Week-7 Head Impact

OBJECTIVE

To create a simulation of a pedestrian head impact and calculate the Head Impact Criterion (HIC) for each of the following cases.

1. Simple head model impacting against rigid wall
2. Child headform dummy model impacting against rigid wall
3. Child headform dummy model impacting against hood

INTRODUCTION

The head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact. The HIC can be used to assess safety related to vehicles, personal protective gear, and sport equipment.

Normally the variable is derived from the measurements of an accelerometer mounted at the center of mass of a crash test dummy’s head, when the dummy is exposed to crash forces.

It is defined as: `HIC = max (t_1, t_2) {[1/(t_2 - t_1) int_(t_1)^(t_2)a(t)dt]^(2.5)(t_2 - t_1)}`

where t₁ and t₂ are the initial and final times (in seconds) chosen to maximize HIC, and acceleration a is measured in gs (standard gravity acceleration).

This means that the HIC includes the effects of head acceleration and the duration of the acceleration. Large accelerations may be tolerated for very short times.

For example, at a HIC of 1000, there is an 18% probability of a severe head injury, a 55% probability of a serious injury and a 90% probability of a moderate head injury to the average adult.

The bumper typically strikes the lower limbs, while the bonnet leading edge strikes in the region of the hip and head impact subsequently occurs on with either the bonnet top or the windscreen area

In this project, a children's headform model is provided. An impact simulation is created to replicate a scenario where this headform will impact on a car bonnet.

MODEL IMAGES

Simple head model

Simple child head model

Meshed hood

PROCEDURE

Case 1: Simple head model impacting against rigid wall:

1. Firstly, on LS PrePost, the .k file containing the FE model is imported via File > Import > LS-DYNA keyword file.

2. The part contains shell elements and we are to assign thickness to it. So, we can access the keyword manager again, select the 'all' option, scroll to SECTION and select SHELL under it. The SHELL section will be used to define the model. We can assign a section ID with ELFORM 2 and thickness 2 mm.

3. Similarly, to create a material, we can go to the keyword manager again, select 'all', and scroll down to the MAT card, under which we can select the 024-PIECEWISE_LINEAR_PLASTICITY. This card is used to define the material. We can assign the material ID (MID), density (RO), Young's Modulus (E), and Poisson's Ratio (PR) here (as of steel in this case). Once we are done, we can click 'accept'. (Here, I have also defined SIGY (Yield stress) and ETAN (Tangent Modulus)).

Now, we can go back to the part card and assign the MID and SECID.

4. Moving on, we can now define the boundary conditions.

A rigid wall is created at a distance of 30 mm in the x-direction of the simple head model.

The simple head model is assumed to be impacting the rigid wall at a velocity of 40 kmph i.e. 11.11 mm/ms in the positive x-direction towards rigid wall.

5. Moving on, we need to define the self-contact for the given model. To do so, we can go back to the keyword manager, choose to show all keywords, go to the CONTACT keyword and select AUTOMATIC_SINGLE_SURFACE. Here, we just need to define the slave of this contact property using the part ID (which would be the simple head part). In addition to naming the contact and assigning a CID (contact ID), we needn't worry about anything else here. We can click 'accept' and then 'done'.

6. Next, we can create a CONTROL card to specify the end time of the simulation. This can be assigned via the TERMINATION card option under the CONTROL keyword in the complete list of keywords in the keyword manager. We can assign a value of 10ms to capture the entire simulation.

7. Finally, we can assign a couple of DATABASE keyword cards for the outputs - namely ASCII_option and BINARY_D3PLOT with a DT (Time interval between outputs) of 0.5ms. The GLSTAT, MATSUM and RWFORC attributes are selected under ASCII_option.

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

DATABASE_HISTORY_NODE card is used to compute the HIC value of the node 69584 in the model.

Once done, we can save it as a .k file and solve it using LS-RUN. The keyword file is inserted and we can click the play button to run the simulation. The number of cores to be utilised can be changed if needed.

Once it is finished without errors (aka Normal Termination), we can reopen LS-Prepost to view the results.

Case 2: Child headform dummy model impacting against rigid wall:

1. To simulate a somewhat realistic representation of a child's head impacting a car hood, the provided child headform model has to be rotated about 50⁰ along Y-axis.  This is done by using *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM as shown:

Using DEFINE_TRANSFORMATION card with TRANID=1, the dummy headform model is rotated by OPTION>ROTATE along Y-axis (A2=1) by 50⁰ (A7=50).

Then, the INCLUDE_TRANSFORM card is used to summon the rotated headform model file to the main file. The main file is saved with ‘.k’ extension.

The main file is then opened in LS-PrePost to add the required keywords to complete the simulation deck.

Note: While adding keywords to main file, it needs to be ensure that the subsystem of the keywords is set to that of the main file.

