Week-7 Head Impact

HEAD IMAPACT COFFICIENT /CRITERION SIMULATION USING LS- DYNA.

Objective: To perform HIC (Head Impact Co-efficient /Criterion) Value and Simulation using Ls-Pre-post.

First we study the head model impact on rigid wall then we further study the head form model on hood to understand the stress, strain and system energy after that we calculate the HIC value in post-processing

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.

Where t1, t2.initial and final time periods and whereas the acceleration (a) during the time period.

We have three cases to analysis and understand the Head impact.

Simple Head Impact on Rigid wall

In challenge simple head model is given which is meshed with shell elements. The Head model

Is going to impact the rigid wall which we need to create as per the model set up. To create rigid wall go to Create Entity > Rigid wall > Planer > 1N+NL> For Z- direction .Rigid wall is created 10 mm away from the Meshed Head model.

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.

Case I: Simple head mode impacting against rigidwall

Simulation

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

A*SECTION_SHELL with a shell element formulation 2 and a shell thickness of 1.5 mm is created and referenced to the head model using part option.

A*MAT_24_PIECEWISE_LINEAR_PLASTICITY cards used as the material property for the head model and it’s referenced.

A planar rigid wall 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 rigid wall is ensured to be against the velocity input direction.

Anode 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 45kmph (12:5 mm/ms) is applied to the created node set of head model in the direction normal to the rigid wall plane.

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.

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

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 0.1ms and ASCII _Outputs like GLSTAT- to calculate energies, NODOUT- for nodal displacement/acceleration and HIC value and the RWFORCE – rigid wall force.

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.

One of the mast 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 any, the models saved.

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

The value is end with 2.77Mpa maximum and in the middle it reached to 399.Mpa

Case II: Pedestrian head foam model impacting against rigid wall

Unlike the previous ate, where the head part was complete here. This model involves translation and rotation of the part hence creating an impact angle of 45 degrees with the rigidwall 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 include before the INCLUDE_TRANSFORM to prevent error. This keyword lets us perform certain transformations like SCALE, ROTATE to the part file reference in the INCLUDE keyword.

For our model, we will rotate the modal about the y-axis at an angle of 45 degrees this can be achieved by specifying the a-a3 parameters in which define the a4-a6 of rotation and 24-36 the point through which the vector passes though and finally a7 where the angle of rotation in degrees can be specified.

INCLUDE_TRANSFORM:

This keyword is modified from of the regular “INCLUDE_INCLLIDE keyword file, that it helps us make changes/ transform the imported model prior to creating it on the viewport.

Certain offsets of ID can also be changed however since our model consist of very few pats we will not use the offset command

Simulation

• A planar rigidwall is created such that pedestrian head foam model impacts the rigid wall at an angle 45deg and placed at an appropriate distance from it the normal direction of the rigid is ensured to be against the velocity input direction.
• 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_VELOCTY card, an input velocity of 45kmph (12.5 mm/ms) is applied to the created node set pedestrian headfoam made against the rigid wall plane,
• Since this head foam part consist of different sub-parts are modeled using different materials with the skull, Accel-block, back-plate modeled using a MAT_20_RIGID material, and the skin with a MAT_77 which is type of hyper-elastic material.
• The head rotate part which has to replace the behavior of a real pedestrian head is modeled using. A SECTION_SOLID with default sold element formulation,
• CONTACT_AUTOMATIC_SURFACE_TO_SURFACE cards established between each of the solid parts as they undergo physical contact during the impact simulation
• There are of the keywords - “CONTROL TERMINATION & ENERGY, DATABASE_ASCI_ Output & BINARY _D3plot & HISTORY_NODE_SET, “RIGIDWALL_PLANAR are created with the same parametric values a5 used for the Simple head model.
• The models Solved for error 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 setups achieved and the result are analyses for post-processing.

Case III: Pedestrian HeadFoam model impacting against Meshed- hood

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

A similar procedure as per the case-II has been followed to orient the pedestrian headfoam model at an angle of 45deg hood using DEFINE_TRANSFORMATION, INCLUDE_TRANSFORM cards

The hood modelled with rigid material card “MAT_24 with Aluminum a material properties the rotational and translation displacement 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-II, except the rigid wall.

The model is solved for errors if any and it is saved in a separate keyword file. The saved keyword is to run in LS-DYNA manager a normal termination of the setup is achieved and the results are analyses for post processing.

Conclusion:

We can observe the HIC factor during the pedestrian head impact simulation because of very short termination time which is given in the card.

Due to the rigid wall thickness in The HIC factor is comparatively more in the case-II (Pedestrian head impacting rigid wall) that that of case-I (Pedestrian head impacting rigidwall hood model).

When the high HIC values, the probability of getting head injury 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 car.

Hence from the above 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 the low injuries severe injuries like hull fractures for the pedestrian involved.