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Lightweight Design worldwide

, Volume 10, Issue 5, pp 32–37 | Cite as

CAE-based concept development for lightweight design of railway vehicles

  • Markus Seitzberger
  • Robert Nedelik
  • Andreas Ruthmeier
  • Thomas Grausgruber
Construction Topology Optimisation
  • 260 Downloads

In the last years lightweight design has become an essential requirement for modern railway vehicles. Therein, under the aspects of high cost pressure and tight schedules the massive application of CAE methods starting already in the early design phase is a key factor for a successful and efficient product development, as demonstrated by Siemens.

Introduction

Continuously increasing requirements for railway vehicles led to the fact that mass reduction has become an important topic in the last years. Amongst others, increased requirements with regard to safety and comfort, but also with regard to higher passenger capacities, lead to higher masses of the vehicles. On the other side, there is a need for light and economic vehicles with low operating and life cycle costs, in order to fulfill economical and environmental protection issues. Thus, lightweight design has become an essential part, in particular for urban- transport rail vehicles (tramways, metros) typically having a high percentage of braking and acceleration cycles.

CAE-based Development of Railway Car-body Structures

The car-body structure is, beside the bogies, the main load carrying structure of a railway vehicle with interfaces to almost all components. For metro vehicles the mass of the car-body structure is about 20 % of the overall vehicle mass, thus being an essential part for lightweight design.

For the development of a car-body structure many requirements have to be considered. In addition to design interface and geometrical requirements there is a large number of functional requirements, amongst others:
  • ▸ sufficient strength for static and operational load cases for the whole life of the vehicles (usually 30 to 35 years of operation)

  • ▸ structural dynamics requirements

  • ▸ stiffness requirements

  • ▸ crashworthiness

  • ▸ acoustic requirements

  • ▸ fire safety issues.

There is a high number of functional issues, which are often influencing each other. Lightweight design is therefore a multidisciplinary topic, which has to be strictly accounted for from the very beginning for a successful vehicle development.

However, under the aspect of increasing cost and time pressure, a successful implementation also requires a paradigm change for product development from a design driven to a simulation driven design process [1], Figure 1.
Figure 1

Comparison of the lead time: Traditional versus CAE based concept development car-body structure (© Siemens)

Main parameters for a product (e.g. mass, quality, costs) are basically determined in the concept phase. It is very advantageous, therefore, when different design variants can be analysed already in this phase, and main criteria can be assessed and optimised. The application of numerical simulation at an early stage is, thus, an essential task for a successful product development within a frontloading process [2, 3].

The application of simulation in the early concept phase, however, requires an adaptation of the classical design approach including the development of corresponding tools and methods. Main demands for these are:
  • ▸ Numerical simulation models have to be set-up independent from a CAD design state.

  • ▸ Conceptual changes have to be realised in a fast and easy way.

  • ▸ Simulation results must be available within short time.

  • ▸ It shall be possible to evaluate different functional requirements, e.g. structural strength, structural dynamics, crashworthiness, and acoustics.

In the following section a “soft-coupled” CAD-CAE method is presented, called Fast FE, which has been developed at Siemens Urban Transport in order to enable an efficient CAE driven concept development based on predefined car-body design principles. Besides a description of the basics of this approach an example is shown revealing the potential of the method. Afterwards results of a basic study are presented, where the possibility to perform a mathematical optimisation of a whole car-body structure in order to find mass optimised concept topologies for future vehicles has been investigated.

Fast FE — CAE-based Concept Design of Railway Car-bodies

Modern lightweight railway car-bodies are typically made of a multitude of thin-walled extrusions and/or sheet metal parts, which are joined by welding, Figure 2. Finite element (FE) analysis models of such thin-walled structures are usually realised using shell elements. The setup of a shell element model of a large structure like a whole rail vehicle car-body is, however, a non-trivial, usually time consuming task involving model abstraction.
Figure 2

Siemens metro platform Inspiro: full train (left), car-body structure in aluminium integral design (right) (© Siemens)

Simplified analysis models may be used, however, the results have limited validity because of their inherent model assumptions. An advantage of using shell meshes right from the start is the possibility of easy and continuous proceeding to finer or more detailed meshes in the further development.

