Automobile Collision Simulation
Automobile Collision: Theoretical Foundations
Collision Simulation Overview
Professor, could you explain the overall picture of automotive collision simulation?
Automotive crash safety simulation is one of the largest application areas of FEM. For a single vehicle, 20 to 50 collision scenarios are evaluated using FEM, with only a few final confirmation tests on actual vehicles.
Collision Physics
The fundamentals of collision are energy absorption and occupant protection:
1. Kinetic Energy โ $E = mv^2/2$. 1500 kg ร (56 km/h)ยฒ / 2 โ 180 kJ
2. Energy Absorption โ The front crash zone absorbs 180 kJ through plastic deformation
3. Deceleration โ The average deceleration during the collision determines the impact on occupants
4. Restraint System โ Seatbelts + airbags decelerate the occupants
So the structure absorbs as much as 180 kJ of energy?
Energy is dissipated through plastic deformation of steel sheets (bending, buckling, tearing). This "controlled destruction" is the design philosophy of the crash zone.
Numerical Methods
The numerical method for collision simulation is explicit FEM (Central Difference Method). LS-DYNA is the world standard.
Features:
- Large Deformation, Plasticity, Contact, and Fracture all occur simultaneously
- Millions of elements in a full vehicle model
- 50 to 200 ms of analysis time
- Strain rate dependent material models are essential
Summary
Key Points:
- 180 kJ Energy Absorption โ Plastic deformation of the crash zone
- Explicit FEM (LS-DYNA) is the world standard
- 20 to 50 collision scenarios evaluated by FEM โ Actual vehicle tests are for final confirmation only
- Material strain rate dependency is important โ Cowper-Symonds law
The Foundation of Crash Safety is Thin-Walled Crushing Theory
The axial crushing theory proposed by Alexander (1960) expresses the energy absorption of corrugated deformation of a thin-walled cylinder as the product of plate thickness, diameter, and yield stress. This theory was utilized by Ford and GM for cross-section optimization of front rails, becoming the analytical foundation for maintaining crash performance while downsizing during the 1980s fuel efficiency regulations. In current vehicle body CAE, a full frontal collision of 10 milliseconds is calculated within 100 ms using the finite element method.
Computational Methods for Automobile Collision
Collision Simulation Setup
Basic LS-DYNA settings:
```
*CONTROL_TERMINATION
0.120 $ 120 ms
*CONTROL_TIMESTEP
0.0, 0.9, 0, 0.0, 0.0, 0, 0, 0
*INITIAL_VELOCITY_GENERATION
1, 0., 0., 0., 0., -15556., 0. $ 56 km/h (in mm/ms units)
*CONTACT_AUTOMATIC_GENERAL
0 $ Full automatic contact
```
So *CONTACT_AUTOMATIC_GENERAL automatically detects contact between all parts.
In collisions, it's impossible to predict what will contact beforehand. Full automatic contact comprehensively detects contact between all parts. LS-DYNA's automatic contact operates stably even with models of millions of elements.
Summary
Explicit Crash Analysis Time Step is 1ฮผs
Crash simulation uses a stable time step determined by element size/sound speed from the CFL stability condition. For typical vehicle body analysis (minimum element 5mm, steel plate C=5000m/s), the time step is about 1ฮผs, calculating a 100ms collision in 100,000 steps. Parallel computing (256 cores) analyzing a full vehicle model of about 7 million elements in 2-4 hours is the industry standard cycle time in the 2020s.
Automobile Collision in Practice
Collision Simulation Practice
Achieving Euro NCAP 5 stars is the development target. All patterns are pre-verified with FEM.
Practice Checklist
NCAP Test is 64km/h Full Overlap Collision
The Euro NCAP full frontal test is a 64 km/h full overlap collision with a fixed barrier, evaluating occupant dummy (Hybrid III 50th percentile) head HIC, chest 3ms acceleration, etc. The 2022 version added the MPDB (Moving Progressive Deformable Barrier) test, making aggressivity evaluation towards the other vehicle in partial overlap collisions also mandatory. Automakers simultaneously calculate all these test modes in LS-DYNA, with some cases exceeding 3000 CPU hours per test cycle.
Automobile Collision: Software & Solver Comparison
Collision Simulation Tools
Related Topics
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