Week-7 Head Impact

Aim-

 To perform the Head Impact Simulation and calculate the Head Impact Criterion (HIC) value for the following cases.

Objective-

To perform head impact analysis and calculate the HIC (Head injury criteria/coefficient) value for the following cases:

• Case1: Simple head model and rigid wall
• Case2: Child head-foam and rigid wall
• Case3: Child head-foam and bonnet/hood

Basic Introduction-

Head Impact :

Fig: Head Impact Test

About 14% of all road fatalities in Europe & rest of world are pedestrians, with children and the elderly being at greatest risk. Pedestrians comprise one of the main categories of vulnerable road users, which also include cyclists and motorcyclists.

The procedure promotes energy absorbing structures, deformation clearance and deployable protection systems such as pop-up bonnets and external airbags.

Child Head Impact:

Car-pedestrian accidents account for a considerable number of automobile accidents in industrialized countries. Head injury continues to be more concerned with Automobile impacts. Because the head is the most seriously injured part in many collisions including in pedestrian automobile collisions. To reduce the severity of such injuries international safety committee have proposed subsystem tests in which head foam impactors are impacted upon the car hood

Currently in India, Automotive Indian Standard (AIS) 100, Amendment 1 is used to evaluate the performance of vehicles against pedestrian safety. This standard has been harmonized from the international evaluation standard Global Technical Regulation No. 9 (GTR 9), whose purpose is to bring about an improvement in the construction of the fronts of vehicles and, in particular, those areas which have been most frequently identified as causing injury when in collision with a pedestrian or other vulnerable road user. The tests required are focused on those elements of the child and adult body most frequently identified as sustaining an injury, i.e. the adult head and leg and the child's head. To achieve the required improvements in the construction of vehicles, the tests are designed in such a way that they will represent the rear world accident scenario.

      

                            Fig: Pedestrian Protection Test Procedures as per AIS 100

The different impactors used in predicting the performance against pedestrian safety are the lower leg form and upper leg form impactors (representative of the adult leg) and the adult and child head form impactors (representative of the adult head and child's head). Head injury is a more life-threatening and most common cause of pedestrian deaths in pedestrian to vehicle collision; it was decided to focus on these impactors and test procedures as a part of this study.

Wrap Around Distance (WAD):

When a car crashes on the pedestrian, the whole human body wraps around the front shape of the car, and the head impacts on the bonnet or the windscreen. The distance at which the head impacts on the car from the ground is mentioned as Wrap Around Distance (WAD).

To be specific the Wrap Around Distance is a measurement of the distance from the ground to the head impact zone over the outer surface of the car. The wrap-around distance is measured longitudinally in the center of the vehicle from the ground.

The severity of the injury caused by the frontal crash depends on the type and shape of the vehicle, the speed of the vehicle, and the movement of the pedestrian relative to the vehicle. In addition to these parameters, the wrap-around distance plays a major role in the safety measures of a pedestrian

During the crash analysis, based on the Wrap Around Distance(WAD), two test areas will be created namely the Child head impact zone and the Adult head impact zone. The child head impact zone is between 1000 to 1700 mm WAD and the adult head impact zone ranges between 1700 to 2100 mm WAD. 

Fig: WAD zone

Head injury criteria (HIC)

The head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact. ... 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 head foam impactors are used to test the behavior on vehicle structures such as the hood. In a pedestrian-vehicle impact, the kinematics and severity of pedestrian injuries are affected by the impact locations on the vehicle and body velocities after impact. The objective of this project is to analyze the pedestrian kinematics in a Pedestrian-Car accident scenario and determine the Head Injury Criteria (HIC) from the head resultant acceleration, for head impacts on the vehicle hood

The equation used for the measurements of the head injury of the whole model for the pedestrian head impact was head injury criteria (HIC). It has been used to predict the risk of engine hood to a pedestrian during the collision.

HIC is calculated according to the below Equation

Where

• a: the resultant acceleration (as a multiple of 10 ms-2 or about 1 g
•  t1, t2: two-time instants (in seconds), which define the start and end of the recording when HIC is at maximum. Values of HIC at the time interval t1-t2 

Note :

For example, At HIC=650,

90% probability of level 1,

55% of level 2,

20% of level 3,

5% of level 4.

Abbreviated Injury Scale (AIS):

Level 1: Slight damage to the brain with headache, dizziness, no loss of consciousness, confusion.

Level 2: Concussion with or without skull fracture, less than 15 minutes of unconsciousness, detached retina, face, and nose fracture.

Level 3: Concussion with or without skull fracture for more than 15 minutes of unconsciousness without severe neurological damage, multiple skull fractures, loss of vision, multiple facial fractures, cervical fracture without damage to the spine.

