Head Impact Simulation using LS-Dyna

OBJECTIVE:

To perform head impact simulation in the given model. The following objectives are to be satisified.

1. The impact velocity should be between 20-40 kmph and the impact angle of pedestrian head should fall in the range of 35-75 degrees.

2. The HIC values should be calculated in post processing.

3. To perform impact simulation of a simple head impact with the Rigid wall with the Stress / Strain plots and acceleration plot

4. To perform the impact simulation of the head form with the Hood with the Stress / Strain plots and acceleration plot. For the hood, the material selection should be Aluminium.

5. The two cases should be compared using their plots.

PROCEDURE:

Case 1: Simple head model impacting against rigid wall

1. Open LS-Dyna Manager>>Start LS-PrePost. Import the given Simple_Head_Model.k extension file.

2. Go to Keyword Manager>>Material>>Choose a material model which shows the elastic behaviour. Either *MAT_001_ELASTIC or *MAT_24_LINEAR_PLASTICITY can be chosen. The values are given as shown in the figure below.

3. Go to Keyword Manager>>Section>>Shell>>Give the section properties as shown in the figure below.

4. Go to Create Entity>>Create a planar rigid wall in a distance of 10mm below the z-axis. This is shown in the figure below.

5. Now create a nodeset which contains all the nodes of the simple head model. This is to define the intial velocity of the head model. We assume a resultant velocity of 40 kmph. To achieve this 28 kmph velocity has to be defined along negative x-axis and y-axis at an angle of 45 degrees which gives a resultant velocity of `28sqrt2` which is 39.59 kmph along z-axis.

Conversion of unit system: 28 kmph = 7.77 mm/ms. The velocity card is shown in the figure below.

6. Now, automatic single surface contact is defined in which the head model is assigned as the slave. The *CONTROL_TERMINATION card is defined with a total runtime of 3 ms.

7. In DATABASE_ASCII option the GLSTAT and NODOUT output requests are placed and the BINARY_D3PLOT is activated to get the animation file. The final model is shown in the figure below.

8. Save the file with suitable name with .k extension. Open LS-Dyna Manager. Run the simualtion.

Case 2: Headform model impacting against rigid wall

1. Create a new folder with suitable name. Copy and paste the Head Impact.k file inside the folder.

2. Open a new LS-PrePost. Go to Keyword Manager>>Define>>Transformation. The head has to be rotated 45 degrees with respect to Y-axis. Enter the values shown in the figure below to achieve the transformation.

3. After defining the transformation, it is necessary to include the path of the original headform model to which transform has to be applied. This is achieved using *INCLUDE_TRANSFORMATION card.

4. Now save the keyword using 'Save Keyword As' option. Select the transformed geometry file name in the window and save it using appropriate file name.

5. Open the saved keyword file and repeat the steps 4-6 as given in case 1. Define automatic single surface contact between the headform and wall with former being the slave.

6. In DATABASE_ASCII option the GLSTAT output request is placed and the BINARY_D3PLOT is activated to get the animation file. The total termination time is given as 4ms. The final model is shown in the figure below.

7. Save the file with suitable name with .k extension. Open LS-Dyna Manager. Run the simualtion.

Case 3: Headform model impacting against hood

1. Create a new folder with suitable name. Copy and paste the .k file which is saved in step 4 of case 2. Also, paste the Meshed-Hood.k file in the same folder.

2. Open LS-PrePost>>Keyword Mananger>>INCLUDE>>INCLUDE. Give the necessary file path. This is shown in the figure below.

3. Now the save this file using suitable name with .k extension. Open the saved file. The meshed hood model is included in the file.

4. The contact interface is defined between the headform and the hood as shown in the figure below.

5. Now go to keyword manager. Create a section and material card for the hood. This is shown in the figure below. Assign the properties to the Hood part ID card. Also, arrest the nodes in the perimeter of the hood along z-axis.

6. In DATABASE_ASCII option the GLSTAT output request is placed and the BINARY_D3PLOT is activated to get the animation file. The total termination time is given as 40ms. The final model is shown in the figure below.

7. Save the file with suitable name with .k extension. Open LS-Dyna Manager. Run the simualtion.

RESULTS AND DISCUSSION:

1. The Von-Mises stress contours for different cases are shown below.

From the Von-Mises plot it can be inferred that the stress value goes higher at the time of impact and after which it decreases steadily.

From the above contour plot, it can be inferred that there is a deformation observed in the headform. This is due to the material properties assigned to the head form. As observed in case 1, the stress value drops after the impact. The stress values are less when compared to case 1.

From the above stress contour, it can be inferred there is a deformation happening both in hood and the headform. Since the hood is of lighter material it takes maximum stress which causes deformation.

2. The energy plots are shown in the figure below.

From the energy plot, the maximum energy is lesser compared to the next 2 cases. This is because of elastic behaviour of the material. At the time of impact around 1.6 ms, the kinetic enrgy drops with an increase in internal energy. 

The energy plots for case 2 and case 3 fairly remains the same. The total energy remains the same since the material of the headform remains the same. The hourglass energy increases for case 3 whereas it remains constant for case 2. In both of the cases we could see that the kinetic energy decreases at the time of impact which caused increase in internal energy.

3. The effective strain plots are shown in the figures below.

The effective plastic strain remains zero throughout the run time for simple head model. This is because of the material properties assigned to the head. Due to this there is no notable defomation observed in the model during the simulation.

From the above plot, it can be inferred that plastic strain drops from 2.25 ms and after a while it reaches a constant value. The constant value denotes the permanent deformation in the headform model.

From the above plot, it is clear that the effective plastic strain is lesser compared to case 2. This is because hood is assigned the aluminium property. During the impact it abosrbs most of the energy thereby transferring lesser impact to the headform. In this case, the hood is deformable as opposed to case 2. (impact was against rigid wall in case 2)

4. The HIC36 values are shown in the images below.

From the above plots, it can be inferred that the HIC36 value obtained for case 1 is higher with a value of 1.143e+07. For case 2, the value obtained is 2.476e+04. For the hood impact the value obtained is 227.3. Due to material property of the hood, the HIC value obtained for case 3 is lesser compared to rest of the cases.

Note:

The HIC36 value obtained for the hood impact simulation without the boundary conditions is shown below.

The HIC36 value obtained is 88.2 which is lesser when compared to 227.3 (with boundary conditions). This is because when the nodes are arrested then the hood behaves as a rigid body which causes increases in the HIC value.

CONCLUSION:

The case setup is done according to the objective and the output requests are placed. The head impact simulation is done and the results are discussed as per the objective.

Drive Link: https://drive.google.com/file/d/15iAdC0tZNrpxgtXDTt8ucgfieaiSq53V/view?usp=sharing