Week - 4 - Crash Box Simulation

#5 // Week - 4 _ Crash Box Simulation.

AIM: To extract the model into LS-DYNA, and to create all the required keywords into the model, by creating a Rigid wall in it, with a crash velocity of 50 kmph which is 13.89 mm/ms, with two different thicknesses needed to be assigned.

Set-up:

A crash box is a highly energy absorbing structure that crashes on application of loads and reduces impact on other components nearby. A full-fledges crashbox is a highly sophisticated design but in this case we will go with a rectuangular channel which can thought of as the most simple crashbox.

Here are the different assumptions and information required:

1. The crashbox should crash into a rigidwall created using *RIGIDWALL_ keyword. Check the manual to understand and create a rigidwall.
2. Material for the crashbox is steel. The simulation needs to be started using elastic material to verify its correctness and later a nonlinear material card data will be provided. Thickness of the crashbox is 1.2mm.

• Initial velocity for the crashbox should be around 50 kmph

1. The units system to be used is gm-mm-ms
2. The simulation should satisfy either of the two situations - (1) It crashes on the rigidwall and rebounces completely or (2) it buckles and folds in the axial direction so as to come in self-contact.
3. Once the simulation is completed and results as asked for below is collected, run another simulation by increasing the thickness of the crashbox from 1.2 to 1.5mm.

OBJECTIVE:

1. Input “.k” file and output files (d3plot, glstat, sleout, rcforc)
2. Animation of the final simulation
3. Cross-sectional force generated in the middle of the crashbox (along its length)
4. Acceleration plot of a node in the middle of the crashbox (along its length)
5. Maximum directional stress and strain along the length of the crashbox (X strain, Y strain, etc)
6. Plot of all energies (total, internal, kinetic, hourglass, sliding)
7. Compare the accelerations and stress/strain plots of 1.2/1.5mm crashboxes.

PROCEDURE:

• Import the model into LS-Dyna as “k.” file.

1. Rigid wall:

Create Entity > Rigidwall > Cre > Planar > 3Nds.

The rigid wall is created at a distance of 10mm away from the crash box.(in X-axis)

> Also we can check the Normals of Rigidwall using the Draw option in Keyword manager.

2. Material properties:

> The material for the crash box is steel. The simulation needs to be started using elastic material to verify its correctness and later a nonlinear material card data will be provided. The thickness of the crash box is 1.2mm.

> So, initially we use an elastic material for a trial run then we change it to Plastic material.

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

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.

> 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	Value
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

3. Keyword manager > HOURGLASS.

To avoid Hourglass energy generation, we will create an *HOURGLASS card with an IHQ value of 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.

4. 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.

5. Part definition:

Keyword manager > PART.

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

6. BOUNDARY CONDITIONS:

Initial Velocity:

Create Entity > Initial > Velocity > Cre.

> The initial velocity for the crash box should be around 50 kmph(13.88mm/ms).

> The crash box is assigned with an initial velocity of V= -13.88 mm/ms applied along the '+ve X' direction to crash against the rigid wall.

7. 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.

8. 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 1 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 0.01 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 7525 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 a 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.

• Keyword manager > DATABASE > ASCII_option.

Output Request: Using database pannel we can request such as animation file energy data, contact forces, etc.

BINARY3DPLOT - The complete state dump interval as given in *DATABASE_BINARY_D3PLOT serves as the time step to be used in the material model driver run. Plotting information is saved in the core for the interactive plotting phase.

ASCII_option- This option is used to read the data of the following ….

GLSTAT - Reade the energy like KE, PE, IE.

NODEOUT - reade node data such as displacement, and acceleration.

MATSUM - Every material part’s energy is due to deformation.

RCFORCE - contact force b/w them.

SECFORC - section force.

SLEOUT - sliding force due to contact.

SPCFORCE - single point contact force.

RESULTS:

Setup the Crash Box Simulation model for impact test using LS-Dyna and simulating the same is done.

• 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..

• The D3plot output File is opened in LS-PrePost using the 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 shown below.

> Crash Box with 1.5 mm Thickness.

> Crash Box with 1.5 mm Thickness.

/ Effective stress (v-m) animation contour of Crash box.

> Crash box with 1.2 mm thickness.

> Maximum Effective stress 2.984e+02 ms is produced.

> Crash box with 1.5 mm thickness.

> Maximum Effective stress 2.984e+02 ms is produced.

/ Effective plastic strain contour of the Crash box.

> Crash box with 1.2 mm thickness.

> Maximum Effective plastic strain 3.336e-02 ms is produced.

> Crash box with 1.5 mm thickness.

> Maximum Effective plastic strain 3.342e-02 ms is produced.

> / Cross-sectional force: By using cross-sectional force we can find what is the force that traveling at a particular place by creating a section.

1. For a 1.2mm thick crash box the maximum cross-sectional force is 6.64e+04 N.

2. For a 1.5mm thick crash box the maximum cross-sectional force is 8.94e+04 N.

> / Acceleration plot of a node in the crash box:

• The resultant acceleration is plotted for node 7525.
• Acceleration remains constant before impact. Due to the impact sudden drop in velocity leads to noise in Acceleration.

> Resultant acceleration plot of a node in a 1.2mm thick crash box.

• For a 1.2mm thick crash box a resultant displacement is developed during the collision with a rigid wall and increases to a maximum of 10.7 mm at a time of 0.8 ms and decreases while the crash box is rebounded.

/ Resultant acceleration = 670mm/ms^2 at 0.9ms.

> Resultant acceleration plot of a node in a 1.5 mm thick crash box.

• For a 1.5mm thick crash box, a resultant displacement is developed during the collision with a rigid wall and increases to a maximum of 10.7 mm at a time of 0.8 ms and decreases while the crash box is rebounded.

/ Resultant acceleration = 536mm/ms^2 at 0.9ms.

From the Resultant acceleration, we can observe that for a less thick 1.2mm crash box node 7525 has higher acceleration than the 1.5mm thick crash box.

> / Maximum directional stress and strain along the length of the crash box:

• For a 1.2 mm thick crash box maximum directional stress & strain along the length of the crash box (X-axis).

> Maximum directional X-stress = 2.405e+02

> Maximum directional X-strain Residual = 7.620e-06

• For a 1.5 mm thick crash box maximum directional stress & strain along the length of the crash box (X-axis).

> Maximum directional X-stress = 2.469e+02

> Maximum directional X-strain Residual = 7.748e-06

For thickness differences, X-stress & X-strain values are higher for a 1.5mm thicker crash box.

This bit of higher value is because more Kinetic Energy energy is generated for a higher-thickness crash box.

> / Energy plots:

The energy plots comprising 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.

> All Energy plots for 1.2 mm thick crash box.

> All Energy plots for 1.5 mm thick crash box.

1. Initially the velocity is maximum so the Kinetic energy is maximum. when impact takes place, the KE is absorbed by part, so that KE is decreasing and internal energy is increasing.
2. At the time of contact the sudden decrease in KE and increase in IE. Total Energy varies as per TE = IE-KE
3. Once deformation reaches max, due to PLAT wall stops velocity TO ZERO and rebound back with some velocity so same time the KE increases,

The Crash box with 1.5mm has more energy than 1.2 because KE is more for 1.5 and due to impact the energy required to stop the crush box is more so that more internal energy is created.

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 run 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 were created from scratch.
• The model was solved successfully and the results are compared for both 1.2 mm and 1.5 mm thick crash boxes.
• The results of the 1.5 mm crash box showed higher energy absorbing capacity and less deformation compared to the 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.