Design of a Lightweight Multifunctional Composite Railway Axle Utilising Coaxial Skins

Article Design of a Lightweight Multifunctional Composite Railway Axle Utilising Coaxial Skins

Preetum J. Mistry * , Michael S. Johnson , Charles A. McRobie and Ivor A. Jones

Composites Research Group, Faculty of Engineering, Jubilee Campus, University of Nottingham, Advanced Manufacturing Building, Nottingham NG8 1BB, UK; [email protected] (M.S.J.); [email protected] (C.A.M.); [email protected] (I.A.J.) * Correspondence: [email protected]

Abstract: The rising economic and environmental pressures associated with the generation and consumption of energy necessitates the need for lightweighting of railway vehicles. The railway axle is a prime candidate for lightweighting of the unsprung mass. The reduction of unsprung mass correlates to reduced track damage, energy consumption and total operating costs. This paper presents the design of a lightweight multifunctional hybrid metallic-composite railway axle utilising coaxial skins. The lightweight axle assembly comprises a carbon fibre reinforced polymer composite tube with steel stub axles bonded into either end. The structural hybrid metallic-composite railway axle is surrounded by coaxial skins each performing a specific function to meet the secondary requirements. A parametric sizing study is conducted to explore the sensitivity of the design parameters of the composite tube and the stub axle interaction through the adhesive joint. The optimised design parameters of the axle consist of a; composite tube outer diameter of 225 mm, composite tube thickness of 7 mm, steel stub axle extension thickness of 10 mm and a bond overlap length of 100 mm. The optimised hybrid metallic-composite railway axle design concept has a mass Citation: Mistry, P.J.; Johnson, M.S.; of 200 kg representing a reduction of 50% over the solid steel version. McRobie, C.A.; Jones, I.A. Design of a Lightweight Multifunctional Keywords: railway axle; lightweighting; composite material; multifunctional; unsprung mass; Composite Railway Axle Utilising coaxial skins Coaxial Skins. J. Compos. Sci. 2021, 5, 77. https://doi.org/10.3390/ jcs5030077 1. Introduction Academic Editor: Frédéric Jacquemin The worldwide imperative to reduce carbon dioxide (CO2) emissions poses a challenge to the transportation sector which accounts for approximately 25% of the total emissions Received: 4 February 2021 generated [1]. Vehicle lightweighting is a means of reducing CO emissions as a conse- Accepted: 5 March 2021 2 Published: 7 March 2021 quence of less energy being required for propulsion. This is recognised by the European Rail Sector Sustainable Mobility Strategy 2010 [2], which identifies fibre reinforced polymer

Publisher’s Note: MDPI stays neutral (FRP) composites as a means of reducing the weight of rail vehicles. with regard to jurisdictional claims in While the structural carbody represents 24% of the rail vehicle mass, bogies are published maps and institutional affil- the single largest mass contributor accounting for 41% of the total tare weight [3]. A iations. bogie is a chassis containing the wheelsets while supporting the carbody through the primary suspension. The wheelset, comprising the axle and wheels, represents the majority of the unsprung mass. This mass gives rise to track impact damage. Renewal of the track represents a significant infrastructure maintenance cost, which contributes to service interruptions and travel downtime. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. A railway axle typically is hot forged from EA1N grade steel [4]. Conventional This article is an open access article steel railway axles are solid. However, lightweighting is common by boring the centre distributed under the terms and of the solid axle with further mass savings being possible using alternative materials, conditions of the Creative Commons including FRP composites [5]. A FRP composite railway axle has yet to be put into service. Attribution (CC BY) license (https:// However, a prototype, filament wound, carbon fibre reinforced polymer (CFRP) tube axle creativecommons.org/licenses/by/ was manufactured for the British Rail Advanced Passenger Train [6]. This initial concept 4.0/). showed a mass savings of 70%. The behaviour of the composite axle was satisfactory in

J. Compos. Sci. 2021, 5, 77. https://doi.org/10.3390/jcs5030077 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, x 2 of 12

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conditions of the Creative Commons a mass savings of 70%. The behaviour of the composite axle was satisfactory in both static Attribution (CC BY) license and fatigue tests. However, the impact behaviour of the composite tube was found to be (http://creativecommons.org/licenses bothpoor. static Current and fatigueresearch tests. into However,a FRP composite the impact railway behaviour axle is of the the subject composite of the tube Horizon was /by/4.0/). found2020, toShift2Rail be poor. programme Current research entitled into NEXTGEAR a FRP composite [7]. Within railway this axle project, is the design subject concepts of the Horizonfor a hybrid 2020, metallic-composite Shift2Rail programme (HMC) entitled railway NEXTGEAR axle were [ 7proposed]. Within this[8]. These project, axles design em- conceptsbodied the for concept a hybrid of metallic-composite a multifunctional CFRP (HMC) railway railway axle. axle were proposed [8]. These axles embodiedThis paper the presents concept the of conceptual a multifunctional design CFRP of a multifunctional railway axle. HMC railway axle This paper presents the conceptual design of a multifunctional HMC railway axle which utilises coaxial skins to meet specific requirements with the overriding aim to min- which utilises coaxial skins to meet speciﬁc requirements with the overriding aim to imise mass. The relevant coaxial skins will be described followed by a parametric sizing minimise mass. The relevant coaxial skins will be described followed by a parametric study of the structural HMC railway axle assembly. sizing study of the structural HMC railway axle assembly.

