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

Introduction:

Head Impact :

Fig: Head Impact Test

About 14 percent of all road fatalities in Europe 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.

Case1: Simple head-model and rigid wall

File Importing:

The simple head model is imported and they have meshed with shell element.

Rigid Wall creation:

• The planar rigid wall is created normal to the Z-axis and placed 20 mm away from the simple head geometry

                                      Fig: Creation of Rigid wall

Section Card :

Since the Simple head foam is meshed using 2d element we will define them with the SECTION_SHELL card 

 

Fig: Defining the Section card for the Simple Head foam

Material Model:

We are using the MAT_ELASTIC card for head foam and The material and section are assigned to their respective part ID’s

Fig: Defining the Material card for the Simple Head foam

Boundary Condition:

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.

Fig: Defining the Initial velocity for the Simple Head foam

Contact Card:

Contact_Automatic_Single_Surface is defined with the only slave as Simple Head-foam and since the Rigid wall is always going to act as very high stiffness with a zero deformation mode.

Fig: Defining the Contact card for the Simple Head foam

Control Card:

Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 2.50ms

Fig: Defining the Control Termination card for specifying the run time 

Database Creation:

• BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.01ms 
• DATABASE_HISTORY_NODE A  nodes on the bottom face of the head were defined the to measure the acceleration and calculated HIC value

ASCII_option keyword

    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

Results:

Effective stress values:

Max Effective Stress: 3.642 GPa

Effective strain values:

Max Effective Strain: 0.006083

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 1.4 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 1.6 ms. The internal energy and Kinetic energy remain constant and the hourglass energy is also under desired limits.

Head Injury Criteria (HIC):

The HIC -15 value for a simple head model which impacted with a Rigid wall is measured from a node on the simple head model. From the graph, it can be seen that resultant acceleration is increased and then decreased after the impact because the body comes to halt and again bounces back from the rigid surface at the bottom.

HIC: 1.18e+07

Note: 

Manual calculation of HIC value for Case-1:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=1.3 ms and t₂=2 ms,

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

                                                                  = 2-1.3

                                                                  = 0.7 ms

So, the average acceleration from the graph is taken as around 12000.

HIC = [(1/0.7)*12000*0.7]^2.5*[0.7/1000]

       =  1.1042e+07, 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!

Final Simulation :

Case2 : Child head-form and rigid wall

Geometry setup:

The test setup as per AIS 100. The release angle was 50⁰ 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.

Fig: Test Setup for Child Headform Impact on Vehicle Hood

Steps for importing Child Headform file:

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.

Fig: Defining an Include_Transform card

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.

Fig: Defining a Define_Transformation card

The Child head FE assembly model is imported and consists of the following parts:

Fig: Child Headform FE Model of Individual Components and Assembly

The accelerometer element was placed on the rear plate inside the inner sphere which is at the same location as its counterpart in the physical head foam.

*CONSTRAINED_EXTRA_NODES_SET was used to assign contact between outer skin and inner sphere of head foam. 

*CONSTRAINED_RIGID_BODIES was used between the backplate and Sphere and accelerometer block.

*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE_ID was used to assign contact between outer skin and inner sphere

Each component of the physical head foam and the complete assembly were weighed to obtain their masses. Density values of each component were adjusted based on the geometry scanned and total mass weighed. The final FE child head foam model masses were 3.51 kg. These were within the allowable tolerance as per AIS 100 (+/- 0.07 kg for Child head foam)

Fig: Creation of Rigid wall

The rigid wall is generated normal to Z-axis as discussed in the previous case and placed 20 mm away from the child head foam model. The material card and section card are already predefined for the child head foam assembly.

Boundary Condition: Initial velocity

The Initial Velocity is defined along X and Z direction to the Head foam to impact the rigid wall

An initial velocity of 11.11mm/ms is with which the Head impacts the rigid wall but here it will be a resultant motion. So, the component motion in the x and z direction has to be calculated.

Fig: Defining the Initial velocity for the Child Head foam

We assume in this case the initial velocity is 40kmph i.e 11.11 mm/ms which is a resultant velocity and so resolve the velocity component in negative x (mag. 7.1399999) and negative z-axis (mag. 8.5100002). In this case, the head is tilted at an angle of 50 deg and the values inputted are as shown above.

Control Cards:

Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 4ms

Fig: Defining the Control Termination card for specifying the run time 

Control Time step: calculates the time step automatically for all the elements and uses the minimum value for the analysis

• TSSFAC (Scale factor for computed time step) = 0.9
• DT2MS(Time step size for mass scaled solutions) = - 0.0011000

DT2MS - Negative: Mass is added to only those elements whose time step would otherwise be less than target time step size =(TSSFAC*|DT2MS|.)

Fig: Defining the Control Timestep card with mass scaling parameter

Database Creation

• 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 Acceleration Block of the head were defined to measure the acceleration and calculated HIC value

ASCII_option keyword

    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

Results:

Effective stress values:

Max Effective Stress: 0.005766 GPa

 

The maximum stress of 0.0045 GPa at 2.4 ms at the element 2954 and after that the head foam rebounds and decreases drastically at the end of the simulation

Effective strain values:

Max Effective Strain: 0.6493 

In this case, the head foam is impacting a rigid wall which is having infinite stiffness and so undeformable, so all the stress acting on the head foam eventually form a deformation on head foam and we can see the total strain increases gradually doing the impact and drops due to Hyperelastic property of the Headfoam at the end of the simulation.

