Crash Box Simulation

Aim:

To simulate the crash test for the given crash box having a thickness of 1.2 and 1.5mm onto a rigid wall at 50kmph and to obtain the following output and data.

• 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.

Theory:

The crash box is a thin-walled component which deforms axially and absorbs the energy during impact. The crash box is widely used in the automobile and aerospace industry to absorb accidental impact energy. So in the case of an automobile, the crash box is a separate component that is mounted in between the mainframe of the car and the front bumper. At the time of the accident, it deforms axially and absorbs accidental energy. Crash box structure provides comfort to the passenger at the time of impact. It works as a safeguard for the costly components behind the bumper like the engine hood and cooling system.

Procedure:

1. Open the new_crash_box.k file in LS-PrePost.

Fig 1: File opened in LS-PrePost

In the opened file, the component (crash box) has meshed with shell elements. The crash box has meshed with 9656 elements/shells and 9792 nodes.

The unit system followed for the unit consistency is gm-mm-ms.

2. Create a Shell section card using the Model and Part > Keyword Manager > SECTION > Select the Shell option with Element formulation=2 and thickness = 1.2mm.

Fig 2: Defined Shell section card

3. Define the material for the crash box component. Select the 024-PIECEWISE_LINEAR_PLASTICITY material card for the crash box component.

Model and Part > Keyword Manager > MAT > Select the Material card

Next, define the material properties like mass density, Young’s modulus, Poisson’s ration, Yield point stress value, and other required quantities for the material cards.

• Density= 0.00783 `(gm)/(mm^3)`
• Young's modulus=2.10 e+05 `(gm)/(mm*ms^2)`
• Poissions ratio=0.33
• Yield strength =310 `(gm)/(mm*ms^2)`
• Tangent modulus = 2100 `(gm)/(mm*ms^2)`

Fig 3: Defined the material card

4. Specifying the respective Material and Section ID to the crash box component part IDs.

Fig 4: Specified the material and section card to the Part ID

5. Next step is to create a Rigid wall in front of the crash box 10 mm away by using the *RIGIDWALL_PLANAR card

This is done by selecting Entity Creation > RigidWall > Create. The rigid wall type is selected as a Planar and defined vector with one point and a normal ie, 1n+NL here selected the one node, and the normal is created as a rigid wall. This rigid wall is translated/offset by 10mm from the crash box component.

The black outer line shows the exact location of the rigid wall from the crash box component.

Fig 5: Created a Rigid wall in front of the crash box

Note: The direction of normal (here x-direction) from the rigid wall and the velocity vector (x-direction) should be in the opposite direction in order to avoid the penetration of the crash box through the rigid wall which is not acceptable since it is a rigid wall and the component is a material which shows the plasticity.

6. Next step is to define a velocity of 50kmph (13.8889 mm/ms) and assigned to each node of the selected crash box component, under Entity Creation >Initial>Velocity>Insert the value. The Crash box is selected by pick option with respect to part as shown below.

Fig 6: Defined a velocity to the crash box

7. Create a Contact using Model and Part > Keyword Manager > Contact > Automatic Single Surface. The slave segment type (SSTYP) is defined by the Part ID, here there is no need to define a master surface so only the slave surface definition is sufficient.

After the automatic contact was created as the crash box elements are getting self contact during crashing ie, Automatic single surface here it was not needed to define the master surface because the crash box itself is master and slave.

Fig 7: Defined an Automatic single surface contact

8. Next the Termination time is to be assigned by using *CONTROL_TERMINATION card. Here the time to end the simulation can be calculated analytically by using the distance between the rigid wall and crash box component ie,10mm, and the velocity of the crash box ie,13.8889 mm/ms.

End time, t = (10mm)/(13.8889 mm/ms) = 0.7199

The calculated time is 0.7199ms but extra time is considered after the impact in simulation ie, the crashing and the bouncing back of the crash box. So, the total time of 1ms is assigned as end time in the *CONTROL_TERMINATION card.