2.2. RailwayRailway AxleAxle ConfigurationsConfigurations AA railway railway axle axle is is designed designed with with the the supporting supporting bearings bearings being being either either outboard outboard of of the the wheelswheels or or inboard. inboard. As As the the track track span span is is fixed, fixed, an an outboard outboard bearing bearing design design has has the the capacity capacity forfor the the greatest greatest mass mass reduction reduction as as it it features features a a longer, longer, plain plain shaft shaft region, region, suitable suitable for for CFRP CFRP replacement.replacement. As a a result, result, an an outboard outboard bearing bearing configuration configuration is isadopted adopted for for this this study. study. An Anaxle axle having having an inboard an inboard bearing bearing architecture architecture generally generally results results in a inlower a lower overall overall bogie bogie mass mass[9] and [9] would and would also alsobenefit benefit from from the themultifunctional multifunctional principles principles described described within within this this pa- paper.per. AA trailertrailer bogiebogie features features the the simplest simplest railway railway axle axle configuration configuration with with no no additional additional drivedrive elements elements mounted mounted onto onto the the shaft. shaft. Furthermore, Furthermore, brake brake discs discs can can be fitted be fitted either either to the to wheelthe wheel webs webs or outboard or outboard of the of wheels. the wheels. A typical, A ty solidpical, steel solid railway steel railway axle for axle a trailer for a bogie,trailer configuredbogie, configured for outboard for outboard bearings, bearings, is shown is in shown Figure in1. Figure The main 1. The features main of features the axle of are the theaxle wheel are the seats, wheel bearing seats, seatsbearing and seats the centraland the plain central shaft plain region. shaft Theregion. mass The of mass this axle of this is approximatelyaxle is approximately 400 kg. 400 kg.

FigureFigure 1.1. AnAn outboardoutboard bearing bearing trailer trailer axle axle with with typical typical dimensions dimensions illustrating illustrating the the wheel wheel seat, seat, bearing seatbearing and seat plain and shaft plain region. shaft The region. global The coordinate global coordinate system shown system is shown in accordance is in accordance with EN 131031-1.with EN 131031-1. 3. Composite Railway Axle Design Incorporating Coaxial Skins 3. CompositeThe concept Railway design Axle of a multifunctional Design Incorporating composite Coaxial railway Skins axle utilising coaxial skins is shownThe inconcept Figure design2. The of design a multifunctional philosophy comp for thisosite axle railway concept axle was utilising derived coaxial from skins the constructionis shown in ofFigure high 2. voltage The design underground philosophy power for this cables. axle These concept cables was are derived comprised from the of severalconstruction layers of to protecthigh voltage the conductor underground and to po meetwer thecables. mechanical, These cables electrical are comprised and thermal of requirementsseveral layers [ 10to]. protect In a similar the conductor way, the structuraland to meet requirements the mechanical, of the electrical axle are and met thermal by the hollowrequirements HMC railway [10]. In axle a similar assembly way, comprising the structural a CFRP requirements tube into whichof the metallicaxle are stubmet by axles the are bonded. Coaxial skins are included to fulﬁl secondary functional requirements. A hollow HMC railway axle assembly comprising a CFRP tube into which metallic stub ax- tensioned tether would run along the axis of the hollow axle attached to the stub axles at les are bonded. Coaxial skins are included to fulfil secondary functional requirements. A either end. tensioned tether would run along the axis of the hollow axle attached to the stub axles at either end.

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Figure 2. Cross-section of a design concept for the hybrid metallic-composite railway axle with a Figurestructural 2. Cross-section composite core ofa and design coaxial concept skinsfor (Note: the hybridtether mechanism metallic-composite not shown). railway axle with a structuralFigure 2. Cross-section composite core of and a design coaxial concept skins (Note:for the tetherhybrid mechanism metallic-composite not shown). railway axle with a structuralConstruction composite coreof the and coaxial coaxial CFRP skins tube(Note: is tether by the mechanism established not roll shown). wrapping technique. EachConstruction layer of the CFRP of the tube coaxial would CFRP be tubeavailable is by in the planar established format, roll for wrapping example as technique. a prepreg Eachtextile layerConstruction reinforcement of the CFRP of the or tube coaxiala conformable would CFRP be available tube foam is bysheet. in the planar establishedThe format, wall thickness forroll example wrapping of the as technique. aCFRP prepreg tube textile reinforcement or a conformable foam sheet. The wall thickness of the CFRP tube Eachcould layer be madeof the inCFRP stages, tube not would only be for available the application in planar of format, the bespoke for example skins, as but a prepreg for the could be made in stages, not only for the application of the bespoke skins, but for the accu- textileaccumulation reinforcement of a sufficient or a conformable thickness. foam This sheet.would The aid wallin adhering thickness to of the the deflection CFRP tube re- mulation of a sufﬁcient thickness. This would aid in adhering to the deﬂection requirement couldquirement be made while in ensuring stages, not minimal only fordefects the applicationwithin the thick of the section bespoke tube. skins, but for the whileaccumulation ensuring of minimal a sufficient defects thickness. within the This thick would section aid tube. in adhering to the deflection requirement while ensuring minimal defects within the thick section tube. 3.1.3.1. The The Structural Structural Hybrid Hybrid Metallic-Composite Metallic-Composite (HMC) (HMC) Railway Railway Axle Axle Assembly Assembly The principal Standard for the structural design of an outboard railway axle is EN 3.1. The Structural principal Hybrid Standard Metallic-Com for the structuralposite (HMC) design Railway of an Axle outboard Assembly railway axle is EN 13103-113103-1 [4 [4].]. Compliance Compliance with with the the load load cases cases within within this this Standard Standard are are met met by by the the structural structural The principal Standard for the structural design of an outboard railway axle is EN HMCHMC railway railway axle axle assembly assembly illustrated illustrated in in Figure Figure3. 3. 13103-1 [4]. Compliance with the load cases within this Standard are met by the structural HMC railway axle assembly illustrated in Figure 3.