Energy Plot: 

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.

Head Injury Criteria (HIC):

The HIC -15 value for head foam impact is measured from an accelerometer placed in the component. From the graph, it can be seen that resultant acceleration is increased and then decreased after the impact because the body comes to halt and again bounces back from the rigid surface at the bottom.

HIC: 3.338e+04

Note: 

Manual calculation of HIC value for Case-2:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=1.9 ms and t₂=2.6 ms,

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

                                                                  = 2.6-1.9

                                                                  = 0.7 ms

So, the average acceleration from the graph is taken as around 1100.

HIC = [(1/0.7)*1100*0.7]^2.5*[0.7/1000]

       =  2.8901e+04, which is nearly equal to the value that we got from solver.

Final Simulation :

Case3 : Child head-form and bonnet/hood

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

Geometry:

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.

Fig: File opened as similar to the Case-2

Since the Material properties and section properties are pre-defined only for  the child head assembly and have to be defined for the car hood

Section Card :

Section Creation Define Section Properties for different element

Since the Car Hood are meshed using 2d element we will define them with SECTION_SHELL card

Fig: Defining the Section card for the bonnet/hood

Material Model:

We will use Aluminium Material Properties for the Car Hood. Although many material models exist for metals, one of the most robust models is *MAT_024 or *MAT_PIECEWISE_LINEAR_PLASTICITY. This material model is the standard workhorse and is the recommended starting point for elastic-plastic simulation of metals and general plastics since it can also handle viscoelastic behavior (i.e., strain-rate dependency).

And for defining this material we are using MAT_024 (MAT_PIECEWISE_LINEAR_PLASTICITY), This is Material Type 24, which is an elastoplastic material with arbitrary stress

 

Fig: Defining the Material card for the bonnet/hood

Contact creation:

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.

 

Fig: Defining the Contact card for the Child Head foam and Hood/bonnet

Boundary Condition: Initial velocity

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. 

Fig: Defining the Initial velocity for the Child Head foam

We assume in this case the initial velocity is 40kmph i.e 11.11 mm/ms which is a resultant velocity and so resolve the velocity component in negative x (mag. 7.1399999) and negative z-axis (mag. 8.5100002). In this case, the head is tilted at an angle of 50 deg and the values inputted are as shown above.

Boundary SPC:

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

Control Cards:

Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 30ms

Fig: Defining the Control Termination card for specifying the run time 

Control Time step: calculates the time step automatically for all the elements and uses the minimum value for the analysis

• TSSFAC (Scale factor for computed time step) = 0.9
• DT2MS(Time step size for mass scaled solutions) = - 0.0011000

DT2MS - Negative: Mass is added to only those elements whose time step would otherwise be less than target time step size = (TSSFAC*|DT2MS|.)

Fig: Defining the Control Timestep card with mass scaling parameter

Database Creation

• *BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.5ms

• *DATABASE_HISTORY_NODE A  nodes on the Acceleration Block of the head were defined to measure the acceleration and calculated HIC value

• *DATABASE_EXTEND_BINARY - this writes the strain tensor to d3plot when STRFLG is set to 1

 ASCII_option keyword

    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

 Results:

Effective stress values in Hood

Max Effective Stress: 0.3089 GPa

Effective stress plot for Hood:

Effective stress plot Head-foam:

The maximum Effective stress value in the head-foam is about 0.0007 GPa.

Effective Plastic strain values:

Max Effective Plastic Strain: 0.4301

In this case, the hood is defined with a piecewise linear plasticity material card which is deformable, so during the head foam impact, the hood deforms significantly and reduces stress and strain on the head foam, and saves the child pedestrian from huge impacts. The effective plastic strain showed on the graph is for the hood part because the deformation in the head form is nearly zero or no deformation 

Energy plot:

      Here since the hood deforms and kinetic energy decreases and the internal energy increases gradually but in the previous study we can able see an abrupt change in these energy values at impact.

Head Injury Criteria (HIC):

The HIC -15 value for head foam impact is measured from an accelerometer placed in the component. From the graph, it can be seen that resultant acceleration is increased and during the initial impact stage decreased after 15 ms because the Hood deforms gradually and slows down the head velocity, not in a quick time but in a margin of time which will save our head from several injuries.

Note: 

Manual calculation of HIC value for Case-3:

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t₁=2.5 ms and t₂=16.5 ms,

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

                                                                  = 16.5-2.5

                                                                  = 14 ms

So, the average acceleration from the graph is taken as around 60.

HIC = [(1/14)*60*14]^2.5*[14/1000]

       =  390.4, which is nearly equal to the value that we got from solver.

Final Simulation :

Conclusion:

Cases	Case-1 : Simple head-model and rigid wall	Case-2 : Child head foam and rigid wall	Case-3 : Child head-foam and bonnet
Stress(Gpa)	3.642	0.005766	0.3089
Strain	0.006083	0.6493	0.4301
HIC	1.18e+07	3.338e+04	387.6
Explanation	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.	With 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.