Week-7 Head Impact

Aim: To perform the Head Impact calculation 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-Form and Rigid Wall

Case3: Child HEAD-Form and Bonet / Hood

Introduction:

Head Impact:

About 14% of all road fatality in Europe are pedestrians, with children and the elderly being at the greatest risk. Pedestrian comprises one of the main categories of vulnerable road user which also include cyclist and motorcyclists.

Most Pedestrian accidents occur within city areas where the speed are moderate. The head, the lower body and the legs are the most frequent injured body regions. To estimate the potential risk of Head injury in the event of Vehicle striking an adult or a child, a series of impact test is carried out at 40kmph using an adult r child Head form impactor. Impact sites are then assessed and the protection offered is rated as good, adequate, weak, marginal and poor.

The procedure promotes energy absorbing structures, deformation clearance and deployable protection system such as popup bonnets and external airbags.

Studies shows that the major source of child and adult pedestrian HEAD injuries is the top surface of the Bonet of the striking vehicle. The Bonet would be impacted with a head-form at 35 to 40kmph. The Bonet would be divided into a “Child Head-from Test area and an “Adult Head-Form test area”. The child Head-Form test area is the area of the Bonet that is likely to be impacted by the Head of a midsize adult male pedestrian is likely to impact. An adult head-form test area corresponds to the area of Bonet that Head of a mid-size adult male pedestrian is likely to impact. An adult head-form is used to test the Bonet I the latter area.

Child Head Impact:  

Car-Pedestrian accidents accounts for a considerable number of automobile accident in industrialized countries. Head injury continue to b more concern with Automobile impacts, because Head is the most seriously sensitive part and injured in many collisions including in pedestrian vehicle collision. To reduce the severity of such injuries international safety committee have proposed system test in which head-form impactor are impacted upon the car hood.

Fig: Pedestrian Protection Test procedure as per AIS 100

 

Currently in India, Automotive Indian Standard (AIS) 100, Amendment 1 is used to evaluate the performance of vehicle against pedestrian safety. This standard has been harmonized from the international evaluation Standard Global Technical Regulation No. 9 (GTR9), whose purpose is to bring about an improvement in the construction of the front of the vehicles and in particular those area 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. To achieved the required improvements in the construction of vehicles, the test are designed in such a way that they will represent the real world accident scenario.

The different impactor 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 an Child Head form impactors (representative of the adult head and the 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 part of this study.

Warp Around Distance (WAD):

When car hits the pedestrian, the whole human body warps 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 Warp Around Distance (WAD). To be specify the Warp-around distance is a measurement of the distance from the ground to the Head impact zone over the outer surface of the car. The warp 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 upon 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, Warp around distance (WAD), plays a major role in the safety measure of the pedestrian.

During the crash analysis, based on the Warp Around Distance (WAD), two test areas will be created namely the child impact zone and the Adult 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.

 

WAD (Euro-NCAP)

 

Test Areas:

The Bonet top is an area bounded by reference lines corresponding to the bonnet leading edge, the sides of the vehicle, and the rear of the bonnet. The gtr divides the bonnet top into test area using a parameter is called the “warp around distance” (WAD). The WAD is the distance from a point on the ground directly below the bumper’s leading edge to a designated point on the bonnet, as measured with flexible device, such as cloth tape measures. A WAD of a specified distance, measured as describe in the gtr, defines points on the vehicle’s bonnet from which test area can be determined.

The WAD is a good indicator of where head impacts are likely to occur the bonnet. Head impact locations on the bonnet are largely explained by the standing height of the pedestrian and the frontal geometry of the striking vehicle. The WAD measurement is based on both pedestrian height and vehicle configuration, By use of the WAD, it can reasonably be estimated where on vehicle a child or adult pedestrian’s head may impact.

Head Injury Criteria (HIC):

The head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact, at HIC of 1000 there is an 18% probability of a severe head injury, a 55% probability of serious injury and 90% probability of moderate head injury to the average adult.

The Head form impactors are used to test the behavior on vehicle structures such as hood. In pedestrian-vehicle impact, the kinematics and severity of pedestrian injuries are affected by the impact location on the vehicle and body velocity after impact.

 

Fig: WAD Zone

The objective of this project is to analyze the pedestrian Kinematics in collision with vehicle and determine the Head injury criteria (HIC) from the head resultant acceleration, for head impact on the vehicle hood.

The equation used for the measurement 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 is the resultant acceleration (as a multiple of 10ms-2 or about 1g)

 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 injury

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 min of unconsciousness, detached retina, face, and nose fracture.

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

Level 4: Multiple Skull fractures with severe neurological damage.