FigureFigure 3. 3.Construction Construction ofof thethe unoptimised structural hybrid hybrid metallic-composite metallic-composite railway railway axle axle de- de-

signsign concept. concept. Figure 3. Construction of the unoptimised structural hybrid metallic-composite railway axle de- 3.1.1.sign3.1.1. concept. Carbon Carbon Fibre Fibre Reinforced Reinforced Polymer Polymer (CFRP) (CFRP) Composite Composite Tube Tube TheThe CFRP CFRP tube tube comprises comprises high high modulus, modulus, carbon carbon fibre fibre reinforcement reinforcement within within an an epoxy epoxy 3.1.1. Carbon Fibre Reinforced Polymer (CFRP) Composite Tube matrix.matrix. The The strength strength of of the the axle axle is is dominated dominated by by the the fatigue fatigue properties properties as as the the axle axle has has a a serviceserviceThe life life CFRP on on the tubethe order order comprises of of 30 30 years highyears ormodulus, or more more [11 [11].ca].rbon This This fibre equates equates reinforcement to to a a high high numberwithin number an of ofepoxy load load 9 cycles,matrix.cycles, on Theon the the strength order order of of 10 the10reverse9 axlereverse is bendingdominated bending cycles. cycles. by the Four-point Four-point fatigue properties bending bending isas is the thethe main axlemain loadhas load a caseservicecase with with life the the on addition addition the order of of torsionalof torsional 30 years loading loading or more under under [11]. specific specific This equates braking braking to conditions conditionsa high number and and dynamic dynamic of load wheelsetcycles,wheelset on hunting. hunting.the order A A highof high 10 axle9 axlereverse stiffness stiffness bending for for the thecycles. CFRP CFRP Four-point tube tube is is necessary necessary bending to tois suppress thesuppress main crack crackload growthcasegrowth with within within the addition the the matrix,matrix, of torsional whilewhile limiting loading deflec deflectionundertion specific at at the the braking bearings bearings conditions and and drive drive and elements elements dynamic for forwheelset powered hunting. axle variants. A high axle The highstiffness stiffness for the is requiredCFRP tube to limitis necessary strain within to suppress the matrix. crack Undergrowth this within low the strain matrix, condition while micro limiting crack deflec initiationtion at isthe inhibited bearings such and thatdrive larger elements cracks for

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powered axle variants. The high stiffness is required to limit strain within the matrix. Un- J. Compos. Sci. 2021, 5, 77 der this low strain condition micro crack initiation is inhibited such that larger cracks4 of do 12 not form. In the event of crack formation with propagation along a fibre, placement of off- axis fibres within the laminate stack act to suppress further growth. do notDynamic form. In conditions the event ofinclude crack formationoperational with frequencies propagation (approximately along a fibre, 60–70 placement Hz tor- of sional,off-axis 90–110 fibres withinHz bending) the laminate as well stack shock act and to suppressvibration furtherlimits. The growth. former can lead to rail corrugationsDynamic if conditionsundamped, include while the operational latter is encountered frequencies (approximatelywhen the wheel 60–70 rolls Hzover tor- a tracksional, irregularity. 90–110 Hz bending)A first indication as well shock of the and vibration vibration level limits. of the The axle former can be can found lead toin railthe Standardcorrugations BS ifEN undamped, 61373 [12]. while Shock the loading latter is e encounteredffects are typically when the assessed wheel rolls at wheelset over a track as- semblyirregularity. level A using first indicationa roller rig of as the part vibration of a laboratory level of the testing axle can programme. be found in The the Standarddynamic testBS ENconditions 61373 [12 specified]. Shock are loading for a steel effects axle are an typicallyd it is deemed assessed appropriate at wheelset to assemblyapply these level to theusing CFRP a roller tube rig as aswell, part to of ensure a laboratory the same testing dynamic programme. test evaluation The dynamic conditions. test conditions specified are for a steel axle and it is deemed appropriate to apply these to the CFRP tube 3.1.2.as well, Steel to ensureStub Axle the same dynamic test evaluation conditions. The stub axle is manufactured from the same EA1N steel as the traditional steel axle. The3.1.2. geometry Steel Stub of Axlethe current steel axle is maintained from the end up to and including the wheelThe seat. stub This axle approach is manufactured allows the from existing the same wheel EA1N and steel bearing as the solutions traditional to steelbe main- axle. tained.The geometry Further of inboard the current of the steel wheel axle seat, ismaintained an extension from is added the end for upinsertion to and into including the CFRP the tube,wheel including seat. This a approach shoulder allows to define theexisting the overall wheel axle and length bearing (not solutions shown toin beFigures). maintained. The geometryFurther inboard of the extension of the wheel effects seat, the an performance extension is addedof the adhesive for insertion joint. into the CFRP tube, including a shoulder to define the overall axle length (not shown in Figures). The geometry 3.1.3.of the Epoxy extension Adhesive effects Joint the performance of the adhesive joint.

3.1.3.Epoxy Epoxy adhesive Adhesive is Jointapplied to the stub axle extensions for bonding into the CFRP tube. Optimisation of the adhesive bonded joint is necessary in relation to: 1. the length of CFRP Epoxy adhesive is applied to the stub axle extensions for bonding into the CFRP tube to stub axle extension overlap, 2. the wall thicknesses of the CFRP tube and the stub tube. Optimisation of the adhesive bonded joint is necessary in relation to: 1. the length axle extension. The adhesive thickness is fixed at 0.2 mm as design best practice [13]. Tech- of CFRP tube to stub axle extension overlap, 2. the wall thicknesses of the CFRP tube niques may be required to limit peel stresses at the ends of the joint. This could include and the stub axle extension. The adhesive thickness is ﬁxed at 0.2 mm as design best localised substrate wall thickening, reinforcing collars, tapering at the ends of the sub- practice [13]. Techniques may be required to limit peel stresses at the ends of the joint. This strates, or a combination of these solutions. could include localised substrate wall thickening, reinforcing collars, tapering at the ends of the substrates, or a combination of these solutions. 3.2. Multifunctional Coaxial Skins 3.2. MultifunctionalThe structural Coaxialand secondary Skins requirements for a railway axle are encapsulated withinThe the structural Standard and EN secondary13103-1 and requirements are illustrated for ain railway Figure axle4. are encapsulated within the Standard EN 13103-1 and are illustrated in Figure4.

