Week - 4 - crash box simulation
Aim: To simulate a crash test for a crash box.
Given:
1. Crashbox FE Model
2. Material is Steel
3. Thickness of Crashboxes are 1.2 mm and 1.5 mm
4. Initial Velocity is 50 kmph
5. Crashbox to be crash into rigidwall using *RIGIDWALL_ keyword
6. The units system to be used is gm-mm-ms
Procedure :
1. Property Creation
- Using *SHECTION_SHELL, thickness is defined for crashbox.
- Fully integrated shell elements are used with shear factor as 0.833 and 5 integration points are used.
- Thickness of 1.2 mm is given in first iteration, for next iteration it is 1.5 mm.
2. Material Creation
- Using *MAT_PIECEWISE_LINEAR_PLASTICITY (MAT24), Steel material is created.
- Appropriate density, Young's Modulus, poissons ratio and yield stress is given.
- Elastic region is defined by Young's Modulus.
- Plastic region is defined by Effective Plastic Strain (EPS) and Effective Stress (ES).
3. Part Creation
- Using *Part, Crashbox is defined.
- Section and material definitions are called in this card.
4. Rigidwall Creation
- Using *RIGIDWALL_GEOMETRIC_FLAT, Rigidwall is defined.
- All nodes of crashbox are given as slaves to rigidwall for contact puurpose.
- Distance between Rigidwall and crashbox is kept as 15 mm.
5. Boundary Condition in the form of Initial Velocity
- Using *INITIAL_VELOCITY, initial velocity is given to crashbox.
- Initial velocity of 13.9 mm/ms (50 kmph) is given in negative X-direction.
- Nodeset is created having all nodes of crashbox is called in this card.
6. Control Cards
1. *CONTROL_TERMINATION :- Termination time is set to 3 ms
2. *CONTROL_ENERGY :- HGEN=2, in order to calculate Hourglass Energy
7. Database Cards
1. *DATABASE_BINARY_D3PLOT :- DT=0.1, in order to get animation after every 0.1ms
2. *DATABASE_OPTIONS :- ASCII output request such as GLSTAT, MATSUM, RWFOC, ELOUT, NODOUT are called after every 0.01 msec
GLSTAT: To get global energy plots such as Internal, Kinetic, Total, Hourglass Energy vs Time.
MATSUM: To get Energy values on part by part basis.
RWFORC: To get Rigidwall force (resistance force offered by rigidwall to crashbox)
ELOUT: To plot and compare Element output for crashbox of thickness 1.2mm and 1.5mm
Element is defined using *DATABASE_HISTORY_SHELL.
NODOUT: To plot and compare acceleration at mid node of crashbox of thickness 1.2mm and 1.5mm
Node is defined using *DATABASE_HISTORY_NODE_SET.
SECFORC: To get Sectional Force at middle of crashbox
3. *DATABASE_CROS_SECTION_PLANE :- To define cross section plane definition
Post- Processing Results:
1. Cross-sectional force generated in the middle of the crashbox of thickness of 1.2 mm along its length
- Maximum cross sectional force for crashbox of thickness of 1.2 mm is around 325 KN at around 1.14 MS
- Maximum cross sectional force for crashbox of thickness of 1.5 mm is around 410 KN at around 1.14 MS
2. Energy Plots for crashbox of thickness of 1.2 mm
- Before impact, Kinetic Energy is maximum, crashbox crashes into rigidwall and its velocity decreases hence Kinetic energy decreases when crashbox hits the wall at around 1.06 ms. After Crashing, It bounces back and kinetic energy increases again.
- During Crash, Internal Energy is stored in crashbox, hence spike in graph at time of crash.
- Fully Integrated elemets are used, Hourglass energy is zero throughout.
- Total Energy is almost constant throughout simulation.
3. Rigidwall force for crashbox of thickness of 1.2 mm
- When Crashbox hits rigidwall, rigidwall dont deform as its rigid. It gives reaction force.
- Maximum reaction (for crashbox of thickness 1.2 mm) given by rigidwall is 330 KN at around 1.09 ms
- Maximum reaction (for crashbox of thickness 1.5 mm) given by rigidwall is 413 KN at around 1.09 ms
4. Comparision of Accelerations of crashboxes 1.2mm vs 1.5mm
Acceleration Plot For Crashbox of thickness 1.2 mm
Acceleration Plot For Crashbox of thickness 1.5 mm
- In both cases acceleration graph is similar, peak is at 1.15 ms
- Acceleration of mid node for crashbox with thickness of 1.2 mm is 6960.6 mm/ms^2
- Acceleration of mid node for crashbox with thickness of 1.5 mm is 6802.9 mm/ms^2
- Acceleration is less due to increase in thickness.
5. Comparision of maximum directional stress and strain along the length of the crashbox
| Stresses | Crashbox @ 1.2mm | Crashbox @ 1.5mm |
| X-Stress | 494.4 Mpa | 495.7 Mpa |
| Y-Stress | 646.1 Mpa | 655.2 Mpa |
| Z-Stress | 702.2 Mpa | 624.6 Mpa |
| Von-Mises Stress | 722.7 Mpa | 720.4 Mpa |
- Directional stresses in Z directions are drastically reduced due to increased thickness.
- Von Mises Stresses reduced slightly due to increase of thickness.
| Strains (Rate) | Crashbox @ 1.2mm | Crashbox @ 1.5mm |
| X-Strains | 1.36 | 1.47 |
| Y-Strains | 0.41 | 0.43 |
| Z-Strains | 0.55 | 0.35 |
| Effective Strain (V-M) | 1.3 | 1.3 |
- Directional Strain Rates in Z directions are drastically reduced due to increased thickness.
- Von Mises Strain rates dont have much variation.
Conclusion
- Cross sectional as well as rigid wall force is more for crashbox with higher thickness, It means resistance is more hence high strength with higher thickness of crashbox.
- Acceleration is less for crashbox with higher thickness.
- Effective stresses are less for crashbox with higher thickness, however there is no variation in strain rate.