CRASH BOX SIMULATION IN LS-DYNA
OBJECTIVES :
To simulate the crash box(1.2 and 1.5mm thick) onto a rigid wall at 50kmph and obtain the following output.
Deliverables:
- Input .k file and output files (d3plot, glstat, sleout, rcforc)
- Animation of the final simulation
- Cross-sectional force generated in the middle of the crashbox (along its length)
- Acceleration plot of a node in the middle of the crashbox (along its length)
- Maximum directional stress and strain along the length of the crashbox (X strain, Y strain etc)
- Plot of all energies (total, internal, kinetic, hourglass, sliding)
- Compare the accelerations and stress/strain plots of 1.2/1.5mm crashboxes.
PROCEDURE :
1. Import crash_box.k file in LS-prepost.
FIG 1 : Crushbox FE model
2. Create a rigid wall planar using create entity and tranlate it to suitable location.
3. Create a section ID for shell with ELFORM 2 and thickness 1.2mm for case1 and after simulation change to 1.5mm in .k file and save it in another folder for case2.
4. Create a material 024 Piecewise Linear Plasticity and assign properties of Steel in g mm ms. Assign Material ID and section ID to Part card.
5. Create a keyword for initial velocity of 13.88mm/ms (50kmph) along Vx .
6. Create contact keyword with Automatic_Single_Surface and select slaves as the crush box nodes and no master since rigid body is automatically considered as master.
7. Create a Hourglass keyword with IHQ=2 and a Control_hourglasss keyword with 1HQ=2 , QH=0.1.
8. Create Database keyword for ASCII and select ELOUT, GLSTAT, RCFORCE, SECFORCE, SLEOUT with DT 0.5. Binary D3plot with DT 0.05 is assigned.
9. Create crosssection_plane under Create entity> database in the middle of the component. Create History_node and history_shell database.
10. Assign control termination with 1 ms simulation time. Save the keyword file and run in LS-Dyna Manager.
11. After normal termination CPU clocks 418 nanoseconds, Open the Binary plot file in LS-Dyna and run the simulation.
12. Vonmisses stresses
13. As per output requests, plot cross sectional forces acting on the middle of component.
14. Acceleration plot of node 5118.
It is evident from the plot that the 1.2mm thick plot experience drasic acceleration of 135mm/ms2 whereas 1.5mm crush tube experiences 35mm/ms2 when the crush box hits the rigid wall at 50kmph(13.88mm/ms).
15. Maximum Directional stress along X-axis.
Max dir x-stress in 1.2mm thick crash box = 179.3 MPa
Max dir x-stress in 1.5mm thick crash box = 208.2 MPa
16. Maximum Y stress
Max dir y-stress in 1.2mm thick crash box = 215.8 MPa
Max dir y-stress in 1.5mm thick crash box = 198.6 MPa
17. Maximum Xstrain
Max dir x-strain in 1.2mm thick crash box = 5.482e-4
Max dir x-strain in 1.5mm thick crash box = 5.341e-4
18. Maximum Ystrain
Max dir Y-strain in 1.2mm thick crash box = 7.721e-3
Max dir Y-strain in 1.2mm thick crash box = 7.948e-3
19. Energies plot
The total energy acting on the 1.2mm thick crush box is 110 KJ. At 0.5ms of simulation Kinetic energy drops from 110KJ to 5000J and Internal energy increases to 100KJ. Hourglass energy and sliding energy is negligible.
The total energy acting on the 1.5mm thick crush box is 135 KJ. At 0.5ms of simulation, Kinetic energy drops from 135KJ to 5000J and Internal energy increases to 130KJ. Hourglass energy and sliding energy is negligible.
20. Comparison of stress plots.
| Comparison of X-stress |
1.2mm thick crash box |
1.5mm thick crash box |
| Element 6640 |
30 MPa |
55MPa |
When component hits the rigid wall at 0.5ms of simulation it produces a stress of 30 MPa and 55MPa in x-direction on the element.
21. Comparison of Acceleration plot.
| Node |
1.2mm thick crash box |
1.5mm thick crash box |
| 5118 |
140 mm/ms2 |
35 mm/ms2 |
Due to less thickness the acceleration of node 5118 in 1.2mm thick crash box is higher when compared.
22. Crosssectional force.
| Section |
1.2mm thick crash box |
1.5mm thick crash box |
| Middle section of crush box |
55KN |
55KN |
Cross sectional forces generated on both the crush box models are almost same.
23. Maximum Directional stress and strain.
| Maximum Directional stress/strain |
1.2mm thick crash box |
1.5mm thick crash box |
| X-stress |
179.3 MPa |
208.2 MPa |
| Y-Stress |
215.8 |
198.6 |
| X-strain |
5.482e-4 |
5.381e-4 |
| Y-strain |
7.721e-3 |
7.948e-3 |
"Based on the stress and strain values we can conclude that the 1.5mm thick box is deformed less when compared with 1.2mm thick crash box when 13.88mm/ms (50kmph) initial velocity is applied. Th internal energy is high, reduced acceleration. "
LINK for simulation files 1.2 mm thick crash box : https://drive.google.com/drive/folders/1hsUD0R5RY7KkVK3W0YRBDF4czGbGfWXy?usp=sharing
LINK for simulation files 1.5 mm thick crash box : https://drive.google.com/drive/folders/1s8rUiW2Fe-UvA7WHsochrOMpe78HJxFB?usp=sharing
CONCLUSION :
1. The keywords are properly defined for simulation.
2. The Simulations are represented clearly.
3. Plots for various outputs are compared.
4. The results are reported.