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
Technische Universität München
Folie 3 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
Technische Universität München
Folie 4 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
Technische Universität München
Folie 5 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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!
Technische Universität München
Folie 6 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
Technische Universität München
Folie 7 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
Technische Universität München
Folie 8 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
Technische Universität München
Folie 9 Lehrstuhl für Leichtbau (LLB)
Institute of Lightweight Structures
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