2. Moving on to boundary conditions, we can apply the initial velocity to the headform. Since it is at an angle, the velocity is broken down into two directional components as shown:

The initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 50°. The horizontal and vertical components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.

A rigid wall is created at a distance of about 80 mm from the bottom of the standard dummy headform model:

3. Just as in the previous case we can create a self-contact card for the headform. The same termination time and database options are selected as well. The only difference being the DATABASE_HISTORY_NODE card, which is used to compute the HIC value of node 25679 in the model.

4. The keyword file is saved to main file using ‘save keyword as’ option as shown.

Once done, we can save the main file as a .k file and solve it using LS-RUN. The keyword file is inserted and we can click the play button to run the simulation. The number of cores to be utilised can be changed if needed.

Once it is finished without errors (aka Normal Termination), we can reopen LS-Prepost to view the results.

Case 3: Child headform dummy model impacting against hood:

1. After importing the provided meshed hood model, we can define the section property for it. The following section card is applied:

2. MAT24 (Piecewise linear plasticity) card is applied for the material of the hood. Aluminium material properties are assigned to the hood.

3. The keyword file of hood is saved using ‘.k’ extension. The keyword file of hood is pulled into main file using keywords *INCLUDE (just like in the previous case). The child headform is pulled into main file by using *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM which is similar to what was done in case 2.

The main keyword file is opened in LS-PrePost and the remaining boundary conditions are added.

4. The initial velocity just as in the previous case is applied.

Additionally, the nodes on the edges of the hood are constrained using SPCs. The nodes are constrained in all directions.

5. Also, contact card is assigned. We shall be using AUTOMATIC_SURFACE_TO_SURFACE here since the contact between the headform and the hood is to be defined.

The headform model is taken as the slave and hood as the master.

Additionally, self contact is defined for the headform:

6. The control card and database cards are the same as that of case (2) scenario.

7. Once done, we can save the main file as a .k file and solve it using LS-RUN. The keyword file is inserted and we can click the play button to run the simulation. The number of cores to be utilised can be changed if needed.

Once it is finished without errors (aka Normal Termination), we can reopen LS-Prepost to view the results.

RESULTS

CASE 1

Stresses (view from below)

Strain (view from below)

Energy plots

HIC

CASE 2

Stresses (View from below)

Strain (View from below)

Energy plots

HIC

CASE 3

Stresses

Strain

Energy Plots

HIC

HIC CALCULATIONS

The HIC value is calculated using the following formula:

`HIC = max (t_1, t_2) {[1/(t_2 - t_1) int_(t_1)^(t_2)a(t)dt]^(2.5)(t_2 - t_1)}`

We shall be considering case 3 for these calculations.

From the plot,

The average value of acceleration for the time interval of t₁=2.5 ms and t₂=10 ms is given by the following formula: `[1/(t_2 - t_1) int_(t_1)^(t_2)a(t)dt]` which is equal to 65.

Therefore, HIC = `{[65]^(2.5)(10 - 2.5)}`= 255.472

HIC(d) = `0.75446(Free Motion Headform HIC)+166.4` = 0.75446(255.472) + 166.4 =`359.14

The HIC and HIC(d) values obtained from the plot in case 3 are 270.4 and 370.4 respectively. The manually calculated values are 255.472 and 359.14 - they are not too far off from the plotted values and are hence acceptable.

OBSERVATIONS

The obvious difference between the cases (especially case 3 and the others) is the rigid wall and its effects. In cases 1 and 2, due to the non-deformable nature of the rigid wall, the headform has to absorb most of the impact. Whereas, that is not the case in case 3. The hood absorbs most of the impact and hence the stresses are generated in the hood, compared to stresses being generated in the headform in cases 1 & 2.

With the rigid wall not softening the impact, the energy plots also experience sudden changes. For example, in cases 1 & 2, we can see that there is a massive drop in kinetic energy after impact, whereas it's more gradual in case 3.

The hood being deformable also plays a big factor in the HIC value for case 3. As we can see, the HIC value is 270.4, which is much lesser than the NCAP threshold of 650. Whereas, the HIC values are much higher and past the threshold in cases 1 & 2 (1.386e+08 & 2.746e+04), meaning they are completely unsafe. The lower the HIC value, higher the impact absorption by the body colliding with the headform.

RESULT

The given headform models were simulated in three cases, with the first case involving a simple headform coming in contact with a rigid wall, the second case involving the provided child headform coming in contact with a rigid wall at an angle mimicking real world conditions and the third case contained simulation of the headform coming in contact with the provided meshed hood at an angle.

The HIC value in case 3 was lower than the NCAP threshold of 650, meaning the impact was mostly absorbed by the hood. This is why hoods are designed to deform and not cause damage to pedestrians when accidents happen. If the hood is not deformable, it could be fatal to the pedestrian and can cause deadly head injuries (as seen by the extremely high HIC values in cases 1 & 2). The deformation is key as it absorbs the impact.