Technische Universität München

Lehrstuhl für Leichtbau (LLB)

Institute of Lightweight Structures

Multistage Structural Optimization in the

Design of the Lightweight Electrical

Vehicle VisioM

Dipl.-Ing. Bernhard Sauerer, Dipl.-Ing. Markus Schatz, Erich Wehrle M.Sc.,

Prof. Horst Baier

26.06.2014

Technische Universität München

Folie 2 Lehrstuhl für Leichtbau (LLB)

Institute of Lightweight Structures

Outline of the presentation

1. Design of the structure of the electrical vehicle VisioM

– Goals of the design process

– Load cases, that were considered

– Geometry/ design space

2. Topology optimization of the complete car structure

– Identifying loadpaths

– Results and interpretation

3. Sizing of the CFRP monocoque

– Definition of reinforcing patches

– Sizing of the patches

4. Conclusion

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Design Task Goal:

• Determination of an weight optimal structure,

satisfying the constraints

• Identification of favorable structure

• Improving the existing design

Modeling:

Design space:

• Consideration of package (battery, motor,...)

• Outer surface from Mute (predecessor)

• Volume between the crashsystems

• Discretization with 1.5x106 solid elements

Load cases:

• Stiffness (Bending, Torsion)

• Chassis loads

• Crash loads quasistatic with inertia relief

Project of the TUM

Working together with:

• BMW

• IAV

• Several TUM Institutes

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𝜎 ≤ 𝜎𝑚𝑎𝑥

∆𝑥 ≤ 𝑥𝑚𝑎𝑥

Crash: Quasistatic loads

Chasis loads

Stiffness Load Cases and Constraints

• Crash loads (4):

– Front, rear, sidecrash using

inertia relief

– Quasi static modeling of the

transient crash behavior

• Stiffness load cases (2):

– Bending and Torsional Stiffnes

– Stiffness was demanded to be

higher than in the preceeding

design

• Chassis loads (3):

– Breaking

– Maximum acceleration

– Curve combined with a bump

• Stress constraint

• Minimal member size

• 1-plane symmetry

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Topology Optimization Results for the Structure

Shell geometry Truss structure

Result: Density distribution in the design space shows important load paths

Defining a density threshold

A monocoque is a favorable

choice under the given loads!

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Optimization of the Monocoque

• Monocoque: shell structure

• Several optimization steps were performed

1. Topology opt.: Identifying locations for

reinforcements

2. Sizing of shell elements: Dimensions of patches

3. Sizing of fiber patches: CFRP design

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Optimizing the Monocoque

1. Identifying heavily stressed regions (Free Size)

– Free sizing with isotropic material (vonMises)

– Defining patches to reinforce the areas

2. Sizing optimization of ply thickness

– Global basic material woven CFRP

– Additional unidirectional patches

– 3 crash load cases (Inertia relief)

– Constraint: Tsai-Wu failure criteria

– Use of global search option (GSO)

Unidirectional fiber patches

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Sizing Optimization of the Fiber Monocoque

Result: Patch thicknesses

Strength evaluation via Tsai-Wu failure index

Critical areas

Force application areas

Mass: 61,8 kg

(Improvement 20 kg)

Local stress

concentrations

determine the

design

• Woven thickness at

lower bound

• Local unidirectional

patches upt to 11.6 mm

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Specific challenges

1. High computational cost of topology opt.

Iterations: ~120 using 16 CPU (2.9 GHz) -> ~30 hours

2. Loading due to passenger mass in different

load cases

3. Finding appropriate constraining mass

4. High sensitivity of the topology to changes of:

– Boundary conditions and forces

– Mass constraint

m<120 kg m<150 kg

Technische Universität München

Folie 10 Lehrstuhl für Leichtbau (LLB)

Institute of Lightweight Structures

Conclusion

• The design process was accompanied by a series of different

optimizations, in OptiStruct

• Different optimization goals:

– identifying loadpathes/ locations for reinforcements

– increasing the stiffness

– reducing mass

• Optimization provides a learning process about the performance of

the structure, that one would not get from a pure analysis.

• The mass of the monocoque was drastically reduced!

• The additional effort for defining the optimization is justifiable

Technische Universität München

Lehrstuhl für Leichtbau (LLB)

Institute of Lightweight Structures

Thank you for your attention