Week - 4 - crash box simulation

OBJECTIVE: To simulate a crash test for the given FE model of a crash box using LS-Dyna. Request for the given deliverables and compare the results for a 1.2 mm thick and 1.5 mm thick crash box.

INTRODUCTION:

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

1. Solver_deck set-up
   1. Open the given FE model of the crash box in LSPP(ls-prepost).

      Figure.1. Opening the keyword file in LSPP

   2. The imported keyword file only contains a part. The rest of the details or properties of the part have not been defined. We need to define materials and sections for the crash box part and also set up a rigid wall for the crash test.

      Figure.2. The imported file

   3. First, we will try to complete the Part definition. Use the Keyword manager to open the part card. We will now create a *SECTION_SHELL_TITLE card and assign it to the part. While defining the section, the thickness to be assigned is 1.2 mm. For material, we will use the *MAT_PIECEWISE_LINEAR_PLASTICITY_TITLE card for creating a material card for steel. Note: Extra care should be given to the units used. Here, we have been asked to use the g-mm-ms unit system. Thus, the E value for steel becomes 2.10E+5 MPa.

      Figure.3. Completely defining the crash_box Part

   4. Now we will create the Rigid wall using the create entity option. Note that the rigid wall can also be created using the Keyword manager(*RIGIDWALL_PLANAR_ID card). Create a planar rigid wall by choosing two nodes to define the base and the normal direction to the rigid wall. Create a node-set containing all the nodes of the Crash_box using the same Create entity option and assign this node-set as the slave Node set ID(NSID) for the Rigid wall.

      Figure.4. The node-set(NSID 1) containing the crash box nodes

      Figure.5. The Rigid wall card

   5. Now we need to define an initial velocity to the crash_box so that it impacts the created rigid wall. We have been asked to set the Initial velocity as 50Kmph which converts into 13.8 mm/ms. This initial velocity can be applied by using the Keyword manager and creating an *INITIAL_VELOCITY card with X-velocity as 13.8 mm/ms. Note that the initial velocity can also be assigned from the Create entity panel.

      Figure.6. The INITIAL VELOCITY Defined

   6. We need to define self contact for the crash box. We will use the Keyword manager and create a *CONTACT_AUTOMATIC_SINGLE_SURFACE_ID card to define it.

      Figure.7. Contact definition

   7. Create a *CONTROL_TERMINATION card and set the runtime as 1ms.

      Figure.8. The Control_Termination card

   8. 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.

      Figure.9. The Hourglass card

   9. Now the solver deck set-up is complete. We need to run the analysis for 2 thicknesses, one 1.2mm and the other, 1.5 mm. The one 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. We will do this after we have completed the total solver deck set-up including the Output requests as well.
2. Output_Request
   1. Create output requests using *DATABASE_GLSTAT, *DATABASE_MATSUM, *DATABASE_RCFORC, *DATABASE_RWFORC, *DATABASE_SLEOUT, and *DATABASE_BINARY_D3PLOT. Set 0.01 as the time interval between successive recorded values for all.
   2. We also need to plot the acceleration for a node midway the length of the crash box. For this, we will create *DATABASE_HISTORY_NODE using the create entity option and then create *DATABASE_NODOUT under the database ASCII for recording all the values related to this node(5041, in this case).
   3. We also need to find the section force at a section midway the length of the crash box. For this, we will create a section using *DATABASE_CROSS_SECTION_PLANE_ID and then create *DATABASE_SECFORC to record all the values related to the created section.

      Figure.10. The Output requests made

   4. 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, 1.5 mm. 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.

      Figure.11. Changing the thickness to 1.5mm

RESULTS AND DISCUSSION:

1. CRASH ANIMATION

   Figure.12. Crash animation

2. STRESS CONTOUR ANIMATION

   Figure.13. Stress contour animation

3. STRAIN CONTOUR ANIMATION

   Figure.14. Strain contour animation

4. STRESS-STRAIN PLOT

   Figure.15. Stress-Strain plot

   The Yield strength is approx 250 Mpa. After 250 MPa, the plastic region begins.

5. DIRECTIONAL STRESS AND STRAIN VALUE COMPARISON

   	Max-Stress(MPa)			Max-Strain
   Model Thickness	X-Direction	Y-Direction	Z-Direction	X-Direction	Y-Direction	Z-Direction
   1.2 mm	276.08 on Ele 5706	264.5 on Ele 5984	263.01 on Ele 9523	0.00118 on Ele 5707	0.00127 on Ele 5980	0.00127 on Ele 1498
   1.5 mm	247.45 on Ele 3012	236.33 on Ele 6973	227.69 on Ele 105	0.00126 on Ele 3000	0.00126 on Ele 5987	0.00123 on Ele 1263

6. GLOBAL ENERGIES PLOT

   (a)	(b)
   Figure.16. Global Energies Plot (a) 1.2mmt (b) 1.5mmt
   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 107000 N-mm and 134000 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 Rigidwall 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 0.44 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.

7. ACCELERATION PLOT

   (a)	(b)
   Figure.17. Acceleration Plot (a) 1.2mmt (b) 1.5mmt
   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.

8. VELOCITY PLOT

   (a)	(b)
   Figure.18. Velocity Plot (a) 1.2mmt (b) 1.5mmt
   The velocity plot shows a constant velocity of 13.8 mm-per-ms up to 0.36 ms. The velocity reduces as the impact occurs and reaches a minimum value at ~0.45 ms when the crash box is on the verge of getting rebounded. Thereafter it moves in the opposite direction with a reduced velocity oscillating about a mean value of ~ 5 mm-per-ms.

9. SECTIONAL FORCE

   (a)	(b)
   Figure.19. Sectional Force Plot (a) 1.2mmt (b) 1.5mmt
   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.

10. STRESS PLOT

    (a)	(b)
    Figure.20. Stress Plot (a) 1.2mmt (b) 1.5mmt
    The plot shows the stress variation on Element number 8140 as the simulation progressed.

11. STRAIN PLOT

    (a)	(b)
    Figure.21. Strain Plot (a) 1.2mmt (b) 1.5mmt
    The plot shows the strain variation on Element number 8140 as the simulation progressed. The 1.5 mm thick model recorded more strain compared to the 1.2 mm thick model.

CONCLUSION:

1. The solver deck was set-up for the crash simulation and the required results were requested using the appropriate *DATABASE keyword cards.
2. 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.

Click here to view the completed files.