 

Case1: Simple Head Model and Rigid wall

The simple head model is imported and they had already meshed with shell elements.

• Now next step is to create the Rigid wall, for that go to entity creation, and create a rigid wall at 5 to 7 mm below the spherical head, here we are using the planar rigid wall, means infinite wall, also make sure to define the slave node for the rigid wall and also the normal of rigid wall should be opposite to the direction of impact, if it is not then there nodes of sphere will penetrate the rigid wall.

Creation of Rigid Wall

 

• Section card need to defined, Since the Simple Head form is meshed with 2D elements, we will define them with the SECTION_SHELL card

Defining the Section card for the simple Head Form model

 

• Now next step is to defining a Material, here we are using the MAT_ElASTIC card for head form

Defining the material for the Head Form

 

• After defining section and material the next step is to assign that material and section to the part.

 

Defining a section and material to the Head form part

 

Boundary Condition:

The next step is to define the boundary condition, we need to provide the initial velocity to the head form, so that it impacts on the rigid wall, we taken the initial velocity of 40kmph i.e. -11.11 mm/ms in the direction to the wall.

 

Defining an initial velocity to the Head form

 

Contact card:

Contact_Automatic_Single_Surface which is basically a self contact is defining for the Head form, because on the other side we have Rigid wall which is going to be very stiff with zero deformation mode.

 

Defining a Self Contact for the Head form

 

Control Card:

 

• To run the simulation properly we need to create the control card, without creating control card simulation will not start, and the most important one is termination card, which will define the running time for the simulation. Control Hourglass energy, Accuracy, Energy.

 

• Now our initial setup is done, now we need to define the output, for that go to Keyword >> binary d3 plot, this is the main animation file also we need to request the ASCII output as well and in it contains the result of GLSTAT, MATSUM, RCFORC, etc.

Defining Database History node for extracting the information from the node

 

Model Checking and Solving the setup

One of the most important pre-processing task is to ensure the validity of CAE model before submitting it to the solver for solution.

A good and rich suite of check reduces engineering errors, increases confidence in the simulation results and minimize the duration of simulation cycle by eliminating multiple back and forth from the solver to pre-processor

• Now our deck setup is completely, now one thing left is only to check the file, whether there is any error present or not, if present then remove those error before giving them to solve in solver.

 

• After that we can save the file, but before that one important thing is to check the error before solving the file, if error is present then file is not able to solved by the solver, so make sure to solve all the error and after that save the file.

No Warning and Errors which is good sign for us

 

Results:

 

 

Maximum von-misses stress is 1.336Gpa

 

Internal Energy vs Kinetic Energy vs Total energy

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 around 1.9ms, here initially the KE of the head form is high at 1.9ms the impact occur and there is sudden drop in the KE and rise in the internal energy, so from here we can say that there is an abrupt change in the KE which is not good, because there is no such energy dissipation, so injuries chances are there is more, obviously it is occurring because we have rigid wall and the Head form is elastic, it will simply hit the wall and bounce back.

Head Injury Criteria:

The HIC-15 value for simple head model which is impacted with rigid wall is measured from node which is defined on the simple head form model, from the graph it is seen that resultant acceleration is increased and then decreased after the impact.

 

HIC-15

 

HIC-36

The value obtained from the HIC-15 & HIC-36 is 1.078 e+07

 

Manual calculation for HIC value

The expression to calculate HIC value is

From the plot

The average value of acceleration for the time interval of t1 = 1.70ms and t2 = 9.90

So time span is equal to t2 – t1 = 9.90 – 1.70 = 8.2

The average acceleration from the graph is taken as 4420

HIC = [(1/8.2)*4420*8.2]^2.5*[8.2/1000]

1.065e+07 which is nearly equal to the value that we got from the solver.

 

Both HIC 15 and HIC 36 will gives us same result, but HIC 15 is more precise than HIC 36

 

Case2:

Child Head-From and Rigid-Wall

Geometry Setup:

The test is as per AIS 100, the release angle setup was 50 degree for the child head form and the velocity of head form is around kmph (11.11mm/ms), in this case the Head form impact on the rigidwall.

Steps for importing the Child Headform:

Note:  Here we aren’t directly use the Headform model, because it is standard model and it is not recommended to do any change in them, so basically we create a main file and using *INCLUDE keyword we import the file into them, so that wahtever conditions we specify on main file, it doesn’t impact the include file.

Here *INCUDE_TRANSFORMATION card is necessary for the file to be transformed and setup for the analysis. In this card the original geometry is called through file name and defined transformation is carried out on that geometry. Transformation  ID (TRANID) is set for 1 for this card. The originlal input file is not  changed but only called for transformation.

