Multistage Structural Optimization in the Design of a Lightweight Electrical Vehicle in the Project VisioM

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The Technical University of Munich has developed an electrical vehicle in the framework of VisioM. The Institute of Lightweight Structures has contributed to the layout process of the vehicle’s structure by performing optimization in consecutive stages of development.

Topology optimization was applied at an early design phase to identify an advantageous geometry for the structure. For creating a safe and efficient vehicle, the performance of its structure is vital, as it has to withstand the loads while being lightweight. Hence, several load cases had to be considered in the optimization. Nonlinear Simulation of the crash behavior raises the computational costs in an unacceptable way. Therefore, quasi static loads via inertia relief analyses were used. In addition chassis loads as well as torsional and bending stiffness were considered.

The results showed regions of plane geometry and regions of bar structures. This contributed to the decision of creating a hybrid construction, of a carbon fiber laminate moncoque combined with an aluminum space frame for the front, rear and roof of the vehicle. In this second phase the idea was to improve the monocoque by insertion of stiffeners. Again topology optimization was used to identify promising locations for fortifications. Based on the results the stiffeners were modeled as shell structure and a size optimization was performed, to define a manufacturable geometry.

To obtain a reliable result for the design of the monocoque, it was important to include the orthotropic material properties of the laminate. Based on the monocoque a shell structure was modeled and sequentially several unidirectional patches were defined. Due to manufacturing constraints the monocoque was modeled as a symmetric fabric that was enforced by the patches. In this way a weight optimal design was found, that fulfilled the failure criterion under the given loads.

Using optimization in the development of the structure has the advantage, to be able to better judge and understand the performance of the design, as one is always seeking the frontiers of the design instead of developing a structure that only satisfies the specifications. In this way challenges can be revealed earlier in the design process. Finding an optimal design is much more meaningful than just finding a better or satisfying design.

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Multistage Structural Optimization in the Design of a Lightweight Electrical Vehicle in the Project VisioM

  1. 1. 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
  2. 2. Technische Universität München Folie 2Lehrstuhl 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
  3. 3. Technische Universität München Folie 3Lehrstuhl 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
  4. 4. Technische Universität München Folie 4Lehrstuhl für Leichtbau (LLB) Institute of Lightweight Structures 𝜎 ≤ 𝜎 𝑚𝑎𝑥 ∆𝑥 ≤ 𝑥 𝑚𝑎𝑥 Crash: Quasistatic loads Chasis loads StiffnessLoad 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
  5. 5. Technische Universität München Folie 5Lehrstuhl für Leichtbau (LLB) Institute of Lightweight Structures Topology Optimization Results for the Structure Shell geometryTruss 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!
  6. 6. Technische Universität München Folie 6Lehrstuhl 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
  7. 7. Technische Universität München Folie 7Lehrstuhl 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
  8. 8. Technische Universität München Folie 8Lehrstuhl 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
  9. 9. Technische Universität München Folie 9Lehrstuhl 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
  10. 10. Technische Universität München Folie 10Lehrstuhl 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
  11. 11. Technische Universität München Lehrstuhl für Leichtbau (LLB) Institute of Lightweight Structures Thank you for your attention

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