HEAD IMPACT SIMULATION USING LS-DYNA

Objective:

The objective of this project is to perform a head impact simulation of a pedestrian headfoam model against a rigid wall and a hood of a car using LS-DYNA.

Also, the head impact coefficient (HIC) should be calculated for each case using LS Prepost.

Pedestrian head impact:

Pedestrian protection CAE is a part of vehicle safety engineering from a crash perspective. In pedestrian protection, we look for imparting minimum injury to head, upper leg (thigh), and lower leg (including knee) in case of an accident with a human subject.

These tests are usually done at a lower velocity from 20-40 kmph as accidents tend to result in more severe circumstances above these velocities and braking in case of an accident often results in collision at low velocities.

Many pedestrian crashes involve a forward moving car (as opposed to cars and other vehicles with a vertical hood/bonnet).

In such a crash, a standing or walking pedestrian is struck and accelerated to the speed of the car and then continues forward as the car brakes to a halt.

The pedestrian is impacted twice, first by the car and then by the ground, but most of the fatal injuries occur due to interaction with the car.

Vehicle designers usually focus on understanding the car-pedestrian interaction, which is characterized by the following sequence of events: the vehicle bumper first contacts the lower limbs of the pedestrian, the leading edge of the hood hits the upper thigh, and the head and upper torso are struck by the top surface of the hood and/or windshield.

HIC:

Crash safety engineers consider the head which is more prone to injury that could lead to concussions or even death to be their main priority during crash analysis.

The factor that is used by engineers to determine the level of damage imparted onto the head region is determined using the HIC factor which stands for Head Injury Criterion / Coefficient.

Normally the variable is derived from the measurements of an accelerometer mounted at the centre of a crash dummy's head, when the dummy is exposed to crash forces.

Generally, it is defined by

where t₁ and t₂ are the initial and final time period chosen to maximize HIC and a is the accelaration over the period of time from t1 to t2.

Case 1: Simple head model impacting against rigidwall

Simulation setup:

1. In the first case, a sphere shaped shell element is used as a simple head model and made to impact on a rigidwall.

2. A *SECTION_SHELL with a shell element formulation 2 and a shell thickness of 3 mm is created and referenced to the head model using part option.

3. A *MAT_24_PIECEWISE_LINEAR_PLASTICITY card is used as the material property for the head model and it is referenced.

4. A planar rigidwall is created normal to one of the global axes of the spherical head model at an appropriate distance from it.The normal direction of the rigidwall is ensured to be against the velocity input direction.

5. A node set is created for the spherical head model by selecting all of its nodes using *SET_NODE option to apply input velocity to the head model.Using *INITIAL_VELOCITY card, an input velocity of 40 kmph (11.11 mm/ms) is applied to the created node set of head model in the direction normal to the rigidwall plane.

6. During impact, if huge deformation occurs it could potentially lead to contact of elements with the adjacent elements in the same shell component. Hence, to impart a self-contact among each element of the part a CONTACT_AUTOMATIC_SINGLE_SURFACE card is used. The only parameter that needs to be initialized in this keyword is the slave set ID, which in our case will be the entire shell part.

7. A *CONTROL_TERMINATION keyword is specified to initialize the end time of the simulation which is about 5 ms.

8. Finally, the DATABASE keyword is created specifying the output parameters that need to be computed during the simulation including the animation d3plot with a time interval of 0.1 ms and ASCII_Outputs like GLSTAT- to calculate energies, NODOUT- for nodal displacement/acceleration and HIC value and the RWFORCE - rigid wall force.

9. The required nodes from which the results need to be extracted are specified in a SET_NODES and reference in a *DATABASE_HISTORY_NODE_SET card.

10. One of the most important pre-processing tasks is to ensure the validity of a CAE model before submitting it to the solver for analysis.After getting cleared up with the errors if any, the model is saved.

11. The saved keyword file is set to run in LS-DYNA manager. A normal termination of the setup is achieved and the results are analysed for post-processing.

Post-processing:

From the binary plot file of the solved keyword, the results for Von-mises stress distribution is analyzed in the head model.

This is the static Von-mises stress distribution of the simple head model impacting a rigidwall.

It is found that the maximum stress produced in the head model is `374.27MPa`

Case 2: Pedestrian headfoam model impacting against rigidwall

Unlike the previous case, where the head part was a complete sphere, this model involves translation and rotation of the part hence creating an impact angle of 45 degrees with the rigid wall and the bonnet.

This can be done using the *INCLUDE_TRANSFORM and *DEFINE_TRANSFORMATION keywords in a separate keyword file and referenced to the headfoam model using the above-mentioned keywords such that any changes made don't directly affect the headfoam model which can act as a basis for all pedestrian safety analysis

DEFINE_TRANSFORMATION :

This keyword has to be initialized before the INCLUDE_TRANSFORM to prevent errors. This keyword lets us perform certain transformations like SCALE, ROTATE, etc to the part file reference in the INCLUDE keyword.

For our model, we will rotate the model about the x-axis at an angle of 45 degrees. This can be achieved by specifying the a1-a3 parameters which define the axis of rotation and a4-a6 the point through which the vector passes through and finally a7 where the angle of rotation in degrees can be specified.

INCLUDE_TRANSFORM :

This keyword is a modified form of the regular *INCLUDE_INCLUDE keyword file, that it helps us make changes/ transform the imported model prior to creating it on the viewport.

