CAE-based concept development for lightweight design of railway vehicles
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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.
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.
▸ 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
▸ 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.
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].
▸ 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
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.
▸ 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.
A successful implementation requires a paradigm change from a design driven to a simulation driven design process.
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.
▸ 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 .
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.
Topology optimisation of 3-D-solid- elements with fixed properties of shell elements
Topology optimisation of 3-D-solid- elements and concurrent topology optimisation of shell elements
Topology optimisation of 3-D-solid- elements and concurrent free size optimisation  of shell elements.
The combined optimisation results in a theoretical mass saving potential of 32 %.
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.
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 .
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