Week - 4 - Crash Box Simulation

                                                                                  CRASH BOX SIMULATION USING LS-DYNA

                                                                                                     WEEK - 4 

AIM: To simulate a crash test simulation of crash box impacting a rigid wall in LS-Dyna.

OBJECTIVES:

• To create a rigid wall in LS-Dyna
• Apply initial velocity of 50 kmph to the crash box.
• Run the simulation and compare results of crash box having thickness 1.2 mm and 1.5 mm respectively.
• The unit system followed is gm-mm-ms
• Plot the cross-sectional force generated in the middle of the crash box.

1. Import:

                                                                                  Imported Model

2. Modelling rigid wall:

• Rigid walls mimic surfaces or volumes that are frequently used to represent stiffer structures that are either stationary or in motion.
• A stationary planar rigid wall is created ahead of the frontal portion of the crash box by selecting a node on the edge, Select planar option and Geo-Vector >> 1n+NL
• Now translate the rigid wall to some distance in the direction of the velocity of crash box.
• The crash box is made to hit the stiffer rigid wall at an impact velocity of 13.88 mm/ms.

                                                                                 Rigid Wall Created

                                                                                  Rigid Wall Translated

3. Initial Velocity:

• Before creating the initial velocity keyword, we must create node set (NSID) for all the shell elements of the crash box.
• Go to Create Entity >> Under Set Data >> create a new SET_NODEby selecting all the elements of the box.
• Now assign that NSID in the initial velocity as shown below
• 50 kmph = 13.89 mm/ms

                                                                        Initial Velocity Assigned To Elements

4. Shell Card selection:

• Shell section card is required to define section properties for shell elements.
• Go to Keyword Manager
• Select section >> shell >> section ID and title.
• Choose Element Formulation value in ELFORM as 2 (Belytschko-Tsay type elements).
• Mention the thickness of the shell elements as 1.2 mm for one simulation and then 1.5 mm for the other simulation.

5. Material Card:

• For crash box, we will assign the material as steel which is a non-linear characteristic material.
• Go to Keyword Manager
• Select MAT >> 024-PIECEWISE_LINEAR_PLASTICITY material card. Give it a material ID and title.
• As we are using gm-mm-ms unit system, we will provide the characteristic values of steel as follows:
• Density of Steel (RO) – 8.05 x 10^-3 gm/mm³
• Young’s Modulus (E) – 210 x 10³ MPa
• Poisson’s Ratio (PR) – 0.3
• Yield Stress (SIGY) – 250 MPa
• Tangent Modulus (ETAN) – 1000 MPa

6. Hourglass:

• Hourglass can be defined using HGID. This keyword is used to reduce or control hourglass energy during the simulation.
• Go to Keyword Manager and select the values as shown below.

• Now assign all these characteristics to the crash box part in PART keyword as shown below.

7. Contact:

• Contact card can be used to define self-contact for crash box as the crash box elements can have contact between themselves after the deformation. Therefore, define AUTOMATIC_SINGLE_SURFACE contact with coefficient of friction 0.2as shown below.

8. Control:

• Define the termination time of the simulation which is necessary to run the simulation. The termination time is given as 3 msin CONTROL SIMULATION Keyword as shown below.

• Now define CONTROL_ENERGYcard for energy dissipation as shown below

9. Cross-section:

• Cross-section can be created under DATA BASE keyword to measure all cross sectional forces.
• Usually, two rows of elements are selected as a section or a simple plane creation can also be done.

• History node can be used to know the acceleration at the node during the simulation

10. Post-Processing Results:

• The requested outputs are d3plot, history node and ASCII options are
• ELOUT:Element Data
• GLSTAT:Global Data
• MATSUM:Material Energies
• SLEOUT:Sliding Interface Energies
• NODOUT:Nodal point data
• SECFORC:Cross-section forces

11. Model Checker

• Since there are no errors, we can run the simulation.

• After creating all the necessary files for simulation and checking the model, we will save the model as Keywordfile with .K extension. To run the model, we will go to File > Run LS-DYNA. We will browse this keyword file and then run the simulation for shell thickness 2 mm and then for shell thickness 1.5 mm as shown below.

                                                                               Simulation For 1.2mm thickness is done 

• Both simulations ended with Normal Termination. Therefore, all the models are simulated successfully.

12. RESULTS AND PLOTS:

• To run the simulation, we will open the binary d3plot file form specified path. Then we will click on Play button on Animation Toolbar. The resulting animation is shown below.

1st CASE:                                  

A. Von Mises, X- Stress, Y- Stress, simulations for 1.2 mm thickness Crash box:

• From above animation, we can see that the stresses are developed throughout the crashbox after impact. Initially the maximum stress occur near the rigidwall. we can see that the stress is migrated throughout the crash box. The stress distribution is uneven because of vibrations induced in the crashbox after the impact. The maximum stress induced here is 285.356 MPa.

• The stresses generated on crash box in X-axis is 250.3 Mpa.

• The maximum value of stress in Y-axis is around 200.1 MPa

B. Plots for 1.2mm thickness:

• Initially we can say that the velocity is maximum therefore kinetic energy is also maximum. But during the impact i.e. at 1ms, kinetic energy is absorbed by the part and it decreases, simultaneously internal energy increases (in the middle).
• Total K.E is converted into I.E, and also the hour glass produced is negligible. this is why the total energy remains constant.

• The maximum force of 65 kN is experienced at 1.20 ms after the impact. This resultant force is nothing but the force created at the section which we have created. earlier.

• The maximum value of the acceleration at Node ID 6895 which is in the middle of the crash box is seen at 1.70 ms

2nd CASE: 

 A. Von Mises, X- Stress, Y- Stress, simulations for 1.5 mm thickness Crash box:

• The maximum stress induced here is 285.393 MPa. It is almost same as in case of 1.2mm thickness.

• The stresses generated on crash box in X-axis is 231.9 Mpa which is lower than the 1st case.

• The maximum value of stress in Y-axis is around 214.6 MPa which is higher than case 1.

B. Plots for 1.5mm thickness:

• If you carefully compare the energy graphs, total energy is slightly increased as the thickness increases simultaneously, K.E. and total energy also increases(slightly).

• The maximum force of 86 kN is experienced at 1.70 ms after the impact. as the thickness increases, mass will increase which inturn increases magnitude during the impact.

• If you compare the two acceleration plots, the changes are negligible.

Conclusion:

Crashbox simulation is done successfully with 1.2mm and 1.5mm thicknesses respectively. All the objectives are fulfilled. From comparing the plots and animations we can say that simulation results will change as the thickness of the model changes.