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APPLICATIONS OF OPTISTRUCT OPTIMIZATION TO BODY IN Kingdom... · PDF file architecture, and chassis layout and occupant package is not always available. Hard p oints are not yet defined

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  • Copyright © Altair Engineering Ltd, 2002 5 / 1


    Carl Reed Jaguar Cars Limited Body and trim CAE Engineering Centre Coventry CV3 4LF

    Abstract: The application of topology and size and shape optimization for the design of efficient automotive

    components in terms of mass and stiffness has developed into a well practiced discipline. Subjecting a vehicle body structure to this type of development enables the analyst to optimise the size, shape and placement of load bearing members. By defining the most efficient topology, structural targets can be achieved with fewer design iterations, therefore, leading to reduced cycle times and lower development costs. The Altair OptiStruct software suite provides the analyst with an optimization environment which realises this method by providing a ‘right first time’ approach to body design.

    This paper presents an insight into the development of an efficient body structure subjected to various load

    cases. Based upon the topology of the load paths, which OptiStruct defines, the paper then details the development of the concept through to a detailed design.

    Keywords: Mass, Stiffness, Optimization, Topology, Optistruct, BIW, FEM 1.0 INTRODUCTION Increasing competition between automotive manufacturers has forced the development of new

    products to focus upon more efficient design. This leads to competitive product, increased sales and ultimately increased profits. Each of the vehicle’s systems contributes to its overall efficiency. By assuming the body in white system behaves as a single component, topology optimization can be applied to ensure that material is distributed throughout the structure in an efficient manner to maximize stiffness and minimize mass. The application of topology optimization to structural design provides an extremely fast route to ensuring the vehicle’s body structure meets or exceeds the relevant structural targets.

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    The objective of this exercise is to develop a ‘right first time’ approach to the design of an automotive body structure. Once the method has been defined, a detailed concept, which matches or exceeds the structural targets, can be developed from preliminary styling surfaces with minimal design iterations. A design process has been developed, which enables the rapid development of BIW design at the concept stage of a vehicle lifecycle. The process developed below summarizes the key stages in the design cycle. (Figure 1). The details of the process are overviewed in the remainder of the paper.

    Define External Boundaries (Styled A-Surface Concept)

    Define space for occupant, power train, chassis

    Define designable, non- designable space

    Apply appropriate loads and weighting factors

    Perform OPTISTRUCT topology optimization

    Create simplified beam and shell model

    Perform OPTISTRUCT size and gauge optimization (NVH)

    Create CAD model

    Define section sizes (crash)

    Create detailed FEM

    Figure 1: Optimization Design Process

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    When developing a new vehicle, the first data available to the engineering community is usually in the form of exterior surfaces released from Styling (Figure 2.0). At this stage of a vehicle’s development, information regarding its systems such as engine configuration, power train architecture, and chassis layout and occupant package is not always available. Hard points are not yet defined so assumptions based on the current vehicle architecture and or preferred use of components needs to be made in order to aid packaging structure. The more traditional approach to body development has largely been due to trial and error. By attempting to fit structure around a given package, components are tested in isolation and accepted or rejected due to their efficiency. Often the component does not work as well in isolation as it does in conjunction with others. When trying to find the optimum combination of load paths, the number of solutions required multiplies resulting in a time-consuming development matrix. Using OptiStruct topology optimization, the optimum material configuration of the BIW within the packaging space available can be determined. In order to achieve this, the available design space for the BIW must be generated as a solid FE model. At Jaguar a process has been developed to rapidly create this design space. Once the Styling surface is available, the first stage is to develop a solid mesh FEM which loads and boundary conditions can be applied to. By coating the released surfaces with a shell mesh, the total available volume for the structure can be created. From this, a sensible designable space model can now be constructed. (Figure 2.1)

    Next, the areas from which the body structure must be excluded need to be defined. These

    include the occupant cavity and space occupied by the engine power train and chassis. (Figure 2.2)

    Figure 2.0: Typical Scan Data Released from Styling

    Figure 2.1: Initial Shell Mesh Enclosing The Surfaces

    Total available volume for optimization

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    Obviously all that exists at this point is shell mesh which cannot be used for topology

    optimization. This now has to be converted into a solid mesh. The fastest method at jaguar to convert the closed volumes into a workable solid mesh is to use AKUSMOD.

    AKUSMOD is used extensively within the NVH analysis community to model fluid cavities

    within the occupant space for noise transfer function and sound pressure level predictions. In this application the meshing capability has been inverted (Figure 2.3). The solid mesh which is defined (the gaps between the exclusion zones and outer surfaces) becomes the allowable space for the body structure. (Figure 2.4). If the analyst does not have access to AKUSMOD, then the model could be generated with a solid meshing algorithm. However, this could prove to be time consuming as close attention would have to be paid to the surrounding shell mesh. AKUSMOD is good for this application because its meshing algorithm does not seed from the underlying shell mesh. Consequently, the analyst does not have to ensure that the volumes are fully enclosed. A mainly brick element mesh is generated, making it easier to perform any necessary hand editing.

    Figure 2.3: Model Set Up for AKUSMOD Meshing

    Cavities excluded during AKUSMOD meshing.

    Figure 2.4: AKUSMOD Mesh

    Figure 2.2: Exclusion Zones – Driveline, Engine, Occupant, Wheel and Boot Space

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    To finish the solid model, some hand manipulation is necessary. By removing material around the doors, windshields, bonnet and boot space apertures, the basic OptiStruct topology is ready. (Figure 2.5)

    To complete the Optistruct model, the analyst is free to define areas of the mesh which are not

    to be modified by the optimization algorithm (i.e. essential structure). All vehicles require stiff longitudinal members for front and rear crashworthiness. These areas are therefore assigned to a non-designable Property ID. (Figure 2.6). When all of these have been defined, the Optistruct model is ready.

    Non-designable space definitions

    Figure 2.6 Finalised Topology Definition

    Designable space definition

    Remaining redundant material cut by hand

    Figure 2.5: Finalised AKUSMOD mesh

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    4.0 SELECTION OF LOAD CASES FOR OPTIMIZATION An automotive body structure is subject to hundreds of different forces during every day of

    operation. However, many load cases are localised in nature and as such require detailed modelling in order to maximise their stiffness. When developing any structure it is always prudent to concentrate on the dominating load cases and their associated failure modes. As a consequence of selecting these, many of the local requirements will in turn be improved. The main load cases for a typical automotive structure can be categorised into two areas.

    1. NVH type load cases - encompassing local and global body static stiffness. 2. Crashworthiness load cases - encompassing energy load path management. 4.1 LINEAR AND NON-LINEAR DOMAIN OPTIMIZATION Optimising topology for NVH type load cases is intuitive as the displacements involved are

    small. Since OptiStruct works within the linear domain it does not consider the failure mechanisms associated with crashworthiness such as large displacement, contact interactions and material behaviour beyond the elastic limit. In this instance, the analyst must recognise that the topology generated for any non-linear load cases must be regarded with engineering judgement. However, automotive crashworthiness targets dictate that the occupant cell acts largely as a rigid entity and therefore should not encounter any large displacement or material yield. This effectively renders it a

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