 

Defining an include Transformation card

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

Define Transformation card

 

The child head FE assembly model is imported and consist of the following part

 

Head-Form Model

 

Rear Plate

 

Inner Sphere and Outer Skin

 

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 form.

 

*CONSTRAINED_EXTRA_NODESET was used to assign contact between outer skin and inner sphere of head form.

*CONSTRAINED_RIGID_BODIES was used betwwen the backplate and sphere and accelerometer block.

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

Each component of the physical Head Form and the complete assembly were weighted to obtain their masses. Density value of each component were adjusted based on the geometry scanned and total mass weighed. The final FE child head form model mass is 3.51kg. These were with in allowable tolerance as per AIS 100 (+/- 0.07kg) for child Head-form.

Control Time Step: defines in the Head-Form file which calcualte the time steps automatically for all the element and uses the minimum value for the analysis.

TSSFAC (Scale factor for computed time step) = 0.9

DT2MS (Time step size for mass scaled solution) = -0.001100

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

 

• Now we imported or included our model sucessfully with defined transformation we rotate it by an angle of 50 degree around y-axis to meet the regulation. Now the next step is to create the rigid wall by following same step as mentioned above

Creation of Rigid-wall

 

Boundary Condition Setup:

• Now we need to define the initial velocity to the Head-Form along X and Z direction so that it impact with the rigid wall, an initial velocity of 11.11 mm/ms is with which the Head impact 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, so we need to resolve the velocity in -x and -z direction, with angle of 50 degree, because head is tilted by 50 degree from the Y-axis.

 

Defining the Initial velocity for the Child Head-Form

 

Control cards setup:

 

• To run the simulation properly we need to create the control card, without creating control card simulation will not start, and the most important one is termination card, which will define the running time for the simulation. Control Hourglass energy, Accuracy, Energy.

 

• Now our initial setup is done, now we need to define the output, for that go to Keyword >> binary d3 plot, this is the main animation file also we need to request the ASCII output as well and in it contains the result of GLSTAT, MATSUM, RCFORC, NODOUT etc.

Control and Database Keyword is defined

 

Model Checking and Solving the setup

One of the most important pre-processing task is to ensure the validity of CAE model before submitting it to the solver for solution.

A good and rich suite of check reduces engineering errors, increases confidence in the simulation results and minimize the duration of simulation cycle by eliminating multiple back and forth from the solver to pre-processor

• Now our deck setup is completely, now one thing left is only to check the file, whether there is any error present or not, if present then remove those error before giving them to solve in solver.

Termination time = 6min 57 sec

 

Results:

Maximum von-misses stress is equal to 0.013Gpa (No Transormation perpendicular to surface)

 

Maximum effective plastic strain: is 0.00732mm

 

The HIC Value obtained from here is 2.87 e+04

 

Defining Transformation card and Rotate it by an angle 50 degree

 

50 Degree rotated arround Y-Axis

 

 

 

The maximum von-misses stress value is 0.01018Gpa

Internal Energy vs Kinetic energy vs Total energy

 

The HIC value should arround 2.434e+04

 

Manual calculation for HIC value

The expression to calculate HIC value is

From the plot

The average value of acceleration for the time interval of t1 = 7.5ms and t2 = 9

So time span is equal to t2 – t1 = 9 – 7.5 = 1.5

The average acceleration from the graph is taken as 787

HIC = [(1/1.5)*787*1.5]^2.5*[1.5/1000]

2.606e+04 which is nearly equal to the value that we got from the solver.

 

Case 3: Child head form and Bonnet/hood

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

Geometry: The meshed hood model is imported and the test is setup as per AIS 100 with release angle of 50 degree for the child head form, so for that we use the -DEFINE_TRANSFORMATION and *INCLUDE_TRANSFORM to define the positions of the child head.

Case Setup is ready

 

 

 

• Now we have to define the Material and property to the Hood section and also need to constrained the hood so that it should not move during impact.

 

Note: Duplicate elements (need to fix this mistake)

 

Section Card: Since car hood is meshed with 2D elements, we will define them with SECTION_SHELL CARD

Section is defined

 

Material Model:

In this case we use the Aluminum Material property for the car hood, although many material models exist for metals, but one of the robust model is *MAT_024 or*MAT_PIECEWISE_LINEAR_PLASTICITY. This material model is the standard work horse and is the recommended starting point for the elastic-plastic simulation of metals and general plastics, since it can also handle viscoelastic behavior (i.e., strain-rate dependency).