Level 4: Multiple skull fractures with severe neurological damage.

 

Procedure-

Note: Kg/mm/ms unit system is used throughout the analysis.

Case-1- Simple head Model

• We have to open the given LS-Dyna keyword (.k file) file in LS-PrePost, using option File>Open>LS-Dyna Keyword File as shown in below snap.

• Initially we will open the simple head model in LS-Dyna.
• First we will start with section card, to create section card from keyword>>all>>section>>solid>>here we will input id, elform type as shown below.

• Now we will create the MAT card for it.

• Finally we have to assign the section and material to the part.

• Now we have to create the rigidwall for impact the head model.
• A stationary planar rigid wall is created ahead of the frontal portion of the spotweld box by selecting a node on the edge, Select planar option and Geo-Vector >> 1n+NL
• Now translate the rigid wall to some distance in the direction of the velocity of head model.
• The rigid wall is created at a distance of 20mm away from the boxes.

• The Initial Velocity is defined as the Simple Head foam to impact the rigid wall. We assume in this case the initial velocity is 40kmph i.e - 11.11 mm/ms in the negative z-axis which is toward the rigid wall.

• Now we have to create the contact for the for the box, AUTOMATIC_SINGLE_SURFACE contact, consedring it as frictionless, here we have only select the slave nodes no master is going to be present.

• Now we have to create the control cards for finalisation of output.
• First we will create the control_energy card.

• Now we will create the control_ternimation card.

• Now we will create the database file.
• BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.1ms 
• DATABASE_HISTORY_NODE A  nodes on the bottom face of the head were defined the to measure the acceleration and calculated HIC value.
• Now we will create the ASCII files.

    The output request in ASCII format, The following keyword are activated

1. GLSTAT- Global Data
2. MATSUM- Material Energies and
3. NODOUT- Nodal point data
4. RWFORC: Rigid wall force
5. RCFORC: Resultant Interface Forces

• Now we will check the model and then head for simulation.

• Since there is no error we can proceed further to save and run the keyword file.

• As simulation ended with Normal Termination. Therefore, the model is simulated successfully.
• Now we can open this files one by one using LS-post processor.

Results & Plots-

Effective Stress (V-M)-

Resultant Acceleration-

Energy Plot-

The above plot depicts the energy plot for a simple head impact on rigid wall simulation. It is visible that the kinetic energy decreases and internal energy increases at the instant of impact and they both meet at around 1.9 ms. This is due to the reduction of the velocity during impact. The simple head geometry being elastic material bounces back without major loss of energy and attains velocity in other direction. This causes again an increase in kinetic energy and a decrease in internal energy further and again they meet at around 2.1 ms. The internal energy and Kinetic energy remain constant and the hourglass energy is also under desired limits.

Head Injury Criteria (HIC)-

Manual calculation of HIC value:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=1.8 ms and t₂=10 ms,

From the above graph we can take time span = t2 - t1 

                                                                  = 10-1.8

                                                                  = 8.2 ms

So, the average acceleration from the graph is taken as around 3000.

HIC = [(1/8.2)*3000*8.2]^2.5*[8.2/1000]

       =  4.042e+06, which is nearly equal to the value that we got from solver.

Note: Why HIC-15 used here rather than HIC-36?

This is because the simulation span that we have taken into consideration is very minimal so that we need to go with the HIC-15 rather than HIC-36. Here both the terms will try to capture the time span of a specific peak acceleration area in the graph. So selected this way!

Animations-

Effective Stress (V-M)-

Resultant Acceleration-

Case-2-Child head-form and rigid wall

The test setup as per AIS 100. The release angle was 55⁰ for the Child head foam. The head-foam velocity at impact was 11.1mm/ms (40 km/h) for head foam. Here in this case impact point on the Rigidwall instead of a car hood.

• The standard dummy model of child headform was provided with necessary keyword for impact simulation as shown in the fig. below.

• Since the transformation of the original geometry is necessary to replicate the real impact situation. So *DEFINE_TRANSFORMATION card is used to rotate the original body about Y-axis and the A1 to A6 are direction cosines values.

• Here the *INCLUDE_TRANSFORM card is necessary for the file to be transformed and step up for the analysis. In this card, the original geometry is called through filename and defined transformation is carried out on that geometry. Transformation ID (TRANID) is set to 1 for this card. The original input file is not changed but only called for transformation.

• The main file is opened in LS-PrePost as shown in fig. 14 to add necessary keywords to complete the simulation setup.

Note: While adding keywords to main file, ensure the Subsys: is set to the main file .k extension.

• As per Euro NCAP standards, the initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 55⁰. The vertical and horizontal components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.

