Crash Box Simulation Using LS-DYNA

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 crashbox 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 unit’s system used is gm-mm-ms The simulation should satisfy either of the two situations - (1) It crashes on the rigid wall and debounces 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

Section properties: Keyword manager>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. Keyword manager>MAT>024-PIECEWISE_LINEAR_PLASTICITY.     MAT24 (Piece wise linear plasticity) material card is used to assign the steel material properties to the crash box. The MAT24 represent Piecewise linearisotropic plasticity. With this material model it is possible to consider the effect of the strain rate.  

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

Steel material properties

Mass Density [gm/mm]

7.85e-3

Young’s Modulus [MPa]

210e3

Poisson's ratio

0.3

Yield stress [MPa]

355

Tangent modulus [MPa]

1.3e3

Keyword manager> HOURGLASS.  To avoid Hourglass energy generation, we will create an *HOURGLASS card with IHQ value as 2 and the rest as default. Now assign this Hourglass card to the Crash_box part.Viscous hourglass control is recommended for problems deforming with high.

Part definition: Keyword manager>PART

The section, material and hourglass ID are assigned to the part card respectively as shown. BOUNDARY CONDITIONS: Initial Velocity: Create Entity>Initial>Velocity>cre   The crash box is assigned with initial velocity of V= 11 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 coefficient with a value of 0.08 is 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 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  

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 ‘.k’ extension and is made to run in the solver by getting normal termination message as shown  

  Now the solver deck setup is complete and ready to run the Analysis. We need to run the analysis for 2 thicknesses, one 1.2mm and the other, 1.5mm. The Solver deck that we have created here is for a 1.2 mm thick model. Thus, we need to create another solver deck for the 1.5 mm thick model. This can be done very easily by simply changing the thickness value under the section definition in the Keyword file by opening it in Notepad. Then save it as a new file.  

Normal Termination

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.    

ANIMATION:

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 1.5 mm thick crash box  

 

 Effective strain contour of 1.2 mm thick crash box  

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.  

X-stress in 1.2mmt crash box.  

X-stress in 1.5mmt crash box.

Y-stress in 1.2 mmt crash box

Y-stress in 1.5mmt crash box.

Y-Strain 1.2 mm crash box

X-strain 1.2mm thickness

X-Strain 1.5mm thick

Y-strain 1.5mm thick 

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.  

Energy Plots for 1.2 mm thickness
 

Energy plot for 1.5mm thickness

The Global Energy plots show the Kinetic energy, internal energy, Total energy, Sliding energy, and Hourglass energy values for the crash simulation. The Energies are higher for the 1.5 mm thick crash box. The total energy for the 1.2 mm thick and 1.5 mm thick crash box was recorded as 0.11E+6 N-mm and 0.14E+6 N-mm respectively.   At 0 ms, the whole of the total energy is Kinetic energy. But as the simulation progresses and the impact between the Crash box and the Rigid wall takes place, at approx 0.36 ms, the Kinetic energy starts getting converted into Internal energy. The Internal energy reaches its peak and the kinetic energy reaches its lowest value at about 1.52 ms when the crash box is on the verge of getting rebounded from the rigid wall. The crash box then moves with a constant rebound velocity which is reflected in the energy plot as internal energy gets converted to kinetic energy and remains constant through the rest of the simulation.

ACCELERATION PLOTS:

The acceleration plot shows 0 acceleration up to 0.36 ms as the crash box is moving with a uniform velocity until this point. Thereafter, it becomes non-zero as the impact takes place and goes on to a maximum value of 201000 mm-per-(ms)"2 at -0.48 ms when the crash box starts moving in the opposite direction on rebounding.

VELOCITY PLOTS:

               1.2 mm thickness model                                                                         1.5mm thickness model

The sectional force recorded is higher for the 1.5mm thick model(201000N compared to 161000N recorded for the 1.2 mm thick model) The sectional forces are zero until the impact happens. On impact, the force can be seen to have suddenly increased to its peak value. Thereafter, the force gets reduced and oscillates about a mean value much lesser than the peak force.

CONCLUSION:

The solver deck was set-up for the crash simulation and the required results were requested using the appropriate *DATABASE keyword cards. The analysis ran successfully. Post-processing of the results was done using the D3plots and the other output ASCII result files, to produce the deliverables.

The keywords necessary to build the crash box analysis was created from scratch.

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