Head Impact using LS DYNA
Introduction:
Pedestrian safety simulation is a part of vehicle safety engineering. This study focuses on car and pedestrian interaction. The main focus of this is to reduce pedestrian injury in case of an accident (head injury, leg injury, etc). Usually, these tests are performed in the lower velocity range from 20-40KmpH. HIC(Head injury criteria/coefficient) is a measure of the likelihood of the head injury caused due to the impact after an accident. This is basically a measure of average acceleration during a small interval of the crash.
OBJECTIVE:
To perform head impact simulation and calculate the HIC(Head injury criteria/coefficient) value in the LS-PrePost.
In this study, the simple head impact on the rigid wall is studied at first. Then the impact of the given headform model with the rigid body is studied further followed by head form impact on the hood. Important results such as stress, strain, system energy, HIC values are derived in post-processing.
Procedure:
The Following analysis is divided into three cases and each case discussed below.
1) Case-1 Simple head impact on the rigid wall:
A simple head geometry is given with a problem statement. This model has meshed only shell elements. In this case, the model is impacted on rigid wall. The rigid wall is created by using following step, Create Entity > Rigid wall >Planer> 1N+NL > For Z Orientation. The rigid wall is created 10 mm away from the bottom-most node.
In this analysis, the unit system is taken kg/mm/ms is used.
Fig. 1.1 Simple Head Model with a planar rigid wall
The section properties for the simple head are given by using *SECTION_SHELL card. All the properties are set to default and a thickness of 1.0mm is assigned to the shell element. The material a card is used MAT_24_Piecewise_Linear_Plasticity and assigning the aluminum material.
Fig. 1.2 Simple Head model Section
Fig. 1.3, Material Assigned For Simple Head Model
The Geometry is assumed to have 40 kmph velocity along the traveling path at 45 degrees along '-Z and X-axis'. In this case, the geometry is circular so, The Transform option is not working. The geometry is traveling 45 kmph velocity into -Z direction so The velocity is converted into KMPH into M/S, The velocity is assigned 12.5 m/s in the X direction and -12.5 m/s in -Z direction. Automatic single surface contact is defined for this simulation, only the slave segment definition is required for this contact type. The simple head geometry is assigned as the slave segment and the master segment column is kept empty.
The HIC value is extracted through the NODOUT card in *DATABASE_ASCII_options. But for extracting a node statistics, the desired node should be selected. Here the bottom-most node is selected for extracting results. Figure 1.4 shows the *DATABASE_HISTORY_NODE card used to select the bottom-most node. Binary D3Plot is activated in the database section to get the final impact animation.
Fig. 1.4, Database_History_Node
The termination time for the problem is defined by *CONTROL_TERMINATION card. The distance between the head geometry and the rigid wall is calculated and the approximate time taken to hit the rigid wall and some additional time after the impact is calculated. This cumulative time (10ms) is defined as termination time in the card.
Fig. 1.5, Control_Terminination
2) Case_2-Impact simulation of a head foam with a Rigid wall.
In this case, a child-head model is been provided, so we would be replacing it with the spherical ball used in the above-mentioned simulation. Now, we don't want the original mode to change in the .K file provided, but I want to incline the head model at 45 degrees to the rigid wall because generally the angle of head hitting the car hood in at an angle of (35-75 degrees.) So, in order to change the angle of the model, I used DEFINE_TRANSFORMATION, the details of which are mentioned below, In this case, I can define angle is22.5 degree and used *INCLUDE_TRANSFORM option twice.
Fig. 1.6, Define_Transformation
Fig. 1.7,Transformed_model
Now, As mentioned in the above paragraph, I don't want to save these changes in the original model, so I use *INCLUDE_TRANSFORM, in order to call the rotated model in a .k file and perform the simulation their with the rigid wall.
The headframe section was selected as a solid section
The other keywords such as *Automatic_Single_surface, Control_Termination and ASCII outputs, GLSTAT, MATSUM, NODOUT, RCFORCE was kept the same as used in Simulation 1 shown above.
3) Case 3- Impact simulation of a headframe with a car hood.
In this simulation, the rigid wall of the previous two simulations are replaced by the car hood model,
Define Transformation and Include transform are used as shown in the previous simulation.
Section Shell and MAT24 with properties of aluminum are used and defined in the car hood.
Automatic surface to surface contact has been used with the CAR HOOD as the MASTER and the HEADFOAM as the SLAVE
The rest of the definitions of the simulation is the same as Case 2 as mentioned above.
Fig. 1.8, Head_Impact_On_Hood
Post-Processing:
Stress, strain, energy plots, HIC, etc will be shown below with discussions. Also, the Simulation video is attached.
1) Case-1 Simple head impact on the rigid wall:
Anim 1.1 Simple Head Model
Graph 1.1 Internal, Kinetic, and Hourglass Energies
Graph 1.2, HIC36
HIC (d) was plotted along with resultant acceleration, from the output results of the history node selected from *NODOUT. We can see it has a very high HIC, which is normal as it was just a simple elastic ball hitting a rigid wall at a speed of 40km/hr.
2) Case_2-Impact simulation of a head foam with a Rigid wall.
Anim 1.2 Von-Mises Stress
During the Point of Impact, in the initial stages of the simulation, the maximum stress and strain value is achieved, after which it remains constant during the rest of the part of the simulation.
Graph 1.3 Internal, Kinetic, and Hourglass Energies
Graph 1.4, HIC36
3) Case 3- Impact simulation of a headframe with a car hood.
Anim 1.3 Von-Mises Stress
From the energy plot, we could see that during the impact point the stress increases and thus the internal energy gradually increases and Kinetic energy gradually decreases, but then remains constant throughout the simulation after the point of impact and thus remains stable. change but then remains constant.
Graph 1.5 Internal, Kinetic, and Hourglass Energies
Graph 1.6, HIC36
From the above plot, we can see the spike in resultant acceleration at the time of impact, the HIC(d) value is also found, it looks more acceptable as the Car Hood(aluminum and mat24) is been used in comparision to the rigid wall used in previous two simulations.
From the above plots, we can say that During the Point of Impact, in the initial stages of the simulation the maximum stress and strain value is achieved, after which it remains constant during the rest of the part of the simulation.
Conclusion -
The HIC value is very large for SIMULATION 1 and SIMULATION 2, and considerably realistic for SIMULATION 3, This is because of the use of more realistic material properties and models in SIMULATION 3.
The Aluminium hood in SIMULATION 3 absorbs stress in comparison to the RIGID WALL in SIMULATION 2, due to which HIC is higher when the RIGID wall is used.
With this project, I learned how by changing the various factors in the models, such as impact velocity, car hood material, impact angles, the simulation results can be achieved for future predictions, and also how to calculate HIC, and also incorporate and manage different files together.
Head Impact on hood
https://drive.google.com/file/d/1oRd9n45C4g4rGyTnsJ2yR_bho0sVl7tC/view?usp=sharing
Head impact on rigid wall
https://drive.google.com/file/d/19vikDaV69-tg9VBwWWAEkL4dTdjBAOCy/view?usp=sharing
SIMPLE_HEAD_MODEL
https://drive.google.com/file/d/1052EJ2Bj5NC6J3onc79NhEn7REcPl35t/view?usp=sharing