Figure 4. Structural and secondary requirements for the design of a hybrid metallic- Figure 4. Structural and secondary requirements for the design of a hybrid metallic-composite composite wheelset. wheelset. Compliance with the secondary requirements is achieved through the use of coaxial skins integrated with the structural HMC railway axle assembly. These skins are illustrated in the main design concept shown in Figure2. Clearly, other skins with unique functions could be incorporated into the overall design.

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3.2.1. Layer 1-Structural Health Monitoring A means of measuring changes in integrity of the structural HMC railway axle assembly will be required for risk mitigation and product certification. This, in the form of a quantitative inspection capability, will ensure that the challenging 30 year service life of the axle can be met. Non-destructive testing (NDT) is preferred for inspection with ultrasonic testing (UT) being common for hollow steel axles. For a HMC axle, attenuation of the UT source is a concern as defects may not be detectable. Furthermore, steps within the axle bore present complexity in sensing using ultrasonics. As an alternative, a structural health monitoring (SHM) layer in the form of strain sensing fibres would provide continuous feedback of the integrity of the CFRP tube. In addition, localised use of “leaky” optical fibres along the bond lines could provide UV curing of the adhesive joint and the potential for bond degradation at the end of life. A visual, trackside measure of structural integrity for the HMC axle is proposed using a tether apparatus. This comprises a tensioned cable which is secured at one end of the axle while being fastened to a force gauge at the other end. The gauge is mounted within the stub axle body giving visual feedback of the tether tension (for example green, amber or red). A reduction in tension (indicated as yellow or red on the gauge) would prompt inspection to resolve whether damage had occurred to either the CFRP tube or the bond between the tube and stub axle. Furthermore, the tether would be engineered to prevent detachment of the wheel should a catastrophic failure to the axle occur.

3.2.2. Layer 2-Fire Protection The operating temperature (BS EN 50125-1) for the axle is from−40 ◦C to +70 ◦C[14]. Limited exposure to higher temperatures via disc braking, heat radiation and cleaning fluid applications demand a glass transition temperature (Tg) for the CFRP tube in excess of 100 ◦C. The railway requirements for fire, smoke and toxicity (FST) performance are demanding and are described in the Standard EN 45545-2 [15]. This criterion is a challenge for thermosetting polymers, especially when operating in enclosed areas such as tunnels. The allowable operating temperature of the CFRP tube is on the order of 120 ◦C. While standard operating conditions can be met, barrier protection is required to adhere to the FST requirements. Approaches to FST management include protecting the CFRP tube surface from high temperature damage using insulation, or actively dispersing heat from a localised heat source. Insulation is available in mat or blanket form, comprising ceramic-based fibres. Thermal break insulation is afforded by an aramid honeycomb with an external skin of glass or aramid fibre in a phenolic matrix. Alternatively, a unidirectional carbon fibre epoxy laminate has good thermal conduction (~7 W/mK [16]) in the axial direction. These 0◦ fibres could be used for both structural performance and to provide axial heat dispersion along the CFRP tube to the metallic stub axles.

3.2.3. Layer 3-Impact Protection Ballast impact, an issue for steel axles, is a greater concern for a HMC axle as it can lead to ﬁbre breakage, delamination and can accelerate matrix crack growth under fatigue. Localised damage may occur while undertaking maintenance. Currently, a coating called “LURSAK” developed collaboratively by Lucchini RS and Akzo Nobel Aerospace is used widely for ballast protection [17]. This solution equally may be applicable to a HMC axle. An alternative approach is to employ a layer of tough reinforcement such as Kevlar or Dyneema. This could be enhanced further using a thin foam beneath the durable reinforcement. This would provide a lightweight, conformable zone around the structural CFRP tube.

3.2.4. Layer 4-Environmental Protection The exterior skin of the tube acts as a chemically inert barrier against corrosive elements such as degreasing ﬂuids or lubricants used on the railway. This is based on a J. Compos. Sci. 2021, 5, 77 6 of 12

similar rationale to the use of an outer sheath around high voltage underground power cables to provide mechanical resistance and protection from the environment [18]. For the axle, a tough, thermoplastic material is recommended. Importantly, this barrier should be sealed to prevent moisture ingress to the interior layers, particularly the structural CFRP tube. Ultraviolet (UV) protection to avoid long term polymer degradation could be a feature of this skin. Embedding a copper mesh within the skin would provide electromagnetic compatibility (EMC) shielding to prevent interference with the SHM equipment. Lastly, pigmentation of this layer could be related to different axle weights, maintenance indicators, manufacture date or other bespoke categories.

4. Sizing of the Structural HMC Railway Axle The axle is subject to four-point bending with the maximum bending moment oc- curring at the centreline of the wheels and remaining constant between those running surfaces. The joint between the stub axle extension and the composite tube is subjected to this maximum bending moment. For this reason, the joint is at a critical section. Therefore, this study will focus on the parameters of the CFRP tube and the stub axle interaction through the adhesive joint. A parametric sizing study is undertaken to arrive at the outer and inner diameters of the composite tube along with the overlap length between the composite tube and the steel stub axle. As stated, the strength requirements of the HMC railway axle will be met through compliance with loadings in EN 13103-1. Direct shear occurs in the axle between the bearing and the wheel. While this will give rise to transverse shear, the dimensions of the existing steel axle are maintained and so were not the focus of this study. A secondary sizing would be necessary to meet stiffness requirements for limiting fatigue crack growth within the matrix and to ensure angular misalignment at the bearings is within speciﬁcation. Furthermore, the parametric study assumes no structural beneﬁts through addition of the multifunctional layers around the CFRP tube.

