Week - 4 - crash box simulation

                                                                                                                        CRASH BOX SIMULATION

AIM: To create a complete simulation file for crash analysis of crash box having two different thickness of 1.2 mm and 1.5 mm from the given FE model of crash box and produce the following 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 crash box (along its length)
• Acceleration plot of a node in the middle of the crash box (along its length)
• Maximum directional stress and strain along the length of the crash box (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 crash boxes.

INTRODUCTION:

A crash box is a highly energy absorbing structure that crashes on application of loads and reduces impact on other components nearby. A full-fledged crash box is a highly sophisticated design but, in this case, a rectangular channel structure is considered as most simple crash box.

Following are the different assumptions and information required:

• The crash box should crash into a rigid wall created using *RIGIDWALL_ keyword.
• Material for the crash box is steel.
• Thickness of the crash box is 1.2 mm.
• Initial velocity for the crash box is around 50 kmph
• The units system used is gm-mm-ms
• The simulation should satisfy either of the two situations - (1) It crashes on the rigid wall and rebounces completely or (2) it buckles and folds in the axial direction so as to come in self-contact. 
• Once the simulation is completed results are collected and another simulation is executed by increasing the thickness of the crash box from 1.2 mm to 1.5 mm.

PROCEDURE:

The given LS-Dyna keyword file is opened in LS-PrePost using option File>Open>LS-Dyna Keyword File as shown in the fig. 1.

                                                                                                                            Fig.1. Imported FE model of Crash box.

PART DEFINITION:

The part is defined by assigning the section properties and material properties.

Section properties:

Keyword manager>SECTION>SHELL.

                                                                                                                         Fig.2.Section shell.

The section properties of crash box are assigned as shell element with 1.2 mm thickness and ELFORM=2, Belytschko-Tsay element formulation.

Material properties:

The crash box is assigned with steel material with following parameters as shown in table.

Steel material properties	Value
Mass Density [gm/mm3]	7.85e-3
Young’s Modulus [MPa]	210e3
Poisson's ratio	0.3
Yield stress [MPa]	355
Tangent modulus [MPa]	1.3e3

Keyword manager>MAT>024-PIECEWISE_LINEAR_PLASTICITY.

                                                                                                                 Fig.3. Steel material

MAT24 (Piece wise linear plasticity) material card is used to assign the steel material properties to the crash box. The MAT24 represent Piecewise linear isotropic plasticity. With this material model it is possible to consider the effect of the strain rate.

Keyword manager> HOURGLASS.

                                                                                                                    Fig.4. Hourglass.

Viscous hourglass control is recommended for problems deforming with high velocities i.e, Hourglass control type=IHQ=2, Flanagan-Belytschko viscous form.

Part definition:

Keyword manager>PART

                                                                                                                      Fig.5. Part definition of crash box.

The section, material and hourglass ID are assigned to the part card respectively as shown in fig 5.

BOUNDARY CONDITIONS:

Initial Velocity:

Create Entity>Initial>Velocity>cre

                                                                                                                    Fig.6. Initial velocity.

The crash box is assigned with initial velocity of V_x= -13.88 mm/ms is applied along -ve x direction to crash against the rigid wall.

Rigid wall:

Create Entity>Rigidwall>cre>Planar>1n+NL

                                                                                                                   Fig.7. Rigid wall.

The rigid wall is created at a distance of 10mm away from the crash box.

CONTACT CONDITION:

Keyword manager>CONTACT>AUTOMATIC_SINGLE_SURFACE

                                                                                                             Fig.8. Contact between Rigid wall and crash box.

The contact type selected is AUTOMATIC_SINGLE_SURFACE. It is quite helpful to apply this contact method in the crash models because all the elements are included in one single set and LS-DYNA considers also when a part comes into contact with itself. The FS and FD that are static and dynamic friction coefficient with a value of 0.08 is entered in the contact card.

CONTROL FUNCTION:

Keyword manager>CONTROL>ENERGY

                                                                                                              Fig.9. Control energy.

The control energy function is enabled for computing the hourglass energy, stonewall energy and sliding energy.

Keyword manager>CONTROL>TERMINATION

                                                                                                       Fig.10. Control termination.

