Week-7 Head Impact
Objective : To perform the head impact analysis of a head model against different scenarios and calculate the head injury criteria as well as the stress - strain plots of them.
Procedure :
Setting up the simulation :
Simple Head v/s Rigid wall :
First we will create the simulation of the simple head with a rigid wall to learn how to create the simulation for a head impact.
Create a new file in LS-DYNA.
- Define Transformation : To define a transformation file go to keyword manager-->Define-->Transformation
Select rotate, give the axis about which it should be rotated as 1 (in this case all axes are selected), enter the co-ordinates of the simple head about which it is to be rotated (in this case, geometric centre is chosen).
- Include Transformation : Next we include the transformation card to our simple head model. Move the simple head model to the local file.
Next select the transformation ID as has been created earlier in the include_transform card.
- Material property : Next we create The material property card is, go to MAT. Inputted unit system is g, ms, mm.
The values were taken from this study https://www.researchgate.net/publication/277331969_Investigating_the_dynamic_response_of_a_punch_to_human_head_using_finite_element_analysis
- Creation of Section : Next we create a section by going to Section and selecting the following options in the card for solid elements. Select ELFORM as 1 for the first case.
A thickness of 2mm is taken for the simple head.
- Assigning to part : Next, we assign the material and section to the shell_blue and shell_red which share the same material and section by entering the material and section IDs in MID and SECID respectively.
- Creating the rigid wall : To create rigid wall go to create entity-->rigid wall-->create, choose the options as planar, select the tail node, click on translate option and give a translational of about 20mm.
Hence the rigid wall will appear like this ahead of the simple head.
Defining Contact : Next we define the contact for the crash box to collide with itself. We go to contact and choose
CONTACT_AUTOMATIC_SINGLE_SURFACE, and enter in the following values and define the slave as partid of the box. We can aslo use SINGLE_SURFACE_AUTOMATIC for this purpose. Give the name as "self_ball".
Assigning Initial Velocity :
Go to create entity-->Inital Velocity-->Select by part-->add all the nodes to which velocity is to be added of the simple head, give a velocity of 11.11 mm/ms in the negative z-direction.
Creation of central node : A node is created in the centre of the crush tube using CNRB to later on measure its acceleration in the post-processing and to obtain the hic values.
- Control parameters : Next we select the control parameters, which govern the conditions in which the simulation will be computed.
Control_termination : This option decides the termination time which is set at 7 milliseconds
- Database options : Next we select the options which we want to see in the output characteristics. The following options are selected in Database_ascii. The options GLSTAT, NODOUT, ELOUT, SSTAT are shosen to show the energy curves, sliding energies, interface forces, nodal accn/disp/velocity and element forces respectively.
In database, history node is chosen and Node ID of the cnetral node is chosen to obtain the acceleration values of the simple head during impact.
The following options are selected in Database_binary_plot. The timestep is kept at 0.2 milliseconds as shown below.
Headform v/s Rigid wall :
Next we will create the simulation of the head form with a rigid wall.
The procedure is same as above. Only the differences in procedure as compared to above are given below.
- Define Transformation : To define a transformation file go to keyword manager-->Define-->Transformation
The origin about which it is rotated is changed and it is rotated 45 degrees with respect to x-axis.
The headform model is aligned like this with the rigid wall.
- Database_history_node_id : The node is chosen whose acceleration is to be calculated and obtain the HIC values.
- Contact Definition :
An additional contact of self contact is added for the skin layer of the head so that its contact with itsels is defined as it will be the one undergoing the deformation.
The slave type is chosen as part ID and part ID of skin is entered as input.
Headform v/s Car Hood :
In this simulation, we include the car hood in place of the rigid wall. As a result, 2 include keywords are used, include transform for the headform and include for the car hood.
We include the meshed hood file which is placed in the same directory as the headform and main file.
Next we assign material property to the head and section to the part
Next select the transformation ID as has been created earlier in the include_transform card.
- Material property : Next we create The material property card is, go to MAT. Inputted unit system is kg, ms, mm, as this is used in the headform model card.
The values were taken from a study of aluminium applications in aircraft door material.
- Creation of Section : Next we create a section by going to Section and selecting the following options in the card for solid elements. Select ELFORM as 16 for the first case, a thickness of 4mm is taken.
- Assigning to part : Next, we assign the material and section to the shell_blue and shell_red which share the same material and section by entering the material and section IDs in MID and SECID respectively.
Make sure that none of the IDs used in the two include files are similar as this can cause errors while running the simulation.
- Contact Definition :
An additional contact of surface to surface is added for the contact of the head form with the hood to define the interface between the two.
The slave is taken as the skin layer of the head whereas the master is taken as the hood.
- Boundary :
Boundary SPCs are added all around of the edge of the hood to lock all the translational motions of freedom so that the hood stays in place and does not move under impact. This is to simulate its presence when fitted to an actual car.