4.1. Material Properties Material properties and performance criterion are established for the three structural components of the composite axle: 1. the carbon ﬁbre reinforced epoxy composite tube; 2. the steel stub axles; and 3. the epoxy adhesive. The Cambridge Engineering Selector (CES EduPack software [19], of Granta Design Limited was used to identify materials for the HMC railway axle following the CES approach to material selection illustrated by Ashby and Cebon [20]. The EA1N grade railway steel is not present in the CES database, therefore, ‘carbon steel, AISI 1030, normalised’ is used as a close approximation by composition. A listing of the material properties is provided in the AppendixA.

4.2. CFRP Tube Modelling Conditions A composite layup was created using Abaqus CAE [21] with alignment to the global railway axle coordinates shown in Figure1. Based on the low torsional loading being applied in comparison to the bending load on the axle, a layup was specified comprising 90% of fibres along the axis of the tube (0◦) and the remaining 10% of fibres in the hoop direction (90◦). Further layers of ±45◦ fibres could be added subsequently to align with torsional inputs, to provide protection to the 0◦ fibres and facilitate a progressive fibre angle variation through the laminate stack. After conducting a sensitivity study, a global mesh size of 7.5 mm with 36 radial elements was chosen to accurately, and efficiently capture the feature details. A computational run time of approximately 2.5 min was achieved which allowed convergence to optimal model parameters within a manageable time frame. J. Compos. Sci. 2021, 5, x 7 of 12

After conducting a sensitivity study, a global mesh size of 7.5 mm with 36 radial elements was chosen to accurately, and efficiently capture the feature details. A computa- J. Compos. Sci. 2021, 5, 77 7 of 12 tional run time of approximately 2.5 min was achieved which allowed convergence to optimal model parameters within a manageable time frame.

4.3. Parametric4.3. Study Parametric of the Stru Studyctural of theHMC Structural Railway HMC Axle RailwayAssembly Axle Assembly The most relevantThe most parameters relevant to parameters be varied tofor be the varied HMC for railway the HMC axle railwayassembly axle are assembly are the CFRP tubethe outer CFRP diameter tube outer (𝐷 diameter), CFRP tube (Do), thickness CFRP tube (𝑡) thickness, steel stub (tt )axle, steel extension stub axle extension thickness (𝑡),thickness and the bond (ts), and overlap the bond length, overlap (𝐿), as length, shown (L bin), Figure as shown 5. in Figure5.

Figure 5. IllustrationFigure 5. of Illustration the parameters of the for parameters optimisation for ofoptimisati the structuralon of the hybrid structural metallic-composite hybrid metallic-compo- railway axle assembly. site railway axle assembly. For a fixed CFRP outer diameter, the inner diameter (Di), and hence thickness, follows For a fixedthe CFRP analytical outer solution diameter, whereby the inner a larger diameter diameter (𝐷), and tube hence requires thickness, a smaller fol- thickness to lows the analyticalsustain solution an equivalent whereby maximum a larger diameter stress. This tube is requires evident a from smaller the thickness equation for bending to sustain anstress equivalent [22]: maximum stress. This is evident from the equation for bending My D π stress [22]: σ = , where y = o and I = D4 − D4 (1) bending I 2 64 o i 𝑀𝑦 𝐷 𝜋 𝜎 = The,𝑤ℎ𝑒𝑟𝑒 maximum 𝑦= bending 𝑎𝑛𝑑 stress, 𝐼= σbending(𝐷 −𝐷, occurs ) under the applied bending(1) moment, M, a distance,𝐼 y, from the2 neutral axis64 for the CFRP tube having a second moment of area, I. The maximumThe CFRP bending tube isstress, defined 𝜎 by the, occurs inner and under outer the diameters, applied bendingDi and moment,Do, respectively. 𝑀, a distance, 𝑦, fromApplying the neutral the maximum axis for the allowable CFRP tu compressivebe having a second stress of moment 250 MPa of forarea, the CFRP tube, 𝐼. The CFRP tubethe corresponding is defined by the inner inner diameter and outer and diameters, thickness are𝐷 and calculated. 𝐷, respectively. Using these dimensions Applyingalong the maximum with the density allowable of the compressive CFRP tube stress material, of 250 the MPa mass for of the the CFRP CFRP tube, tube is found to the correspondingreduce inner as the diameter outer diameter and thickn increasesess are as calculated. shown in Figure Using6. these The mass dimensions of the HMC railway along with theaxle density assembly, of the however, CFRP tube increases material, with the increasing mass of the CFRP CFRP tube tube diameter. is found This to is a result of reduce as thethe outer mass diameter of the increases steel stub as axle shown extension in Figure (fixed 6. The at amass length of the of HMC 200 mm) railway correspondingly axle assembly,increasing however, with increases the CFRP with tubeincreasing diameter. CFRP This tube suggests diameter. that This to minimiseis a result theof mass of the the mass of theHMC steel railway stub axle,axle extension the entire assembly,(fixed at a including length of the 200 stub mm) axle correspondingly mass, requires consideration, increasing withrather the thanCFRP simply tube diameter. the mass ofThis the suggests CFRP tube that itself. to minimise the mass of the HMC railway axle,The the outer entire diameter assembly, of including the CFRP the tube stub is fixedaxle mass, at 225 requires mm and considera- the mass of the stub tion, rather thanaxle simply is minimised. the mass This of the is achieved CFRP tube by itself. hollowing the stub axle extension as it increases in volume proportionally with an increase in the CFRP tube diameter. Hollowing the stub axle extension to reduce the thickness of the steel at the overlap leads to an associated increase in stress within the steel. This is indicated in Figure7, which shows that, as the thickness of the steel increases in the stub axle extension, the stress reduces. However, beyond approximately 30 mm a plateau in the stress value is reached whereby further increases in thickness does not reduce the stress further. Note that, as expected, the stress in the CFRP tube is not affected by changes to the thickness in the overlap of the stub axle. However, stress in the adhesive reduces as the thickness of the steel reduces. This suggests that the wall of the steel stub axle extension is deflecting, relieving an edge binding effect caused by the stub axle locally compressing the adhesive layer. J. Compos. Sci. 2021, 5, x 8 of 12

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Figure 6. Reduction in the carbon fibre reinforced polymer (CFRP) composite tube mass with increasing diameter and the competing gain in the railway axle assembly mass.