Fig 8: Created *CONTROL_TERMINATION card

calculate the energy dissipation, Hourglass energy, rigid wall energy, sliding interface energy that can be done by *CONTROL_ENERGY card.

Finally create a *CONTROL_HOURGLASS card for control hourglass effect in the analysis with 1HQ=2, QH=0.1.

9. Now, the following output data is generated by using the *DATABASE card with a time step of 0.01ms are assigned for generating output data.

DATABASE_BINARY_D3PLOT --> Used to find out the animation of the process,

DATABASE_ASCII_NODOUT --> To find the Nodal motions,

DATABASE_ASCII_ELOUT --> To find the element data,

DATABASE_ASCII_GLSTAT --> To find the global statistics and energies,

DATABASE_ASCII_MATSUM --> To find the material energies,

DATABASE_ASCII_RCFORC --> To find the Resultant contact interface forces,

DATABASE_ASCII_SLEOUT --> To find the Contact interface energies

DATABASE_ASCII_SECFORC --> To find the Cross-section forces data

For generating section forces creating a specific node and element/shell list which defines the desired section of component is necessary. The selection of nodes and elements/shell lists is done by picking each node and element through Creating Entity>Set Data>*SET_NODE for node-set and Creating Entity>Set Data>*SET_SHELL for element/shell-set.

Fig 9: Node and shell set created

*DATABASE_CROSS_SECTION_SET card is used so as get nodes and element/shell set from the component and are assigned for node-set ID (NSID) and shell element set ID (SSID) respectively.

Fig 10: Created *DATABASE_CROSS_SECTION_SET card

The acceleration plot of a node and element of the crash box can be done by picking the desired node and element by using *DATABASE_HISTORY_NODE and *DATABASE_HISTORY_SHELL card.

Fig 11: Created *DATABASE_HISTORY_NODE card

Fig 12: Created *DATABASE_HISTORY_SHELL card

10. After assigning all the above-mentioned keywords, the model is ready for the simulation. The keyword file (.k) can be edited in notepad++ so that the comparison study can be done for the part which is having 1.2mm and 1.5mm thickness. Save the keyword for both files and open in the LS-DYNA manager which solves the problem.

Fig 13: Saved keyword file ran in LS-DYNA solver

Output Results:

The cross-sectional force generated in the middle of the crash box:

Fig 14 (a): Cross-sectional force generated in x, y, z direction along with resultant force for 1.2 mm shell thickness crash box

Fig 14 (b): Cross-sectional force generated in x, y, z direction along with resultant force for 1.5 mm shell thickness crash box

Note: From the above figure 14(a) & 14(b) we can able to see that the force value at the Y and Z direction after impact is very low since the velocity value is in the X-direction. At the impact time, we can see that the force value in X-direction peaks up and goes down after bounce-back of the crash box. The Resultant force of a 1.5 mm thickness crash box is higher than that of a 1.2 mm thickness crash box since as per Newton's second law we know that F=ma, here 1.5 mm thickness crash box is having higher mass compared to the 1.2 mm thickness crash box and here the acceleration is same in both the case.

Acceleration plot of a node in the middle of the crash box:

Fig 15(a): Acceleration plot of a node(7811) in x, y, z direction along with resultant acceleration for 1.2 mm shell thickness crash box:

Fig 15(b): Acceleration plot of a node (7811) in x, y, z direction along with resultant acceleration for 1.5 mm shell thickness crash box:

Note: From the above figure 15(a) & 15(b) we can able to see that the nodal acceleration in the X-direction is negative since the accelerated body when came to impact there a deceleration happened. Here the value of deceleration in X-direction is nearly the same and the resultant acceleration is a magnitude and doesn't depend on the direction so we can see the positive value and almost equals the value of X-direction magnitude since the acceleration value in the Y and Z direction is very small.