Certain offsets of IDs can also be changed however since our model consists of very few parts we will not use the offset command.

Simulation setup:

1. A planar rigidwall is created such that the pedestrain headfoam model impacts the rigidwall at an angle 45 deg and placed at an appropriate distance from it.The normal direction of the rigidwall is ensured to be against the velocity input direction.

2. A node set is created for the pedestrian headfoam model by selecting all of its nodes using *SET_NODE option to apply input velocity to the head model.Using *INITIAL_VELOCITY card, an input velocity of 40 kmph (11.11 mm/ms) is applied to the created node set of pedestrain headfoam model against the rigidwall plane.

3. Since this headfoam part constitutes of different sub-parts, each sub-parts are modeled using different materials with the Skull, Accel block, Backplate modeled using a MAT_20_RIGID material, and the Skin with a MAT_77_O-OGDEN_RUBBER which is a type of hyperelastic material.

4. The headfoam part which has to replicate the behavior of a real pedestrian head is modeled using a SECTION_SOLID with default solid element formulation.

5. *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE card is established between each of the solid parts as they undergo physical contact during the impact simulation.

6. The rest of the keywords - *CONTROL_TERMINATION & ENERGY, DATABASE_ASCII_Output & BINARY_D3plot & HISTORY_NODE_SET, *RIGIDWALL_PLANAR are created with the same parametric values as used for the Simple head model.

7. The model is solved for errors if any and it is saved in a separate keyword file.

8. The saved keyword file is set to run in LS-DYNA manager. A normal termination of the setup is achieved and the results are analysed for post-processing.

Post-processing:

Animation of Head impact simulation

Von-mises stress

From the binary plot file of the solved keyword, the results for Von-mises stress distribution is analyzed in the head model

Effective Stress plot

During pedestrian head impact, the skin part of the head developes major amount of stress as it impacts the rigidwall at a short period of time.

From the Effective stress plot, it is found that the head impact occurs at time `1.7ms` and the maximum stress produced during pedestrain head impact is `0.025 MPa` at `2.9ms`.

Acceleration plot

The acceleration plot is drawn from the database node from the acceleration block in the pedestrian head model.

It is found that the maximum resultant acceleration during the impact happens at `2.9ms` and it is `2.2 (mm)/(ms^2`

Also, from the HIC plot ,it can be seen that the value of HIC (Head Impact Coefiicient) in this case is `8.163 E+04`.

There is such a huge value of HIC factor is that because of the short termination period of the simulation.

Energy plot

From the above energy plot, it is evident that the kinetic energy of the system decreases as the head impact the rigidwall.

Correspondingly, the internal energy increases due to impact energy absorption.

There is a little amount of hourglass energy dissipation in the system which is due to distortion of elements in the model after impact.

Since there is no sliding kind of phenomenon occuring in this simulation, the sliding energy of the system remains zero.

Case 3: Pedestrian headfoam model impacting against rigid hood

This case involves the same simulation but the impact occurs on a hoood similar to real-life pedestrian accidents.

A similar procedure, as per the case 2, has been followed to orient the pedestrain headfoam model at an angle 45 degree to the hood using *DEFINE_TRANSFORMATION and *INCLUDE_TRANSFORMATION cards.

The hood is modelled with rigid material card *MAT_20 with Aluminium as the material properties.The rotational and translational displacements are constrained in the rigid hood material.

All other boundary conditions, contact establishments, section setup, material setup, control setup and database setup have been created in similar way as in case 2, except the rigidwall.

The model is solved for errors if any and it is saved in a separate keyword file.

The saved keyword file is set to run in LS-DYNA manager. A normal termination of the setup is achieved and the results are analysed for post-processing.

Post-processing:

Von-mises stress

From the binary plot file of the solved keyword, the results for Von-mises stress distribution is analyzed in the head model.

Effective Stress plot

During pedestrian head impact on the hood, the skin part of the head developes major amount of stress as it impacts the hood at a short period of time.

From the Effective stress plot, it is found that the head impact occurs at time `2.6ms` and the maximum stress produced during pedestrain head impact is `0.025 MPa` at `3.8ms`.

Acceleration plot

The acceleration plot is drawn from the database node from the acceleration block in the pedestrian head model.

It is found that the maximum resultant acceleration during the impact happens at `3.8ms` and it is `1.9 (mm)/(ms^2`

Also, from the HIC plot ,it can be seen that the value of HIC (Head Impact Coefiicient) in this case is `7.199 E+04`

There is such a huge value of HIC factor is that because of the short termination period of the simulation.

Conclusion:

We are able to see a huge HIC factor during the pedestrian head impact simulation because of very short termination time which is of 5ms.

The HIC factor is comparatively more in the case 2 (Pedestrian head impacting rigidwall) that that of case 1( Pedestrian head impacting rigid hood model). Thus, it concludes that with high HIC values, the probability of getting head injuries is high according to Abbreviated Injury Scale (AIS).

However, in real cases, the damage could be significantly high considering the acceleration factor as it increases at a sudden rate when calculated from a significant distance of the head from the hood of the car.

Hence from the above-gathered simulation data and results, we can conclude that depending on the velocity rate, distance, cross-sectional area, material, and various other factors could alter the impact results causing either low injuries like concussions or severe injuries like skull fractures for the pedestrians involved.