The outer diameter of the CFRP tube is fixed at 225 mm and the mass of the stub axle is minimised. This is achieved by hollowing the stub axle extension as it increases in volume proportionally with an increase in the CFRP tube diameter. Hollowing the stub axle extension to reduce the thickness of the steel at the overlap leads to an associated increase in stress within the steel. This is indicated in Figure 7, which shows that, as the thickness of the steel increases in the stub axle extension, the stress reduces. However, beyond approximately 30 mm a plateau in the stress value is reached whereby further increases in thickness does not reduce the stress further. Note that, as expected, the stress in the CFRP tube is not affected by changes to the thickness in the overlap of the stub axle. However, stress in the adhesive reduces as the thickness of the steel reduces. This suggests that the Figurewall of 6. 6. the ReductionReduction steel stub in inthe axle the carbon carbonextension fibre ﬁbre reinforced is reinforceddeflec ting,polymer polymer relieving (CFRP) (CFRP) an composite edge composite binding tube tubemass effect masswith caused in- with creasingincreasingby the stub diameter diameter axle andlocally and the the compressingcompeting competing gain gain the in intheadhesive the railway railway layer. axle axle assembly assembly mass. mass.

The outer diameter of the CFRP tube is fixed at 225 mm and the mass of the stub axle is minimised. This is achieved by hollowing the stub axle extension as it increases in volume proportionally with an increase in the CFRP tube diameter. Hollowing the stub axle extension to reduce the thickness of the steel at the overlap leads to an associated increase in stress within the steel. This is indicated in Figure 7, which shows that, as the thickness of the steel increases in the stub axle extension, the stress reduces. However, beyond approximately 30 mm a plateau in the stress value is reached whereby further increases in thickness does not reduce the stress further. Note that, as expected, the stress in the CFRP tube is not affected by changes to the thickness in the overlap of the stub axle. However, stress in the adhesive reduces as the thickness of the steel reduces. This suggests that the wall of the steel stub axle extension is deflecting, relieving an edge binding effect caused by the stub axle locally compressing the adhesive layer.

FigureFigure 7.7. MaximumMaximum stressstress inin each each component component of of the the hybrid hybrid metallic-composite metallic-composite railway railway axle axle assembly as- oversembly a range over ofa range stub axleof stub extension axle extension thicknesses. thicknesses.

AA decisiondecision waswas taken taken to to fix fix the the thickness thickness of theof the steel steel within within the bondthe bond overlap overlap to 10 to mm. 10 Atmm. this At thickness, this thickness, the stress the in stress the steel in the is below steel theis below allowable the stressallowable of 185 stress MPa, of minimising 185 MPa, theminimising mass of thethe stubmass axle, of the hence stub theaxle, overall hence HMC the overall railway HMC axle railway assembly axle mass. assembly mass. AnAn analysisanalysis ofof thethe bondbond overlapoverlap isis requiredrequired toto fixfix thethe parametersparameters associatedassociated withwith thethe jointjoint itself.itself. HavingHaving donedone this,this, thethe overalloverall CFRPCFRP tubetube cancan bebe specified.specified. FigureFigure8 8 shows shows the the relationshiprelationship betweenbetween thethe overlapoverlap lengthlength ofof thethe bondbond andand thethe stressesstresses thatthat developdevelop withinwithin thethe components.components. It is evident that the stress inin thethe steelsteel reducesreduces withwith anan increaseincrease inin thethe overlap length up to approximately 100 mm of overlap. The stresses within the CFRP tube and the epoxy adhesive are less sensitive to the overlap length. For this reason, the overlap length of the adhesive bond is fixed at 100 mm. FigureWhile 7. Maximum the stress stress within in each the component steel stub axle of the has hybrid been metallic-composite reduced below the railway design axle allowable as- semblyof 185 MPa,over a the range stress of stub within axlethe extension adhesive thicknesses. remains at 70 MPa which is above the allowable stress of 40 MPa. This peak stress in the adhesive occurs in a small region at the inner end of the overlappingA decision region.was taken To mitigateto fix the this, thickness the thickness of the ofsteel the within CFRP tubethe bond is increased overlap locally to 10 mm.from 7At mm this to thickness, 15 mm, by the overwinding stress in the hoop steel (90 is◦) below fibres. Thisthe allowable reduces the stress compressive of 185 MPa, and minimisingpeel loads significantly the mass of bythe improving stub axle, thehence bending the overall stiffness HMC of therailway CFRP axle tube assembly in the overlap mass. region.An Having analysis stiff of the adherends bond overlap in the jointis required region, to however, fix the parameters increases the associated peak shear with stress. the jointAdding itself. short Having tapers done to the this, outer the diameter overall CFRP of the tube CFRP can tube be specified. and the inner Figure diameter 8 shows of the relationshipsteel stub axle between extension the provides overlap compliancelength of the at bond the edges and the of the stresses joint and that reduces develop the within peak thestress components. in these positions. It is evident that the stress in the steel reduces with an increase in the

J. Compos. Sci. 2021, 5, x 9 of 12

overlap length up to approximately 100 mm of overlap. The stresses within the CFRP tube J. Compos. Sci. 2021, 5, 77 9 of 12 and the epoxy adhesive are less sensitive to the overlap length. For this reason, the overlap length of the adhesive bond is fixed at 100 mm.