Stress plots for the crash box:

Fig 16(a): Effective Stress(v-m) for 1.2 mm shell thickness crash box:

Fig 16(b): Effective Stress(v-m) for 1.5 mm shell thickness crash box:

Fig 16(c): Effective Stress(v-m) for Element (6132) in 1.2 mm shell thickness crash box:

Fig 16(d): Effective Stress(v-m) for Element (6303) in 1.5 mm shell thickness crash box:

Note: From the above figure 16(a) & 16(b) we can able to see the Effective Stress(v-m) for 1.2 mm and 1.5 mm shell thickness crash box. Here (from figure 16(c) & 16(d)) we have taken Element no: 6132 from 1.2mm thickness crash box and Element no: 6303 from 1.5mm thickness crash box. The value of stress in the particular elements that we took for study exceeds the yield strength = 310 `(gm)/(mm*ms^2)` and so there will be permanent deformation in the crash box.

Here in the 1.5 mm shell thickness crash box, we can see the lesser value of stress generation than the 1.2 mm shell thickness crash box since here more resisting area to the impact/load.

Strain plots for the crash box:

Fig 17(a): Effective Plastic Strain for 1.2 mm shell thickness crash box:

Fig 17(b): Effective Plastic Strain for 1.5 mm shell thickness crash box:

Fig 17(c): Effective Plastic Strain for Element (6132) in 1.2 mm shell thickness crash box:

Fig 17(d): Effective Plastic Strain for Element (6303) in 1.5 mm shell thickness crash box:

Note: From the above figure 17(a) & 17(b) we can able to see the Effective Plastic Strain for 1.2 mm and 1.5 mm shell thickness crash box. Here (from figure 17(c) & 17(d)) we have taken Element no: 6132 from 1.2mm thickness crash box and Element no: 6303 from 1.5mm thickness crash box. From the above plot, we can see that there is a permanent deformation on the element that we took for study and this happened due to the higher value of stress which exceeds the yield strength = 310 `(gm)/(mm*ms^2)` and so there will be permanent deformation in the crash box since it shows a constant line in the graph even after the impact. These value of strain was calculated as 0.007 in Element no: 6132 from 1.2mm thickness crash box and 0.003125 in Element no: 6303 from 1.5mm thickness crash box

Energy plots for the crash box (total, internal, kinetic, hourglass, sliding):

Fig 18(a): Energy plot for 1.2 mm shell thickness crash box:

Fig 18(b): Energy plot for 1.5 mm shell thickness crash box:

Note: From the above figure 18(a) & 18(b) we can able to see that the 1.5mm thickness crash box is having higher momentum (ie, p =mv) and higher energies compared to the 1.2 mm thickness crash box. Here the velocity and acceleration are the same at the initial time but the mass value is different.

Initially, the crash box is having kinetic energy (ie, K.E = 1/2mv^2) but after the impact, we can able to see that the kinetic energy is converted to internal energy. Hereafter nearly t=0.8ms that impact we can see that the crash box bounces and gains small velocity is gained that is shown in the graph above figure 18(a) & 18(b).

If you see in the above figure 18(a) & 18(b) we can find out that hourglass energy is nearly zero since we had considered the control keyword for the hourglass problem and within the control.

Hourglass: It is a zero-energy deformation mode ie, the element seems deformed but no energy recorded for particular deformation.

Here the contact keyword is only mentioned as the Automatic_single_surface for the crash box so the force due to contact will not be recorded and if you see the DATABASE_ASCII_RCFORC and the DATABASE_ASCII_SLEOUT will not return any value as plot since the rigid wall part is undefined ie, the master part ID. So we see the value of Sliding energy as zero in the graph.

Here the total energy of the system remains constant since there is no energy loss/gain in the system but there will be small deformation since we took the MAT_24 material property and it will be very small in value.

Conclusion:

• Simulated the crash test for the given crash box having a thickness of 1.2 and 1.5mm onto a rigid wall at 50kmph.
• The material card and all other keywords are properly defined for the simulation.
• The crash box bounces back after impacting on the rigid wall.
• Plots for the various terms are studied.

The link for the keyword file is mentioned below:

https://drive.google.com/drive/folders/1fwsH4o2mtQufwEb2bQMwTP0DMxlQRJcX?usp=sharing