To overcome these difficulties in a timely manner, a method has been developed at Siemens Urban Transport, which allows setting up a finite element analysis model by using the advantages of modern CAD functionality and combine it efficiently with the CAE environment. The developed method is shown in Figure 3. Key points for the realisation are [4]:
  • ▸ For a given design principle of a car-body structure a generalised CAD mid-surface model is built up as a template or base model for later project derivation.

  • ▸ This mid-surface model is defined such that all geometrical abstraction necessary for FE meshing is already included. In addition, further FE relevant information can be defined directly via CAD, e.g. material or wall thickness distributions, assignment of load and boundary groups.

  • ▸ The base model is built up in a flexible, modular and parameterised way. All relevant global dimensions and design characteristics can be changed by simple parameter modifications, to obtain a specific project derivation from a platform template model efficiently.

  • ▸ In addition, fundamental design changes of individual components and subassemblies are possible due to the modular geometry setup. This is particularly important for concept investigations, where typically different design variants, often involving significant topological design changes, have to be assessed in short time.

  • ▸ Once the CAD information is transferred into the CAE environment several steps for model preparation can be performed automatically by using predefined application scripts within the FE preprocessing tool before the analysis is started and the simulation results are evaluated and post-processed.

Figure 3

Fast FE process: Overview of the FE model generation (© Siemens)

A successful implementation requires a paradigm change from a design driven to a simulation driven design process.

Example

An example for CAE based development of a metro car-body is shown in Figure 4. The project derivation started from the platform base model. As a first step the structure was adapted to the specific project requirements (e. g. global dimensions, numbers, sizes, positions and geometries of doors and windows, positions of pivots). That model was used for an initial analysis to provide an overview of the critical areas and natural frequencies of the structure.
Figure 4

Fast FE process: Example (© Siemens)

On the basis of these initial results, design improvements were defined for the recognised problem areas. Additionally, possible mass reduction measures as well as measures for the improvement of the natural frequency behavior were defined and evaluated in a further analysis step. Multiple geometric modifications were introduced directly in the ready to mesh surface model, again utilizing the parameterised and modular CAD model.

These results were afterwards used as a base for further development by design engineers. The first CAD design model thus already contained valuable project-specific improvements with regard to strength and structural dynamics, as well as assessed mass reductions for the car-body concept. Therefore, a high level of concept maturity could be obtained in a very short time.

Numerical Structural Optimisation Tools for Car-body Concept Development

Numerical optimisation tools have been established to be suitable for daily use in the development of lightweight components within the last years. Due to the constantly increasing computing power, but also due to the continuous improvement of optimisation software packages, large FE-models can nowadays be subjected to a structural optimisation. With regard to numerical efficiency finite element analysis codes with an integrated optimisation algorithm are of particular interest. Therein, the optimisation algorithm is directly coupled to the finite element solver, which enables (semi-)analytical calculation of sensitivities by directly referring to the system of equations of the finite element solver. This way gradient based methods can be applied with high numerical efficiency (avoiding the need to numerically evaluate the gradients), which is essential for a suitable optimisation of large-scale structures for industrial applications.

In the following, different topology optimisation approaches are evaluated to find a mass optimised aluminium metro car-body shell, delivering structural concepts with an optimum load path distribution already in the early concept development phase. Main emphasis in this work, which has been developed in close cooperation with the Institute of Lightweight Design and Structural Biomechanics at TU Vienna [5], has been put on the basic analysis of tools and methods to answer the following questions:
  • ▸ Is it possible, with commercially available tools, to optimise whole car-body structures suitable for daily use?

  • ▸ How has the FE model to be defined to obtain a stiffened thin-walled structure instead of a framework design?

  • ▸ Which optimisation algorithms are appropriate?

  • ▸ Which quality of results is available?

For the work presented herein Optistruct from Altair Engineering was used. As optimisation method the ’dual method’ was applied [6].

