Detailed Vehicle Collision Simulation
Detailed Vehicle Collision: Theoretical Foundations
Vehicle Collision Simulation
Professor, car crash safety can't be designed without FEM, right?
Exactly. In modern automotive development, hundreds to thousands of FEM simulations are performed before actual vehicle crash tests. FEM leads the design of crash safety.
Collision Classification
| Collision Type | Standard | Speed | Characteristics |
|---|---|---|---|
| Full Frontal (Full Overlap) | FMVSS 208, Euro NCAP | 56 km/h | Full-width rigid barrier impact |
| Frontal Offset | Euro NCAP, IIHS | 64 km/h | 40% offset ODB |
| Side Impact | FMVSS 214, Euro NCAP | 50 km/h | Deformable barrier side impact |
| Rear Impact | FMVSS 301 | 80 km/h | Fuel leakage prevention |
| Pole Side Impact | Euro NCAP | 32 km/h | Narrow obstacles like utility poles |
| Pedestrian Protection | Euro NCAP | โ | Head impact on hood |
There are that many collision patterns?
For a single vehicle model, 20 to 50 collision cases are simulated. Each case involves an explicit dynamic calculation of a full-vehicle model with millions of elements for 50 to 200 ms. The computational resources required are enormous.
FEM Model Scale
Typical full-vehicle crash model:
| Item | Value |
|---|---|
| Number of Elements | 3 million to 10 million |
| Number of Nodes | 1 million to 5 million |
| Number of Material Models | 50 to 200 |
| Number of Contact Definitions | Hundreds |
| Calculation Time | 4 to 24 hours (100 to 200 CPUs) |
| Result File Size | 10 to 100 GB |
10 million elements! That's an incredible scale.
It includes everything: BIW (Body-in-White), closures, chassis, powertrain, interior, seats, dummy, airbag... Mesh generation can take weeks, and calculation setup can take days.
Crash Safety Design Philosophy
Energy absorption is the fundamental concept of crash safety:
1. Front Crush Zone โ Absorbs energy through controlled buckling
2. Cabin (Occupant Compartment) โ High-rigidity cage that does not deform
3. Restraint System โ Decelerates occupants with seat belts and airbags
The core of the design is "parts that should crush" and "parts that must not crush," right?
FEM simulates this "controlled buckling." The shape, thickness, and material of the crash box ribs are optimized using FEM to achieve the target energy absorption and deceleration pulse.
Summary
Key Points:
- 20 to 50 collision cases simulated with FEM โ Before physical vehicle tests
- 3 million to 10 million element full-vehicle model โ LS-DYNA explicit method
- Energy absorption and occupant compartment deformation limitation โ Crash safety design philosophy
- Controlled buckling of crash boxes โ Optimized with FEM
- Compliance with standards like Euro NCAP, FMVSS โ Multiple scenarios
Crash Safety Engineering was Founded by Hugh DeHaven
Hugh DeHaven, considered the father of modern crash safety engineering, proposed the concept of a "variable crash zone" in 1942. The idea of intentionally deforming the engine compartment to absorb energy and protect the passenger cabin during a vehicle collision with an obstacle is the prototype for the crushable zone design implemented in all modern vehicles. Ford's first adoption of DeHaven's theory in a mass-produced vehicle, the padded dashboard in 1956, also stems from the same concept.
Computational Methods for Detailed Vehicle Collision
FEM for Collision Simulation
Please tell me the technical details of collision simulation.
Element Types
- BIW (Body) โ Shell elements (mainly Quad4, HEX8R)
- Closures โ Shell elements
- Bumper, Side Members โ Shell + Solid
- Dummy โ Shell + Solid + 1D elements (joints)
- Airbag โ Shell elements + Gas model (ALE/CPM)
Material Models
- Steel Sheet โ MAT24 (Elasto-Plastic) + Strain rate dependence (Cowper-Symonds)
- Aluminum โ MAT24 or MAT125
- Resin โ MAT24 or MAT89
- CFRP โ MAT54/58 (Progressive damage)
- Rubber โ MAT77 (Ogden hyperelastic)
- Foam โ MAT57/63 (Compressible foam)
Strain rate dependence is important, I see.
Strain rate during collision is $10 \sim 1000$ /s. The yield strength of steel increases by 20-50% with strain rate. Ignoring this effect leads to underestimation of energy absorption. Cowper-Symonds law:
Contact
Hundreds of contact definitions are needed in a crash model. LS-DYNA's *CONTACT_AUTOMATIC_GENERAL (global automatic contact) is standard. Prevents penetration using the penalty method.
Summary
The Era Where 10 Million Element Models Solve in 2 Hours
Modern automotive full-vehicle crash models have reached a scale of 7 to 12 million elements, over 5000 material definitions, and over 200 contact pairs. As of 2024, running LS-DYNA MPP on 256 cores (e.g., AMD EPYC 9354 ยท 128 cores ร 2 nodes) completes a 100ms full frontal crash analysis in about 2 to 4 hours. Toyota and VW use a "night run" system where multiple test modes are executed simultaneously overnight, significantly shortening development TAT (Turn Around Time).
Detailed Vehicle Collision in Practice
Collision Simulation Practice
Please tell me the workflow for collision simulation.
Workflow
1. Receive CAD Data โ Integrate CAD of each component
2. Mesh Generation โ Create shell mesh (5-10 mm) with HyperMesh/ANSA
3. Material Definition โ Set MAT24, etc., from material test data
4. Modeling of Joints โ Spot welds (*CONSTRAINED_SPOTWELD), adhesive, bolts
5. Dummy Placement โ Certified dummy models like WorldSID/THOR
6. Restraint System โ Seat belts (*ELEMENT_SEATBELT), air
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