Figure 8. MaximumFigure stress 8. Maximum in each component stress in each of the component hybrid metallic-composite of the hybrid metallic-composite railway axle assembly railway over axle a rangeas- of bond overlap lengths.sembly over a range of bond overlap lengths.

While 4.4.the stress Finalised within Parameters the steel of thestub HMC axle Railwayhas been Axle reduced below the design allowable of 185 MPa,A the structural stress within HMC railwaythe adhesive axle hasremains been designedat 70 MPa to which meet theis above load casesthe with the allowable stressStandard of 40 EN MPa. 13103-1 This withpeak thestress overall in the objective adhesive of occurs reducing in a the small mass. region Parameters at the identified inner end offor the optimisation overlapping of region. the HMC To railwaymitigate axle this, assembly the thickness were of the the CFRP CFRP tube tube outer is diameter increased locally(Do), CFRPfrom 7 composite mm to 15 mm, tube by thickness overwinding (tt), steel hoop stub (90°) axle fibres. extension This reduces thickness the (t s), and the compressivebond and overlappeel loads length significantly (Lb). by improving the bending stiffness of the CFRP tube in the overlapIntuitively, region. Having the diameter stiff adhe of therends composite in the joint axle region, would however, be maximised increases to benefit from the peak shearthe stress. high second Adding moment short tapers of area, to theI, in outer relation diameter to bending. of the CFRP This tube was and expected the to bring inner diameterthelargest of the masssteel stub reduction. axle extens However,ion provides the study compliance revealed that at the as theedges CFRP of the tube diameter joint and reducesincreased, the peak so did stress the diameter in these positions. of the considerably heavier stub axle that was joined with it. While the tube became lighter with increasing diameter, the assembly with stub axles 4.4. Finalisedbecame Parameters heavier. of the Hence HMC the Railway CFRP Axle tube diameter was fixed at 225 mm and the wall thickness A structuralwas set HMC at 7 mm, railway as the axle joint has performance been designed was to relatively meet the insensitive load cases to with this parameter.the Standard EN 13103-1Having with defined the overall the CFRP objectiv tubee of dimensions, reducing the the mass. stub Parameters axle joint identi- parameters were fied for optimisationaddressed. of Increasing the HMCthe railway thickness axle ofassembly the stub were axle extensionthe CFRP didtube reduce outer thediam- internal stress, however, the stress within the adhesive increased. The thickness of the stub axle extension eter (𝐷), CFRP composite tube thickness (𝑡), steel stub axle extension thickness (𝑡), and was fixed at 10 mm as a trade-off. Increasing the stub axle overlap with the CFRP tube the bond overlap length (𝐿). reduced the stress, however, beyond 100 mm there was no appreciable benefit. Intuitively, the diameter of the composite axle would be maximised to benefit from The strength allowable in the adhesive is considerably lower than that of the steel stub the high second moment of area, I, in relation to bending. This was expected to bring the axle or CFRP tube. Adoption of a square unoptimised geometry for the stub axle extension largest mass reduction. However, the study revealed that as the CFRP tube diameter in- resulted in high stresses in the adhesive. creased, so did the diameter of the considerably heavier stub axle that was joined with it. The final dimensions of the structural HMC railway axle assembly are shown in While the tube became lighter with increasing diameter, the assembly with stub axles be- Figure9. These parameters have been defined in observation of the allowable stresses. The came heavier. Hence the CFRP tube diameter was fixed at 225 mm and the wall thickness mass of the optimised axle is 200 kg (50% mass reduction). was set at 7 mm, as the joint performance was relatively insensitive to this parameter. Further optimisation of the design can be carried out to include: Having defined the CFRP tube dimensions, the stub axle joint parameters were ad- • dressed. IncreasingA stiffness the thickness analysis of ofthe the stub HMC axle railway extension axle did assembly reduce tothe minimise internal stress, deflections at the however, the stressbearings, within strains the adhesive within increase the CFRPd. tubeThe thickness matrix and of shaftthe stub whirling axle extension effects. • Addition of off axis fibres to address levels of torsional loading and to protect the load was fixed at 10 mm as a trade-off. Increasing the stub axle overlap with the CFRP tube bearing axial fibres. reduced the stress, however, beyond 100 mm there was no appreciable benefit. • Specifying short, compliant tapers to the stub axle extension and CFRP tube to reduce The strength allowable in the adhesive is considerably lower than that of the steel peak stresses in the bond. stub axle or CFRP tube. Adoption of a square unoptimised geometry for the stub axle extension resulted in high stresses in the adhesive.

J. Compos. Sci. 2021, 5, x 10 of 12

The final dimensions of the structural HMC railway axle assembly are shown in Fig- ure 9. These parameters have been defined in observation of the allowable stresses. The mass of the optimised axle is 200 kg (50% mass reduction). Further optimisation of the design can be carried out to include: • A stiffness analysis of the HMC railway axle assembly to minimise deflections at the bearings, strains within the CFRP tube matrix and shaft whirling effects. • Addition of off axis fibres to address levels of torsional loading and to protect the load bearing axial fibres. J. Compos. Sci. 2021, 5, 77 10 of 12 • Specifying short, compliant tapers to the stub axle extension and CFRP tube to reduce peak stresses in the bond.

FigureFigure 9.9.The The optimisedoptimised structural structural hybrid hybrid metallic-composite metallic-composite railway railway axle axle (outboard) (outboard) design design concept con- showingcept showing ﬁnal dimensionsfinal dimensions with anwith assembly an assembly mass ofmass 200 of kg. 200 kg.