Run the 3 different simulations
Post processing :
Simple Head v/s Rigid wall :
- Simulation Mechanism : We see that the simple head approaches the rigid wall, deforms a bit on impact and then bounces of the rigid wall retaining just a little bit of the deformation.
This is the stress distribution of the simple head at its highest concentration. It reaches a peak value of 114 Mpa at the point of contact with the wall.
This is the deformation upon leaving the wall as shown above with slight residual stresses.
The strain plot of the simple headform is as given below.
The effective plastic strain is very less in the simple head, of the order of around 4.4% at the end of the impact.
- Energy plots :
We can see that the kinetic energy lost is equal to the internal energy gained indicating that no energy is lost in computation as energy error.
- Acceleration plots and HIC 36 value :
Given above we can see that looking at the acceleration plot is consists of multiple peaks and crests after collision indicating not much damping present after the collision with the rigid wall. The point acting as the sensor at the cneter of the simple head experiences a lot of shift in accleeration after collision, and this gives rise to a very high HIc 36 value as if evident in the graph.
The value of HIC is such due to the unit system being in gm, mm, ms.
In post settings we change the value of G and time to match our current units sytem.
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Headform v/s Rigid wall :
- Simulation Mechanism : We see that the simple head approaches the rigid wall, deforms much more on impact and then bounces of the rigid wall retaining a lot of the deformation.
This is the stress distribution of the simple head at its highest concentration. It reaches a peak value of 4.52 Mpa at the point of contact with the wall.
This is the deformation upon leaving the wall. As we can see the material of the skin on the simple headform exhibits plastic behaviour and retains a lot of the deformation after the impact as well.
The strain plot of the simple headform is as given below.
The effective plastic strain is much more in the heaform, this is due to the different material properties of the skin headform which retains most of the deformation.
- Energy plots :
In this we see that after the initial contact the variation in energy is more abrupt as compared to the earlier collision due to the material getting deformed to a greater extent.
- Acceleration plots and HIC 36 value :
Given above we can see that looking at the acceleration plot is consists of a single peak and crest indicating that the headform acts like an impact absorbing material, absorbing the impact and reducing the acceleration immediately in just one peak. The point acting as the sensor at the center of the simple head experiences lesser shift in acceleration after collision, and this gives rise to a a value lower than our intial HIC value of a simple head form and a rigid wall.
Stress v/s strain :
The stress vs strain curve is plotted for a particular element 2436 as shown below.
This particular element lies on the face that makes the collision with the rigid wall, hence it undergoes a large deformation. As we can see the strain value above 0.8 becomes unstable with respect to stress and the stress value reduces above 0.9.
Headform v/s Car Hood :
- Simulation Mechanism : We see that the simple head approaches the car hood, deforms the hood on impact, this reduces the blow on the headform as the hood is more yielding than the material on the top of the headform. Hence the hood of the car acts like a cushion in the case of a pedestrian collision
This is the stress distribution of the simple head at its highest concentration. It reaches a peak value of 0.3343 Gpa on the hood while the headform experience very little stresses and no deformation, as we can see in the simulation.
The strain plot of the simple headform is as given below.
This is the deformation upon leaving the wall. As we can see the material of the hood exhibits plastic behaviour and retains a lot of the deformation after the impact as well. This is by design so that in the case of any collision, the damage to the pedestrian's head is reduced.
- Energy plots :
In this we see that after the initial contact the variation in energy is in a very wave like manner and smooth indicating the role of the hood in cushioning the impact of the headform and deforming by itself to reduce the variation in acceleration of the headform.
- Acceleration plots and HIC 36 value :
Given above we can see that looking at the acceleration plot although the overall number of variations in the acceleration is more, the HIC value is much lower as compared to the previous 2 simulations. Moreover we see the difference in HIC when the headform collides with an automotive hood as compared to a rigid wall. This is because the headform experiences very less changes in acceleration due to the yielding nature of the hood. Hence the injury is reduced to the pedestrian and all vehicles are mandated by law to have such a design for the hood. Also the material chosen for the hood is aluminium which exhibits plastic behaviour.
Stress v/s strain :
The stress vs strain curve is plotted for a particular element 3038 as shown below.
Strain plot
As we can see above the strain value of an element on the headform is very less as compared to the earlier collsiion with a rigid wall. The strain is of the order of 0.1% which is negligeble.
Stress/Strain plot :
This particular element lies on the face that makes the collision with the hood. Uptil the point of nonlinearity the stress-strain curve is quite linear unitl it tapers off.
Conclusion : The hoods of automotive vehicles are made to be yielding and undergo deformation so that in the case of a collision with pedestrian, it will absorb the impact and deform upon contact thereby reducing the rate of change of acceleration and reducing the risk of injury to the pedestrian.
Result :