The control termination function is enabled to specify the end time of simulation. The termination time is set for 2 ms to capture the effect of crash box striking the rigid wall. 

DATABASE OPTION:

Keyword manager>DATABASE>BINARY_D3PLOT

                                                                                                            Fig.11. Database binary_D3plot.

The time step value of 0.02 ms is given for the BINARY_D3PLOT and in the DATABASE_ASCII option for GLSAT, MATSUM, NODOUT, RCFORC, RWFORC, SECFORC and SLEOUT.

Keyword manager>DATABASE>EXTENT_BINARY

                                                                                                         Fig.12. Database binary extent.

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

Keyword manager>DATABASE>HISTORY_NODE

                                                                                                    Fig.13. Database history node.

DATABASE_HISTORY_NODE card is used to compute the acceleration of a 5094 node in the crash box FE model.

Keyword manager>DATABASE>CROSS_SECTION_PLANE

                                                                                                                Fig.14. Data base cross section plane.

DATABASE_CROSS_SECTION_PLANE card is used to compute the sectional force of the crash box.

                                                           Fig.15. Normal termination.

The keyword file created is checked for errors using the option keyword manager>model check. The keyword file is saved using ‘.k’ extension and is made to run in the solver by getting normal termination message as shown in fig. 15.

RESULTS:

The D3plot output file is opened in LS-PrePost using option File>open>LS-Dyna binary plot.

The animation of Von-Mises stress and Effective plastic strain for 1.2 mm and 1.5mm thick crash box is as shown below.

                                 v-m stress animation contour of 1.2 mm thick crash box

                                       v-m stress animation contour of 1.5 mm thick crash box

                                    Effective plastic strain contour of 1.2 mm thick crash box

                                   Effective plastic strain contour of 1.5 mm thick crash box

Sectional force:

The sectional force along the length of crash box is plotted for both 1.2mm and 1.5mm thick crash box.

                                                                                                    Fig.16. Resultant sectional force on 1.2mmt crash box.

For 1.2mm thick crash box a resultant force of 21.9 KN is developed during the collision with rigid wall and increases to a maximum of 43.2 KN at time 0.781 ms and decreases while the crash box is rebounced.

                                                                                                Fig.17. Resultant sectional force on 1.5mmt crash box.

For 1.5mm thick crash box a resultant force of 28.3 KN is developed during the collision with rigid wall and increases to a maximum of 55.5 KN at time 0.781 ms and decreases while the crash box is rebounced.

Acceleration plot of a node in the crash box:

The resultant acceleration is plotted for a node 5094 with close proximity to the rigid wall.

                                                                                          Fig.18. Resultant acceleration plot of a node in 1.2mmt crash box.

For 1.2mm thick crash box a resultant acceleration of 0.687 x 10³ mm/ms² is developed during the collision with rigid wall and increases to a maximum of 8.24 x 10³ mm/ms² at time 1.28 ms and decreases while the crash box is rebounced.

                                                                                       Fig.19. Resultant acceleration plot of a node in 1.5 mmt crash box.

For 1.5mm thick crash box a resultant acceleration of 0.687 x 10³ mm/ms² is developed during the collision with rigid wall and increases to a maximum of 7.69 x 10³ mm/ms² at time 1.30 ms and decreases while the crash box is rebounced.

Directional stress along the length of the crash box.

The directional stress like x-stress, y-stress and z-stress is plotted for the crash box with 1.2 mm and 1.5 mm thickness.

                                                                                              Fig.20. X-stress in 1.2mmt crash box.

                                                                                             Fig.21. X-stress in 1.5mmt crash box.

From the graph it is observed that the x-stress developed is same for both the crash box during collision and is slightly increased for 1.2mm thick crash box while rebounding.

                                                                                           Fig.22. Y-stress in 1.2mmt crash box.

                                                                                             Fig.23. Y-stress in 1.5mmt crash box.

From the graph it is observed that the y-stress developed is fluctuating more in case of 1.2mm thick crash box compared to 1.5mm thick crash box.

                                                                                             Fig.24. Z-stress in 1.2mmt crash box.

                                                                                             Fig.25. Z-stress in 1.5mmt crash box.

