Week-7 Head Impact

PROJECT: Pedestrain Head Impact

Aim: To simulate the impact of headform on an automobile hood and hence calculate head impact coefficient.

Input:

• A simple sphere with shell elements has been provided to replicate the headform.
• Children’s head form
• Meshed hood

Units : kg-mm-ms-kN

Case i): Impact simulation of a simple sphere impact with the Rigid wall.

In this case, a simple sphere with shell elements is imported in the LS-Prepost environment.

Section: The thickness of the headform is assumed to be 2 mm. A SECTION_SHELL keyword is defined with default element formulations.

Material: The material is assumed to be uniform throughout with following properties. As the model is used to replicate the headform, the elastic material properties of skull provided in the input is used to define the MAT_001 keyword.

A MAT_001 elastic material is defined accordingly.

Rigidwall: In the first case to simulate the impact of head onto the floor of car, a rigid wall is created to simulate the bonet/hood of the automobile. A rigidwall is created along __ axis using entity creation tab.

Boundary condition: As the head impacts the floor at an angle 50 deg, initial velocity card is defined by creating a Nodeset using entity creation tab.

As the headform is moving at an angle of 50 deg at 40kmph, accordingly the velocity is converted into units of kg-mm-ms and applied along +x and –z direction.

INITIAL_VELOCITY

Control: Termination time of 3 ms is added under Control keyword. Energy card is added to calculate the hourglass energy and sliding energy as shell elements are included in the model.

CONTROL_TERMINATION

CONTROL_ENERGY

Contact: Automatic single surface card is added to study the penetration, self-contact and contact interface energy.

CONTACT_AUTOMATIC_SINGLE_SURFACE

Database: Binary plot and ASCII plots are requested for every 0.1.

Verification:

Energy of the system is conserved and hourglass energy is within the limits.

Case ii): Impact simulation of the Headform provided against the rigidwall

In the second case, the model of a children's headform is imported into the LS-Prepost by using INCLUDE keyword. Since the impact is to be simulated with headform at an angle, a DEFINE_TRANSFORMATION card is created to define the orientation of the headform after it is imported.

DEFINE_TRANSFORMATION

INCLUDE_TRANSFORM card is used to import and transform/orient the headform using the transform id created when the transformation was defined.

The model of the headform after INCLUDE_TRANSFORM.

A rigidwall is created entity creation tab.

A nodeset is defined by subsystem in the entity creation tab. INITIAL_VELOCITY card is created to simulate the headform travelling at 40kmph.The directions are defined as per the orientation of the headform.

The given input file has all materials predefined along with assignment in PART. Various keywords pertaining to the contact, timestep, hourglass and control settings are already defined and have not been changed.

Control, contact and database similar to the first case are added and solved with default settings.

Verification:

As the total energy of the system is conserved and hourglass energy is within limits, we can infer that simulation ran without any numerical instability.

Von- Mises stress:

Effective Plastic strain:

Acceleration:

The headform moving uniform velocity after impact accelerates and decelerates in a short span of time. It ten continues to move with unifrom velocity. This is due to stiffness offered by the rigidwall and no transfer of energy takes place rather it just deflects, which is not the case in real time.

Stress:

Strain :

Case iii): impact simulation of the head form with the Hood

The file containing the headform and meshed hood are imported into LS-Prepost by defining INCLUDE keyword for meshed hood. And DEFINE_TRANSFORMATION and INCLUDE_TRANSFORM in case of headform to orient the head at the required angle.

Model after DEFINE_TRANSFORMATION and INCLUDE_TRANSFORM

The automobile bonet/hood consists of an outer panel (skin) and inner panel (reinforcements). The thickness of both the panels is around 0.7 mm. Hence the shell thickness in this case of meshed hoodwill be assigned as 1.4 mm to replicate the hood in real time.

Accordingly, MAT_24 Piecewise linear plasticity model is created for the meshed hood with following properties sourced from web of Aluminium.

Similar to the second case, INITIAL_VELOCITY card is created and directions are denoted accordingly.

Control termination card is defined accordingly, as both the parts are designated as flexible. Some extra time is added to check the interaction after the headform gets far from the hood.

All the other control cards such as energy, contact and database keywords are added by retaining the default values.

An AUTOMATIC_SINGLE_SURFACE keyword is added to check the penetration.

Animation:

Verification:

The total energy is conserved, hence it can be inferred that simulation is more or less successful in replicating the impact of headform against the meshed hood. Though the hourglass energy increases to certain level between 20 to 30 ms timeframe and remains constant thereafter. Since, the meshed hood is made up of shell elements of 1.4 mm a different element formulation can be used to limit the hourglass energy.

Stress

Strain

Acceleration plot :

Von-Mises Effective stress:

Skin of the headform on impact.

Effective Plastic strain:

The animation shows an exaggerated view of the hood after the impact, it is to be kept in mind that though the inner and outer panel thickness has already been accounted for in the SECTION_SHELL thickness, but there are other stiffeners in place to minimise the deformation as well protecting the pedestrian at the same time.

Comparison of headform against rigidwall and meshed hood:

Comparing the mean effective stress in both the cases, it can been inferred that skin undergoes maximum stress. The stress value peaks to maximum of 0.01 GPa in case of rigidwall as the headform rebounds after impact against rigidwall. But in case of hood, due to the transfer of energy and hood being flexible absorbs some amount of energy. The stress values in case of hood are small compared to that in rigidwall is because of orientation of hood at an angle as compared to rigidwall lying flat.

There is region in the graph where the stress values fluctuate between a small range due to interaction between the headform and the hood.

Similarly, the effective plastic strain in case of headform against rigidwall is very high compared to that of the headform against hood due to stiffness offered by the rigidwall and orientation with respect to the ground.

HIC Calculation:

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 calculated as the maximum values of averages of deceleration lasting for less than prescribed time period with peak acceleration of 3 ms.

In case of calculating HIC₃₆ the time period is chosen as 36 ms. HIC36 is just the injury criteria, since we are simulating the impact of the headform and not the dummy body as a whole as it is cumbersome and adds a lot of complexity.

To calculate the Head Injury Co-efficient, HIC(d) is calculated, which represents the HIC value that a free motion headform would have measured if it were attached to a body. HIC(d) takes into account the weighted standardized maximum integral value of the head acceleration. The following weight function is used based on experimentation is used which provide close enough direction to evaluate the performance taking into account the injury criteria too.

From the plot as requested for hic36 in LS-Dyna, HIC(d) = 378.4 is obtained. This value of HIC is usually less severe and has more chances of survival.

HIC(d) can also be calculated manually using the Gaussian Mixture Model and fitting a distribution to get the mean and calculate the integral, and then incorporating the weightage which is a tedious method. The maximum of integral between the inerval is already provided by LS-Dyna (281).

This value can be substituted in the formula to obtain the HIC(d) value.

HIC(d) = 0.75446*281+166.4 = 378.4

Conclusion:

The Pedestrain Head Impact was carried out by first simulating a simple sphere impacting rigidwall, and then by adding complexity, a headform provided by LSTC was simulated again using rigidwall and hence a meshed hood.

The comparison of headform against rigidwall and meshed was done to infer the chaning stiffness and geometry for the variation in the results.

Finally, the Head Impact Criterin for H36 was calculated to be 378.4 which is well within the limits, to judge the impact to be not as severe and has high chances of survival. The same experiment can be done by varying the impact angles and at different location of the hood.

Please follow the link to access the input, ouput and graphs.

https://drive.google.com/drive/folders/1L7U1I3Yb7HJ4eaMep-UTI5zZoNL35qNa?usp=sharing