Week - 4 - Crash Box Simulation

AIM: To create a complete simulation file for crash analysis of crash box having two different thicknesses of 1.2 mm and 1.5 mm from the given FE model of the crash box and produce the following deliverables.

• Input .k file and output files (d3plot, glstat, sleout, rcforc)
• Animation of the final simulation
• The 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)
• The 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 high-energy-absorbing structure that crashes on the application of loads and reduces the 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 a most simple crash box.

Following are the different assumptions and information required:

• The crash box should crash into a rigid wall created using the *RIGIDWALL_ keyword.
• The material for the crash box is steel.
• The thickness of the crash box is 1.2 mm.
• The 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 rebounds 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

PART DEFINITION:

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

Section properties:

• Keyword manager>SECTION>SHELL.

• The section properties of the 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
• Keyword manager>MAT>024-PIECEWISE_LINEAR_PLASTICITY.

• MAT24 (Piecewise linear plasticity) material card is used to assign the steel material properties to the crash box. The MAT24 represents Piecewise linear isotropic plasticity. With this material model, it is possible to consider the effect of the strain rate.
• Keyword manager> 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

• The section, material, and hourglass ID are assigned to the part card respectively

BOUNDARY CONDITIONS:

Initial Velocity:

• Create Entity>Initial>Velocity>cre

• The crash box is assigned with an 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

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

CONTACT CONDITION:

• Keyword manager>CONTACT>AUTOMATIC_SINGLE_SURFACE

• 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 coefficients with a value of 0.08 are entered in the contact card.

CONTROL FUNCTION:

• Keyword manager>CONTROL>ENERGY

• The control energy function is enabled for computing the hourglass energy, stonewall energy, and sliding energy.
• Keyword manager>CONTROL>TERMINATION

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

DATABASE OPTION:

• Keyword manager>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

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

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

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

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

• 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 the crash box is plotted for both 1.2mm and 1.5mm thick crash boxes.

• For a 1.2mm thick crash box, a resultant force of 21.9 KN is developed during the collision with a rigid wall and increases to a maximum of 43.2 KN at time 0.781 ms and decreases while the crash box is rebounded.
• For a 1.5mm thick crash box, a resultant force of 28.3 KN is developed during the collision with a rigid wall and increases to a maximum of 55.5 KN at time 0.781 ms and decreases while the crash box is rebounded.

Acceleration plot of a node in the crash box:

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

• For a 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 8.38 x 10³ mm/ms² at time 1.28 ms and decreases while the crash box is rebounded.
• For a 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 7.68 x 10³ mm/ms² at time 1.30 ms and decreases while the crash box is rebounded.

Directional stress along the length of the crash box.

• The directional stress like x-stress, y-stress, and z-stress is plotted for element no-7038 in the crash box with 1.2 mm and 1.5 mm thickness.

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

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

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

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

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

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

• From the energy plot graph, it is observed that the kinetic energy is reduced during the time of the collision to a value of 6.08x10³ N-mm in the case of a 1.2 mm thick crash box and 7.99x10³ N-mm in the case of a 1.5 mm thick crash box. The internal energy is increased to a value of 1.01x10⁵ N-mm in the case of 1.2 mm thick crash box and 1.27x10⁵ N-mm in the case of 1.5 mm thick crash box.

Maximum Resultant Acceleration plot of the 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.
• 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:

• From the graph, it is observed that the maximum v-m stress of 379.16 MPa is occurring in the 346 elements 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.
• From the graph, it is observed that the maximum v-m stress of 380.596 MPa is occurring in the 346 elements 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:

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

CONCLUSION:

1. The keywords necessary to build the crash box analysis were 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 the 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 Link: https://drive.google.com/file/d/1b5oZ3i-ExAxf_YcCofLvRRTj_-ffvsBm/view?usp=sharing