Conceptual Design Study of a Metro Car-body Shell

The main goal of this investigation was the application of a topology optimisation to find a thin-walled stiffened structure for the car-body shell that yields the lowest possible mass while withstanding all required loads. The outer sheet of the car-body side wall is therein considered as a main load bearing structure.

The design space for the conceptual design is shown in Figure 5. It is restricted by gauging analysis, clearances of the bogies and, amongst others, interior. To be able to achieve a finer mesh size while not increasing CPU time the structure is reduced to a quarter model. The load cases were derived from EN12663 [7] and were adapted to the quarter model. The load cases coming from EN12663 were reduced to those load cases having a major structural forming effect. Furthermore, substitute static load cases were introduced to represent the structurally relevant fatigue load cases.
Figure 5

Design space of the car-body (© Siemens)

At first, the design space was solely meshed with 3-D-solid-elements and a topology optimisation was executed. Due to the utilised Solid Isotropic Material with Penalisation (SIMP) approach [6, 8], which enforces each element of the design space to be either a void or fully solid region, the analysis yields a framework structure as shown in Figure 6.
Figure 6

Result of the 3-D-topology-optimisation without considering a load-bearing outer sheet (normalised density values of the design variables) (© Siemens AG)

In the next step the outer sheet of the side wall, roof and front end as well as the upper face sheet of the floor were included in the optimisation as a load-bearing structure by the use of shell elements. Different optimisation variants were analysed:
  1. 1.

    Topology optimisation of 3-D-solid- elements with fixed properties of shell elements

     
  2. 2.

    Topology optimisation of 3-D-solid- elements and concurrent topology optimisation of shell elements

     
  3. 3.

    Topology optimisation of 3-D-solid- elements and concurrent free size optimisation [6] of shell elements.

     

The combined optimisation results in a theoretical mass saving potential of 32 %.

Version 3 yielded the best results as depicted in Figure 7. The results gained with the combined optimisation (variant 3) yield a mass saving of 26 % compared to the result of the optimisation with 3-D-solid-elements only (without shell elements). In relation to an actually manufactured reference car-body shell the combined optimisation results in a (theoretical) mass saving potential of 32 % together with slightly improved stiffness characteristics.
Figure 7

Result of variant 3, combined topology and free size optimisation considering a load-bearing outer sheet: inner reinforcements, visualisation of normalised density values (left); outer sheet, visualisation of element thickness properties (right) (© Siemens)

When taking a closer look at the results it can be seen that important structural parts such as the body bolster and the side sill are confirmed in terms of location and dimension. In addition, interesting findings for potential improvements of current structures can be made. These are for example the pattern of reinforcement structures in the side wall region, the connection of the body bolster to the side sill and coupling plate as well as the position and dimension of the cross beams below the floor plate. The concept design using structural optimisation thus leads to new ideas how components and the assembled structure can be engineered in an innovative lightweight way.

The investigations show that it is possible with commercially available tools to optimise whole car-body structures suitable for daily use. However, it also turned out that the quality of the results very much depends on the model set-up as well as the proper choice of optimisation parameters and algorithms. Combined FE shell-solid models in combination with gradient based optimisation methods deliver a very general and efficient approach for stiffened thin walled shell structures, allowing to find improved concept approaches for innovative car-body structures. The interpretation of the findings into a manufacturable structure is currently in development at Siemens Mobility, Urban Transport.

Notes

Thanks

The work with regard to structural optimisation has been made in close cooperation with the Institute of Lightweight Design and Structural Biomechanics at TU Vienna. Special thanks go to Professor Helmut J. Böhm for the technical and scientific support and supervision. Furthermore the support of Altair Engineering Inc. with regard to support on the application of structural optimisation tasks is gratefully acknowledged. The content of the present article was presented in November 2016 at the “9. Ranshofener Leichtmetalltage” and published in the corresponding conference proceedings [9].

References

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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Markus Seitzberger
    • 1
  • Robert Nedelik
    • 1
  • Andreas Ruthmeier
    • 1
  • Thomas Grausgruber
    • 1
  1. 1.Siemens AG Austria, Division Mobility, Urban TransportViennaGermany

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