5.5. ConclusionsConclusions AnAn approachapproach for the design design of of a amultifunctional multifunctional composite composite railway railway axle axle utilising utilising co- coaxialaxial skins skins has has been been presented. presented. The The basis basis of of the the design design is a structural HMCHMC railwayrailway axleaxle withwith anan outboardoutboard bearingbearing arrangement.arrangement. ThisThis assemblyassembly comprisescomprises aa CFRPCFRP tubetube withwith steelsteel stubstub axlesaxles bonded bonded into into either either end. end. An An objective objective of theof the HMC HMC railway railway axle axle is to is minimize to minimize the massthe mass of the of axle thewhile axle while meeting meeting the loading the load requirementsing requirements within thewithin Standard the Standard EN 13103-1. EN To13103-1. address To the address load casesthe load within cases this within Standard, this Standard, a parametric a parametric sizing study sizing is study conducted is con- toducted explore to explore the sensitivity the sensitivity of the designof the design parameters parameters of the CFRPof the CFRP tube and tube the and stub the axlestub interactionaxle interaction through through the adhesive the adhesive joint. joint. The finiteThe finite element element model model (FEM) (FEM) was particularlywas particu- usefullarly useful in this in regard this regard as ranges as ranges of parameter of parameter values values (rather (rather than than single single values) values) could could be comparedbe compared to highlightto highlight behaviour behaviour trends trends and an regionsd regions where where stresses stresses would would plateau. plateau. As As suchsuch thethe salientsalient dimensionsdimensions ofof thethe CFRPCFRP tubetube andand stubstub axleaxle interactioninteraction werewere sizedsized (see(see FigureFigure9 ).9). Compared Compared to to a a solid solid steel steel axle axle weighing weighing 400 400 kg, kg, the the optimised optimised HMC HMC railway railway axleaxle hashas aa massmass ofof 200200 kgkg representingrepresenting a a reduction reduction of of 50%. 50%. TheThe structuralstructural HMC HMC railway railway axle axle is is surrounded surrounded by by coaxial coaxial skins skins performing performing a specific a spe- functioncific function to meet to meet secondary secondary requirements. requirements These. These include include fire andfire and ballast ballast skins skins to protect to pro- thetect HMC the HMC railway railway axle axle core. core. An outerAn outer pigmented pigmented layer layer provides provides visual visual identification identification of theof the axle axle class class along along with with protection protection against against environmental environmental agents. agents. Integrated Integrated into into thethe railwayrailway axleaxle isis aa SHMSHM layerlayer toto provideprovide feedbackfeedback onon thethe integrityintegrity ofof thethe composite composite tubetube andand adhesiveadhesive bondsbonds duringduring thethe operatingoperating life.life. AnAn additionaladditional tethertether isis addedadded providingproviding aa globalglobal indicationindication of of the the integrity integrity of of the the HMC HMC railway railway axle. axle. ThisThis tethertether willwill havehave thethe furtherfurther benefit of retaining the wheel should the HMC railway axle undergo a catastrophic failure. benefit of retaining the wheel should the HMC railway axle undergo a catastrophic fail- 6.ure. Future Work

A detailed stress analysis of this composite axle is the subject of future work to incor- porate classical laminate theory (CLT) and a comprehensive Tsai-Wu failure assessment. Detailed design and optimisation of the joint is required to include; tapering of the stub axle to match the stiffness at end of stub axle to the composite and different joint conﬁgura- tions to reduce peel stresses. In addition, the detailed analysis of the adhesively bonded joint should include the use of orthotropic elements to more accurately understand the stress distribution. Furthermore, fatigue data of composite materials is generally limited to 107 cycles of rotating bending. Therefore, an experimental programme to address the fatigue of composite tubes at 109 cycles is required using a scaled roller rig or other means to validate the FEA results. In addition, failed specimens from the testing could be compared with the FEA predictions to ensure that the failure modes predicted by the Tsai-Wu criterion were in agreement with the test specimens. J. Compos. Sci. 2021, 5, 77 11 of 12

Author Contributions: P.J.M. and M.S.J. conceived the composite railway axle design and drafted the article, C.A.M. carried out the finite element analysis of the composite axle design and I.A.J. reviewed and corrected the article. All the authors read and approved the final manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The first author would like to acknowledge the funding support of the Engineering and Physical Sciences Research Council through the: “EPSRC Future Composites Manufacturing Research Hub (Grant number: EP/P006701/1)” and “EPSRC Industrial Doctorate Centre in Composites Manufacture (Grant number: EP/L015102/1)”. Conflicts of Interest: The authors declare that they have no competing interests.

Abbreviations

HMC: Hybrid metallic-composite; CO2: Carbon dioxide; CFRP: Carbon ﬁbre reinforced polymer; FRP: Fibre reinforced polymer; NDT: Non-destructive testing; UT: Ultrasonic testing; SHM: Structural health monitoring; FST: Fire, smoke and toxicity; UV: Ultraviolet; EMC: Electromagnetic compatibility; CES: Cambridge Engineering Selector; CAE: Computer-Aided Engineering.

Appendix A Material properties used for modelling the HMC railway axle with coaxial skins. Material Property Value Units CFRP E11 135 GPa CFRP E22,33 10 GPa CFRP tube (Carbon ﬁbre reinforced epoxy) CFRP G12 5 GPa CFRP σallowable, compressive 250 MPa ρCFRP 1600 kgm−3 Esteel 210 GPa Stub axle (EA1N steel approximated as AISI σsteel 185 MPa 1030 steel) allowable ρsteel 8000 kgm−3 Eadhesive 11 GPa Gadhesive 5 GPa Adhesive (epoxy) adhesive σshear 40 MPa ρadhesive 1250 kgm−3

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