in Japan was initiated in the early 1960s also improves yield and tensile strengths, and toughness.20 4.7. Indian standard grades The conventional steel grade used in India for railway axles is low carbon steel, although they have poor cor- rosion resistance.164 Efforts to develop alloyed steels with improved mechanical and corrosion properties led to the development of the two alloy steels; MS3 and MS6 (Table 4) at the Kanpur Institute of Table 8. Mechanical properties of various axle materials. Steel grade Yield strength (MPa) UTS (MPa) Elongation (%) References Conventional 320 550 22 8,9 MS3 400 700 21 164 MS6 404 625 22 AAR 345 607 22 15 EA1N 320 550-650 22 8,9 EA4T 420 650-800 18 35NiCrMoV12 600 800-950 13 19 30NiCrMoV12 860 975 20 35CrMo 863 982 22 167,168 LZ50 330 629 24 169 AS1440/4340 770 980 14 20 SFA55 275 540 23 SFA60/Q 295 590 20 SFA65 345 640 23 JIS S38C (AS10038) 294 539 25 179,180,181 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 181 Technology in 2013.164 The heat treatment is normal- izing, and forged railway axles were produced accord- ing to the Indian Standard ISR16/95. Electrochemical polarization and immersion tests conducted on MS3 and MS6 alloys in seawater (3.5% NaCl solution) revealed less pitting than for conven- tional alloys steels in India.164 The corrosion rates of MS3 and MS6 alloys were lower than conventional axle steel grades although the microstructures were the same, which is ferrite-pearlite. For MS3 steel, the improved corrosion resistance was attributed to the formation of nano-sized SiO4 2- ions, which promoted selective cation permeability reducing chloride ions penetration rates in the rust.187 There was less pitting in the MS3 and MS6 due to the relatively high amounts of Mn, Si, and S than the conventional grade. The MnS inclusions are effective in reducing the formation of corrosion pits due to their poor con- ductivity, which reduces the ability to form corrosion cells in MS3 steel in chloride environments.188,189 Microstructure and alloying elements are important for corrosion resistance of steels (Figure 21).164,190 Increased pearlite in ferrite-pearlite microstructures could increase metal dissolution rates.191 Pearlite reduces the corrosion resistance of steels in chloride saturated environments, due to its high cementite pro- portions. For a ferrite-pearlite microstructure, there is small anode (ferrite) compared to a large cathode (cementite lamellae), which is the exact opposite of the ferrite-bainite microstructure. Generally, fine interlamellar spacings and fine pearlite colonies enhance corrosion resistance as observed for MS3 alloys.164 A summary of the corrosion mechanism is shown in Figure 21. For a ferrite-pearlite microstruc- ture, pearlite corrodes more easily resulting in high corrosion products. The corrosion products possess high induced stresses with associated volume expan- sion which leads to cracking.191 The finer pearlite col- onies and interlamellar spacings in MS3 alloys than in conventional alloys resulted in uniform stress distribu- tions in the corrosion products, which were more sta- ble than products with stress gradients,164,190 and so had less cracking. Low carbon contents decrease pearlite amounts, hence reducing its effect on corrosion. The MS3 alloys possess low carbon contents and finer interlamellar pearlite spacing, hence the improved corrosion resist- ance, which also promotes more uniformity in the stress distribution in the corrosion product than the conventional steel. High corrosion rates in the conven- tional steel were due to relatively high carbon content and coarse interlamellar spacing in the pearlite colonies. The Indian developed alloy MS3 and MS6 steels had improved corrosion resistance under fog, immersion, and polarization tests than conventional alloy steels, as well as improved mechanical properties. Countries such as Egypt and South Africa have rail transport integrated into their transport network Figure 21. Corrosion mechanisms in pearlitic and bainitic steels with different stages of oxide formation.190 182 D. E. P. KLENAM ET AL. system. For example, South Africa has a Government Department of Science and Innovation, although most of their rail axle steel grades are proprietary. The steel grades being used there are comparable to most of the European Standards. 5. Areas for future research Rail axle materials need to meet certain critical mechan- ical and physical properties to ensure structural integ- rity as summarized in Figure 22. These properties can be induced to rail axle material without increasing cost, by exploring low cost low density rail axle steels and coatings.192 However, it is often difficult to improve all desired properties, and ductility could be compromised by increasing strength, Figure 23.193 For a typical rail axle material, the grey polygon with the black outline shows the minimum property requirement or conven- tional grade, and any property should be above the minimum. Hypothetical Alloy A meets all the criteria although thermal conductivity and creep resistance are the same as the rail axle standard compositions. However, thermal conductivity and creep resistance are not critical requirements for rail axles, Figure 22. Hypothetical Alloy B has over 80% of its properties exceeding the conventional grade, but the critical requirement for axle material, plain strain. is very poor. Although Alloy B could be potential alloy, ways to improve the plain strain should be devised. Limitations of the current rail axle steel grades are relatively low strength (�300MPa for yield strength and �600MPa for UTS), poor corrosion resistance and comparative high density.13,14,97,194 Generally, for most one main element alloy systems (where most of the constituents are dilute in concentration), strength and ductility are mutually exclusive; a gain in strength com- promises ductility and toughness.195 Light weight axles are also critical for