CRASH BOX SIMULATION 

CRASH BOX SIMULATION 

Objective: To simulate a crash test for a crash box from the given FE model with 1.20mm thickness  and comparing the acceleration and stress/strain plot with 1.50mm thickness of the same model.

Step 1: The file is loded to Ls-Prepost.

Step 2: *Section cards are created for the Shell elements and thickness of 1.2 mm and element formulation of 2 is assigned.

Step 3: Material is being created with *MAT_PIECEWISE_LINEAR_PLASTICITY (MAT024) card and steel properties are given to the part.   

Steel properties: E=2.1e5, Density=7.85e-3, Pr=0.3, σy=1000 and ETAN=10000 (g,mm ms)

Step 4: Sections and materials are assigned to the respective parts in Parts sections.

Step 5: Planar rigid wall is created with the normal facing opposite to the velocity vector. 7 mm gap is kept between the shell part and the rigid wall.

Step 6: Control Termination has been given to the model with DT=3 ms and control enegry is also defined with hourglass taken into consideration.

Step 7: Database Binary D3plot card has been given to the model with DT=0.1 ms so that the output data will be written every 0.1 milli second of the total run time (3 ms).

Step 8: Database ASCII is also written with GLSTAT, MATSUM, RCFORC, SECFORC with DT as 0.1 ms. 

Step 9: Database crosssec is also defined so that the cross sectional force can also be determined. For that, a node set and a shell set is defined and SECFORC card is opt in the database ASCII options. 

Step 10: Contact has been defined with the card Automatic_single surface by defining slaves as the shell parts so that once the shell part starts to buckle up, the conatct will get initiated and penetrations will be avoided.

Step 11: Intial velocity of 13.88mm/ms is given to the model in positive x direction.

Step 12: An spc set is also given to the intial nodes which touches the Rigid wall by constraining Y and Z translation so that the model will behave like in reality.

Step 13:The model is checked with "model checking" tab and the the file has been saved as Keyword file (.k).

Step 14: The Key file is being loaded into ls-program-Manager and is allowed to run.Normal termination has occured and D3 plots are automatically written and saved to the folder.

Step 15: The D3 plot file is opened in ls-prepost and the output has been checked.

The same simulation is ran again by changing the thickness value to 1.5 and the accelerations and stress/strain plots are compared.

RESULTS OBTAINED:

1.Acceleration plot of a node in the middle of the crashbox (along its length)

Node no: 8265

the maximum acceleration occurs at 0.9ms which is 1.77e3mm/ms2 

2.Cross sectional force generated in the middle of the crashbox (along its length)

The max section force along the length is 1.04e3 N at 2.3ms

3.PLOTS OF ENERGIES

Here from the plot, it is clear that the total energy is constant and as the kinetic energy decreases, internal enegy is rising up. hourglass and sliding energy also lies around zero.

4.Accelerations and stress/strain plots of 1.2/1.5mm crashboxes

Stress

Here from the image it is clear that as thickness increased, the stiffness of the model is increased and effective stress is reduced. the minimun stress is recored in 1.50mm thickness model that is 3.14e2 pa.

The above image shows the effective von mises stress for 1.20mm thckness model.

Tha above image shows the effective von mises stress for 1.50mm thckness model.

Strain

The strain values for both the thickness model lies around zero in this case

Accelerations.

Here from the below plots, the maximum acceleration for the model with thickness 1.20mm is 0.061mm/ms2 at 1.5ms

Here from the below plots, the maximum acceleration for the model with thickness 1.50mm is 0.062mm/ms2 at 1.0ms

5.Maximum directional stress and strain along the length of the crashbox (X strain, Y strain etc)

The maximum directional stress along x axis at the end of simulation is 2.95e2 pa

The maximum directional stress along y axis at the end of simulation is 2.21e2 pa

The maximum strain obtained in both x and y direction is nearly 0.

CONCLUSION

The crash box model is simulated from the given FE model with 1.20mm thickness  and  the acceleration and stress/strain plot with 1.50mm thickness of the same model is compared and the results are obtained. From the results it is clear that the model with 1.50mm showed a reduced stress value.

NOTE: Here in the simulation, the buclking of crash box is not acheived since the velocity on hitting the rigid wall is converted to internal energy and hence it got re bounced back to its original position

The crushing effect is acheived by introducing Boundary Prescribed Motion into the model

Boundary Prescribed Motion is defined by selecting a few top nodes from the part.

The data curve is defined with Abscissa and ordinate values as (0,0),(10.45,145),(10.46,145)

This is calculated so as to get the 13.88mm/ms velocity for the crash box.

speed=distance/time

speed=13.88, distance= 145(i have taken only the 145 mm length fron the part into consideration since the nodes from BPM will not show the result when it hits the rigid wall)

time=?

13.88=145/t

t=145/13.88=10.45ms

In boundary card, BPM is defined as the LCID is also mentioned.

The Buckling model is also attached in the same file path.

Thank you :)