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Lightweighting is becoming an increasingly important issue for rail vehicles because their weight has generally risen over the last 30 years. Rail vehicle components and assemblies that would be good candidates for lightweighting using composites are identified in this paper. The potential for composite materials is examined by highlighting some of the technical, regulatory, economic and cultural aspects that normally influence the design process.
(Published on July 2006 – JEC Magazine #26)
MARK ROBINSON & JOE CARRUTHERS NEWRAIL, UNIVERSITY OF NEWCASTLE UPON TYNE, SCHOOL OF MECHANICAL & SYSTEMS ENGINEERING, UK
Rail vehicle components and assemblies that would be good candidates for lightweighting using composites are identified. The potential for composite materials is examined by high-lighting some of the technical, regulatory, economic and cultural aspects that normally influence the design process.
The example below shows a UK Intercity 125 on the left and a new-generation Super Voyager on the right. The Super Voyager is approximately 30% heavier per seat.
The benefits of lightweighting to the operator, vehicle owner and infrastructure owner are clear:
- Reduced energy consumption; - Lower operating costs; - Less damage to track; - Increased capacity (for passengers or freight).
Lightweighting through material substitution
There are cost and energy benefits associated with lightweighting and, therefore, one might reasonably ask why there hasn’t been a greater use of lightweight materials, such as composites, in rail vehicles. Clearly there are some technical issues that need to be addressed, including the additional complexity of designing and manufacturing with composites. But are there also some more general railway industry barriers that need to be overcome? As a starting point, the MODURBAN lightweighting team has examined the mass distribution of a typical state-of-the-art metro vehicle. This is presented in the following figure: It can be seen that the five elements that contribute most to the mass of a typical metro vehicle are:
- Bogies (approximately 40%); - Bodyshell (approximately 20%); - Passenger interior (approximately 10%); - Heating, ventilation and air conditioning (approximately 5%); - External doors (approximately 5%).
Together, these five elements typically represent 75 - 80% of a vehicle’s tare mass.
An interesting question that can be usefully addressed is: “What proportion of a vehicle’s tare mass can be potentially influenced by material substitutions (e.g. using composites)?”. Clearly, some of the system categories used for this breakdown analysis are going to be readily amenable to material substitutions (e.g. “passenger interior”), whilst others are less likely candidates (e.g. “heating ventilation and air conditioning”). Therefore, to provide an estimated answer to this question, the system categories can be sorted into two groups - those that could potentially be amenable to mass reductions through material substitutions, and those that are less well suited to such treatment. The resulting groups are shown in the following table:
If the groupings presented in the table above are correlated back to their associated masses then an estimate of the proportion of a vehicle’s tare mass that can be potentially influenced by material substitutions is around 80% (i.e. the “Yes” column accounts for approximately 80% of a vehicle’s tare mass).
The specification of composites for lightweight rail vehicles
When examining the practicalities of specifying composite materials for use in rail vehicles, there are several aspects that need to be considered. These include technical, regulatory, financial and cultural issues. The principal points to consider are as follows:
- From a design and manufacturing perspective, if composites are to be specified more widely for rail vehicle applications, then engineers must be able to work with them as routinely as traditional materials. In other words, appropriate tools, training and data need to be made available. In terms of materials, the MODURBAN team is working on a tool that will allow designers to compare different materials on a functional basis.
- In terms of fitness for purpose, everyone with a vested interest (i.e. designers, suppliers, manufacturers, operators, certifying bodies, and passengers) needs to be confident that composite structures will perform in a predictable manner, over the life of a vehicle, without compromising safety.
- Composites need to be cost-competitive with metals within the framework of the rail industry’s normal costing practices. It will be difficult to argue for composites as a special case.
- Operator requirements can often be very prescriptive in terms of material selection, making it difficult to introduce alternative solutions. For example it is quite common to see that certain materials are strictly prescribed in a vehicle’s specification, e.g. “The bodyshell structure will be made in steel or light alloy”. Clearly this represents a significant barrier to the introduction of alternative materials such as composites. The MODURBAN team is seeking to highlight such issues and promote the substitution of prescriptive requirements with functional ones to stimulate lightweight design.
A next-generation rail vehicle cab: a case study in the use of composites for lightweight design
Conventional rail vehicle cab structures based on welded steel assemblies can weigh up to one tonne each. With two cabs per train-set, this represents a significant weight saving opportunity. Furthermore, current cab designs tend to be very complex, high part count assemblies with fragmented material usage. This is because they must meet a wide range of demands including proof loadings, crashworthiness, missile protection, aerodynamics, insulation, etc. Assembly costs are high, and there is little in the way of functional integration.
An innovative all-composite cab design is pictured below. Designed by Ingleton¹ and realised by the HYCOPROD European project, the all-composite cab design has the following features:
- It has been designed to meet UK Railway Group Standard GM/RT2100 - “Structural Requirements for Railway Vehicles”. This specifies mandatory requirements for proof loads, crashworthiness, missile protection, aerodynamic loads, etc. ;
- It is approximately 25% lighter than a traditional steel frame cab design;
- From a crashworthiness perspective, the cab has an estimated energy absorption capability of 1.5 - 2 MJ. This is derived from specialist composite energy absorbing “cells”;
- The highly integrated design significantly reduces the number of parts in the cab assembly from 50-60 parts in a traditional steel frame cab, down to around 10-15 parts for the new composite design (see example illustrated below);
- Furthermore, through greatly reduced assembly and installation costs, the overall cost of the all-composite cab is estimated to be 20 - 30% lower than a traditional steel solution.
1 INGLETON, S., Design of Composite Cabs for Structural Applications in UK Passenger Rolling Stock, PhD Thesis, The University of Sheffield, (2006).