The concept design of a multifunctional composite railway axle utilising coaxial skins is shown in Figure 2. The structural requirements are met by the hollow HMC railway axle assembly comprising a CFRP tube into which metallic stub axles are bonded. Coaxial skins are included to fulfil secondary functional requirements. A tensioned tether would run along the axis of the hollow axle attached to the stub axles at either end.
Construction of the coaxial CFRP tube is by the established roll wrapping technique. Each layer of the CFRP tube would be available in planar format, for example as a prepreg textile reinforcement or a conformable foam sheet. The wall thickness of the CFRP tube could be made in stages, not only for the application of the bespoke skins, but for the accumulation of a sufficient thickness. This would aid in adhering to the deflection requirement while ensuring minimal defects within the thick section tube.
3.1 The structural HMC railway axle assembly
The principal Standard for the structural design of an outboard railway axle is EN 13103-14. Compliance with the load cases within this Standard are met by the structural HMC railway axle assembly illustrated in Figure 3.
Carbon fibre reinforced polymer (CFRP) composite tube
The CFRP tube comprises high modulus, carbon fibre reinforcement within an epoxy matrix. The strength of the axle is dominated by the fatigue properties as the axle has a service life on the order of 30 years. This equates to 109 reverse bending cycles. Four-point bending is the main load case with the addition of torsional loading under specific braking conditions and dynamic wheelset hunting. A high axle stiffness for the CFRP tube is necessary to suppress crack growth within the matrix, while limiting deflection at the bearings and drive elements for powered axle variants.
Dynamic conditions include operational frequencies (approximately 60-70 Hz torsional, 90-110 Hz bending) as well shock and vibration limits. The former can lead to rail corrugations if undamped, while the latter is encountered when the wheel rolls over a track irregularity. A first indication of the vibration level of the axle can be found in the Standard BS EN 6137310. Shock loading effects are typically assessed at wheelset assembly level using a roller rig as part of a laboratory testing programme.
Steel stub axle
The stub axle is manufactured from the same EA1N steel as the traditional steel axle. The geometry of the current steel axle is maintained from the end up to and including the wheel seat. This approach allows the existing wheel and bearing solutions to be maintained. Further inboard of the wheel seat, an extension is added for insertion into the CFRP tube, including a shoulder to define the overall axle length (not shown in Figures). The geometry of the extension effects the performance of the adhesive joint.
Epoxy adhesive joint
Epoxy adhesive is applied 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 tube to stub axle extension overlap, 2. the wall thicknesses of the CFRP tube and the stub axle extension. The adhesive thickness is fixed at 0.2 mm as design best practice11. Techniques may be required to limit peel stresses at the ends of the joint. This 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
The structural and secondary requirements for a railway axle are encapsulated within the Standard EN 13103-1 and are illustrated in Figure 4.
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 Figure 2. Clearly, other skins with unique functions could be incorporated into the overall design.
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.
Layer 2 – Fire protection
The operating temperature (BS EN 50125-1) for the axle is from -40°C to +70°C12. 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 45545-213. 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 14) 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.
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 protection15. 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.
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. A tough, thermoplastic material is recommended for this application. 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.