From the graph it is observed that the z-stress developed is fluctuating more during the time of collision in case of 1.5 mm thick crash box compared with 1.2 mm thick crash box.

Directional strain along the length of the crash box:

The directional strain like x-strain, y-strain and z-strain is plotted for the crash box with 1.2 mm and 1.5 mm thickness.

                                                                                               Fig.26. X-strain in 1.2mmt crash box.

                                                                                             Fig.27. X-strain in 1.5mmt crash box.

From the graph it is observed that the x-strain developed is negative i.e compressive strain is developed slightly more in case of 1.5mm thick crash box compared to 1.2mm thick crash box.

                                                                                              Fig.28. Y-strain in 1.2mmt crash box.

                                                                                                 Fig.29. Y-strain in 1.5mmt crash box.

From the graph it is observed that the y-strain developed is fluctuating more in case of 1.2mm thick crash box compared to 1.5mm thick crash box.

                                                                                                Fig.30. Z-strain in 1.2mmt crash box.

                                                                                                 Fig.31. Z-strain in 1.5mmt crash box.

From the graph it is observed that the z-strain developed is slightly more in case of 1.5mm thick crash box compared to 1.2mm thick crash box.

Energy plots:

The energy plots comprising of kinetic energy, internal energy, total energy, hourglass energy and sliding energy were plotted for 1.2 mm and 1.5 mm thick crash boxes. The graph represents an energy balance of a dynamic test on a crash box and it can be noted that as the kinetic energy decrease, the internal energy increases as expected from the theory. The other important things to consider are that the total energy has to remain constant and the sliding interface must remain low.

                                                                                            Fig.32. The Energy plot of 1.2mmt crash box.

                                                                                       Fig.33. The Energy plot of 1.5mmt crash box.

From the energy plot graph, it is observed that the kinetic energy is reduced during the time of collision to a value of 6.08x10³ N-mm in case of 1.2 mm thick crash box and 7.58x10³ N-mm in case of 1.5 mm thick crash box. The internal energy is increased to a value of 1.01x10⁵ N-mm in case of 1.2 mm thick crash box and 1.27x10⁵ N-mm in case of 1.5 mm thick crash box.

Maximum Resultant Acceleration plot of crash box:

 

                                                                                         Fig.34. The Resultant acceleration plot of 1.2mmt crash box.

From the graph, it is observed that the maximum acceleration of 9.755x10³ mm/ms² is occurring in the 337 node of the crash box at a time of 0.819 ms.

                                                                                          Fig.35. The Resultant acceleration plot of 1.5mmt crash box.

From the graph, it is observed that the maximum acceleration of 8.480x10³ mm/ms² is occurring in the 3425 node of the crash box at a time of 0.819 ms.

Effective (v-m) stress plot:

 

                                                                                           Fig.36. The v-m stress plot of 1.2mmt crash box.

From the graph, it is observed that the maximum v-m stress of 378.72 MPa is occurring in the 347 element of the crash box at a time of 0.799 ms is higher compared to the yield stress value of 355 MPa of the crash box material.

                                                                                Fig.37. The v-m stress plot of 1.5mmt crash box.

From the graph, it is observed that the maximum v-m stress of 380.594 MPa is occurring in the 346 element of the crash box at a time of 1.23 ms is higher compared to the yield stress value of 355 MPa of the crash box material.

Effective plastic strain plot:

                                                                            Fig.38. The Effective plastic strain plot of 1.2mmt crash box.

                                                                             Fig.39. The Effective plastic strain plot of 1.5mmt crash box.

From the graph, it is observed that the maximum Effective plastic strain induced is same for both the crash box during the time of collision and later it gets increased for 1.5mm thick crash box.

CONCLUSION:

1. The keywords necessary to build the crash box analysis was created from scratch.
2. The model was solved successfully and the results are compared for both 1.2 mm and 1.5 mm thick crash box.
3. The results of 1.5 mm crash box showed higher energy absorbing capacity and less deformation compared to 1.2 mm thick crash
4. Learned to build a solver deck for simple crash analysis in LS-Dyna and extract the outputs to interpret and compare the results.

 

Google Drive Link:https://drive.google.com/drive/folders/1JaWK9iJ_mPGmnof7xAFzXgiBvtkT_hor?usp=sharing