• The rigid wall is created at a distance of 20 mm from the bottom of standard dummy headform model.

• Here some of the cards like section, material and contact defination card s are aleardy present as it is a standard model so we do not need to define that.
• So now we will define the control cards and database cards.
• We will define control_Energy cards and Control_termination cards.

 

• Now we will create the database file.
• BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.1ms 
• DATABASE_HISTORY_NODE A  nodes on the bottom face of the head were defined the to measure the acceleration and calculated HIC value.
• Now we will create the ASCII files.

    The output request in ASCII format, The following keyword are activated

1. GLSTAT- Global Data
2. MATSUM- Material Energies and
3. NODOUT- Nodal point data
4. RWFORC: Rigid wall force
5. RCFORC: Resultant Interface Forces

• Now we will check the model and then head for simulation.

• Since there is no error we can proceed further to save and run the keyword file.

Results & Plots-

Effective Stress (V-M)-

Effective Plastic Strain-

Plots-

• In the energy plot for head foam impact on a rigid wall, the kinetic energy decreases as internal energy increases at the impact and since the head foam is modeled with hyper-elastic material there is a gradual change in the energised after impact compared to the simple head model case there some of the energy is absorbed in deformation caused due to impact.

Manual calculation of HIC value:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=4.3 ms and t₂=5.4 ms,

From the above graph we can take time span = t2 - t1 

                                                                  = 5.4-4.3

                                                                  = 1.1 ms

So, the average acceleration from the graph is taken as around 486.

HIC = [(1/1.1)*486*1.1]^2.5*[1.1/1000]

       =  5727.74, which is nearly equal to the value that we got from solver.

Animations-

Effective Stress (V-M)-

Effective Plastic Strain-

 

Case3 : Child head-form and bonnet/hood

• Here now we will define tranformation Card

• In this case, the rigid wall is replaced with a bonnet to study the real-life situation of child pedestrian head impact simulation.

• First we will start with section card, to create section card from keyword>>all>>section>>shell>>here we will input id, elform type as shown below.

• Now we will create Mat card for hood.

• The meshed hood model in imported and The test setup is the same as per AIS 100 with the release angle was 50⁰ for the child head foam so like the previous case we will use *DEFINE_TRANSFORMATION and *INCLUDE_TRANSFORM to define the position of the child's head.

 

• An AUTOMATIC_SURFACE_TO_SURFACE contact has been defined between Hood and headfoamHood as master and the head foam as a slave. As the head foam has 5 components in it, the slave node-set type is given as Part Set ID and hood as master segment type is also given Part set ID.

 

• The Initial Velocity is defined along X and Z direction to the Head foam to impact on the car hood. An initial velocity of 11.11mm/ms is with which the Head impacts the rigid wall but here it will be a resultant direction. So, the component motion in the negative x and negative z-direction has to be calculated. 

• The Boundary SPC is defined for the Hood to constraint all the degree of freedom in translational for the set of nodes (NSID) on the edges of the hood.

• Here some of the cards like section, material and contact defination card s are aleardy present as it is a standard model so we do not need to define that.
• So now we will define the control cards and database cards.
• We will define control_Energy cards and Control_termination cards.

 

1. GLSTAT- Global Data
2. MATSUM- Material Energies and
3. NODOUT- Nodal point data
4. RWFORC: Rigid wall force
5. RCFORC: Resultant Interface Forces

• Now we will check the model and then head for simulation.

• Now we will run the simulation and plot the results.

Reslts & Plots-

Effective Stress (V-M)-

Effective Plastic Strain-

Plots-

Manual calculation of HIC value:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=2.6 ms and t₂=10 ms,

From the above graph we can take time span = t2 - t1 

                                                                  = 10-2.3

                                                                  = 7.7 ms

So, the average acceleration from the graph is taken as around 64.8.

HIC = [(1/7.7)*64.8*7.7]^2.5*[7.7/1000]

       =  260.272, which is nearly equal to the value that we got from solver.

Animations-

Effective Stress (V-M)-

Effective Plastic Strain-

Conclusion-

• The simple head model was defined with MAT_ELASTIC material card which implies the material is pure elastic that’s why the max stress is only there in the time of the impact there is no significant deformation in the simple head geometry. That’s why the HIC values are too high.
• The standard model of the child head foam with proper material was used, Since the rigid wall is undeformable so the total stress is transferred to the head and deforms the head foam geometry severely the HIC values are lower than case 1.
• The standard head model but in this case, it impacts on the car hood to study the real-life situation the hood was defined with a piecewise linear plasticity material model (Mat_024) which is a deformable model so during the impact the hood deforms significantly and reduces stress and strain on the head foam and thus reduces HIC values and the value are lower than case 2. which is what we finally required.