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

Case1:  Simple head model and rigid wall

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

• We need to create a Rigid wall, the planar rigid wall is created normal to the Z-axis and placed 20 mm away from the simple head geometry

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

• We need to define a material card for head foam and The material and section are assigned to their respective part ID’s

• 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.

• 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.

• Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 20ms

• 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 bottom face of the head were defined the to measure the acceleration and calculated HIC value
• The output request is in ASCII format, The following keyword is activated

1. GLSTAT- Global Data
2. NODOUT- Nodal point data
3. RWFORC: Rigid wall force

Results:

Effective stress values:

 Max Effective Stress: 0.8698 GPa

Effective strain values:

Max Effective Strain: 0.006248

 

 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 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.

Note: Why was 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.

Case2: Child head-form and rigid wall

Geometry setup:

• The test setup is 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 is on the Rigidwall instead of a car hood.
•  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.

• 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.

• 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 the 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 the 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)

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

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|.)

• 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

The output request is in ASCII format, The following keyword is Activated

1. GLSTAT- Global Data
2. NODOUT- Nodal point data
3. RWFORC- Rigid wall force

 Results:

Effective stress values:

Max Effective Stress: 0.006487 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: 0.007373

 

 

• In this case, the head foam is impacting a rigid wall which is having infinite stiffness and is 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 energized 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: 2.769e+04

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.

 

• 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 elements we will define them with SECTION_SHELL card

 

• 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

 

• 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.

• 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 degrees of freedom for the set of nodes (NSID) on the edges of the hood.

• Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 20ms

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|.)

• *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

The output request is in ASCII format, The following keyword is Activated

1. GLSTAT- Global Data
2. NODOUT- Nodal point data
3. RWFORC: Rigid wall force

Results:

Effective stress values in Hood

Max Effective Stress: 0.2167 GPa

Effective Plastic strain values:

Max Effective Plastic Strain: 0.0333

 

• 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.

• From the plot, the HIC₁₅ value for the first and second case is beyond the safe value because the head model is impacting non deformable rigid wall. The HIC₁₅ value obtained from LS-PrePost for third case is 235.5 and HIC_(d) value is 344.1 which is less than the safe HIC value of 650 as per Euro NCAP Protocol because the hood absorbs the impact energy and gets deformed.

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=2.5 ms and t₂=17.5 ms is, 

[1t2-t1∫t1t2a(t)dt]=48">  =48

Therefore,`HIC = max_(t1 t2){[48]^2.5 (17.5-2.5)} = 239.438` 

`HIC(d) = 0.75446`(Free Motion HeadForm HIC)`+166.4=0.75446 times 239.438+166.4=347.046`

• The HIC₁₅ value obtained from LS-PrePost for the third case is 235.5 and HIC_(d) value is 344.1. The HIC₁₅ value obtained from the manual calculation for the third case is 239.438 and HIC_(d) value is 347.046. There is a small variation in the HIC value obtained from LS-PrePost and manual calculation which is acceptable.

CONCLUSION:

1. The impact simulation of pedestrian headform is created for a simple case scenario to a complex real-time scenario i.e.,

case (1): Simple head model impacting against a rigid wall

case (2): Child headform model impacting against a rigid wall

case (3): Child headform model impacting against the hood.

2. The computational time required for a simple case is less compared to a real-time case scenario but the accuracy of the results is poor for simple cases.

3. There is a variation in the values for stress/strain and HIC₁₅ for all the cases because of the difference in material properties and boundary conditions.

4. The HIC₁₅ value obtained for case 3 was within the standard safe HIC₁₅ value of the EURO NCAP protocol.

Google Drive Link: https://drive.google.com/drive/folders/1uO_Gq8L0VKly40tOqsIkaJfOCi0SaQD2?usp=sharing