Copyright © Altair Engineering Ltd, 2002 5 / 1
APPLICATIONS OF OPTISTRUCT
OPTIMIZATION TO BODY IN WHITE DESIGN
Carl Reed
Jaguar Cars Limited
Body and trim CAE
Engineering Centre
Coventry CV3 4LF
creed19@jaguar.com
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.
Copyright © Altair Engineering Ltd, 2002 5 / 2
2.0 OBJECTIVE
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
Copyright © Altair Engineering Ltd, 2002 5 / 3
3.0 TOPOLOGY FEM AND DESIGNABLE SPACE DEFINITION
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
Copyright © Altair Engineering Ltd, 2002 5 / 4
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
Copyright © Altair Engineering Ltd, 2002 5 / 5
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
Copyright © Altair Engineering Ltd, 2002 5 / 6
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