development of trains of the future, to reduce both energy and material consumption,196 to become enablers and drivers of an eco-friendly trans- portation system. Thus, other structural materials than steels could be considered, where better performance and mechanical properties would have to be traded for higher cost and greater reliability.76,197–199 Figure 22. Critical mechanical properties of typical material for rail axles. Figure 23. Spider web chart showing how improvement of new alloys are benchmarked against commercially available alloys.193 CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 183 There are many combinations of metallic materials which could fit the requirements for rail axle materials of the future.112,181,186,196–204 However, the develop- ments in computational materials design using machine learning (ML) and artificial intelligence systems could be advantageous to reduce the choices. The traditional Edisonian approaches have been expensive and designed and desired properties are not always met. Thus, ML combined with calculation of phase diagram (CALPHAD) techniques (e.g. Thermo-Calc) should be explored to define new classes of alloys by their phases to reduce time by identifying likely compositions quickly. Targeted alloy design could lead to solving light weighting and corrosion resistance requirements of future rail axle steels. Low density steels 205–207 and complex concentrated alloys 197–199,208–211 could be explored for potential rail axle materials. “Complex concentrated alloys” (CCAs) are based on multiple principal elements with near equiatomic compo- sitions. Alloys of this type could be developed to solve the strength-ductility tradeoff due to the simultaneous devel- opment and co-existence of strengthening and toughen- ing mechanisms.212–220 This is not the case in typical carbon steels used for rail axles where there is an inverse relationship between strength and ductility.195 Some unique properties have been achieved for this new paradigm in alloy design using computational and experi- mental approaches at the nano 197–199,208,221–225 and micro-scales.212,215,226–233 For example, the strength-duc- tility synergy is achievable due to the complementary effects of toughening and strengthening mechanisms. These mechanisms include solid solution strengthening, interstitial strengthening, dislocation interactions, trans- formation induced plasticity, twining induced plasticity, stacking faults, nano-precipitation and nano twinning.234 It is generally difficult to have so many active mecha- nisms in conventional dilute alloys, hence the tradeoffs between properties.195 Similarly, the equiatomic and non- equiatomic compositions of CCAs have high flexibility to design high strength and lightweight materi- als,197–199,208,211,222 because different elements can be used, including lightweight alloying elements. Currently, research around CCAs is still developing and is based on the composition criterion, although very high cost impli- cations could discourage their use.198,208 Alloying ele- ments such as Cr, Ni, Mo, V, Nb and Ru are very expensive and using them in very high quantities increases overall cost. This was the main reason why most expensive elements were used in minute propor- tions in one main element alloys to reduce cost. Coating of rail axles using cold spraying should be explored for improved surface properties,235–237 which would increase fatigue resistance, as well as corrosion resistance. Cold spraying does not require applied heat, so there are no phase transformations to change the microstructure as well as great potential for repair.238,239 This technique has been successful for coatings on bulk titanium alloys,240,241 aluminum alloys 242,243 and on complex concentrated alloys.244,245 Cold sprayed coatings can enhance the mechanical properties at the axle surfaces, to reduce surface defects, and help prevent sites for corrosion and fatigue. The cold spraying equipment is not expensive, and the weight of the coatings increases the axle weight negligibly. 6. Summary and concluding remarks Many railway axle failures are attributed to corrosion fatigue, where corrosion of the surface provides defects for surface crack nucleation, and inclusions could lead to sub-surface crack nucleation. However, most railway axles have not been designed for corrosion resistance because priority was on strength. Thus, corrosion resist- ance of many current axle materials is not satisfactory. This is due to the generally poor corrosion resistance of carbon steels used for rail axles, and ballast impact that generate micro surface defects. Corrosion and corrosion fatigue are the main precursors of surface defect induced axle failures. Coatings have been employed to reduce surface defects, although they can be detrimental if debonding or delamination wear occurs. However, recent advances in alloy development have included corrosion properties. Finally, ways to improve efficiency and develop tools for enhanced structural integrity of rail axle materials such as coatings using cold spraying were highlighted. The need to explore complex concen- trated alloy coating materials and techniques to enhance mechanical properties of rail axles was also suggested. The implication of these results on struc- tural integrity of rail axles and areas for future research directions were discussed. ORCID D. E. P. Klenam http://orcid.org/0000-0003-1914-9633 L. H. Chown http://orcid.org/0000-0001-9699-6065 L. A. Cornish http://orcid.org/0000-0002-9291-4584 References 1. Robinson, M.; Kapoor, A. Fatigue in Railway Infrastructure; Woodhead Publishing Limited, CRC Press, Great Abington, Cambridge, UK, 2009. 184 D. E. P. KLENAM ET AL. 2. Molemaker, R.; Pauer, A. The Economic Footprint of Railway Transport in Europe, Community of European Railway, ECORYS Brussels, Belgium, 2014. 3. Human Development Report. 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