 

 

Material Model is defined

 

• Now after defining the Material and Section, now the next step is to assign those material and section to the hood part.

Assign a Material and Section to the Hood part

 

• Now we need to define the contact between the Head form and the node, here we use Automatic surface to surface contact between the Head form and the Hood

Contact between Head Form and Hood

 

 Boundary Condition:

• Now we need to define the initial velocity to the Head-Form along X and Z direction so that it impact with the rigid wall, an initial velocity of 11.11 mm/ms is with which the Head impact 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, so we need to resolve the velocity in -x and -z direction, with angle of 50 degree, because head is tilted by 50 degree from the Y-axis.

 

Defining an Initial velocity

 

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 edge of the Hood

 

Applying SPC to the Hood

 

Control cards setup:

 

• To run the simulation properly we need to create the control card, without creating control card simulation will not start, and the most important one is termination card, which will define the running time for the simulation. Control Hourglass energy, Accuracy, Energy.

 

• Now our initial setup is done, now we need to define the output, for that go to Keyword >> binary d3 plot, this is the main animation file also we need to request the ASCII output as well and in it contains the result of GLSTAT, MATSUM, RCFORC, NODOUT etc.

Control and Database Keyword is defined

 

Control Time Step: defines in the Head-Form file which calcualte the time steps automatically for all the element and uses the minimum value for the analysis.

TSSFAC (Scale factor for computed time step) = 0.9

DT2MS (Time step size for mass scaled solution) = -0.001100

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

 

Model Checking and Solving the setup

One of the most important pre-processing task is to ensure the validity of CAE model before submitting it to the solver for solution.

A good and rich suite of check reduces engineering errors, increases confidence in the simulation results and minimize the duration of simulation cycle by eliminating multiple back and forth from the solver to pre-processor

• Now our deck setup is completely, now one thing left is only to check the file, whether there is any error present or not, if present then remove those error before giving them to solve in solver.

 

No Errors which is good sign for us

 

 

Complete Deck-Setup

 

Result:

Maimum Effective Von-Misses stress is equal to 0.1946 Gpa

 

Effective Plastic strain value is equal to 0.0074134

Internal Energy vs Kinetic Energy vs Total Energy

The Max HIC value obtained fromhere is 438.7

 

Manual Calculation of HIC

The expression to calculate HIC value is

From the plot

The average value of acceleration for the time interval of t1 = 1.5ms and t2 = 16

So time span is equal to t2 – t1 = 16 – 1.5 = 14.5

The average acceleration from the graph is taken as 61.9

HIC = [(1/14.5)*61.9*14.5]^2.5*[14.5/1000] = 437.11

437.11 which is nearly equal to the value that we got from the solver.

 

 

Defining Transformation card and Rotate it by an angle 50 degree

 

The Maximum HIC value obtain from Here is 693.9

 

 

Head Impact at the differnt location of Bonet 

Source: EURO-NCAP

 

The maximum HIC value obtained from here is 438.7

 

Head Impact at Bonet Edge:

 

Permanent deformation of Bonet ( Higher HIC value and more chances of Injury)

 

 

HEAD Impact at WAD 775mm (Bonet Leading Edge)

Source: EURO-NCAP

 

Head Impact at Bonet Leading Edge

The maximum value of HIC at Bonet Leading edge is 1571

 

Head Impact at center of Hood:

 

Here it can be clearly seen that the valuer of HIC recorded on the Accelerometer of the head was 438.7 which is very less as compared to the case1 and case2, the lower value of HIC is because the introduction of deformable part i.e. hood, which help in absorbing most amount of energy.

 

Refering Probabilty of Injury % vs HIC plot

 

Refering to the above plot when we compared our HIC value, it was found that with 40kmph of speed at 50 degree inclination, when head of the child hit the car aluminum hood, there is 60% probability of damage with level AIS, 20% of Damage with level AIS2, 7-8% probability of damge with AIS3, 2-3% probability of damage with AIS4 and level of damage with AIS5 and AIS6 is negligible.

 

Conclusion: In this challenge we learn about the procedure to carrying out the Head impact analysis, however manyconclusion can be drawn from this work. In case 1 the HIC value was 1.078e+07 and in case 2 HIC value was 2.434e+04, in both cases Rigidwall is used but in ist case simple Spherical head form is used which strike the Rigidwall prendicular to it and in case 2 the actual child head form is used also headform is inclined 50 degree and strike with intial velocity of 40kmph. In both cases dure to present of rigid wall the value of HIC is very high, However in case 2 when the rigid wall is replaced with the Hood model which having aluminum property the HIC was significantly reduced due to fact that Hood will absorb lot energy during impact, so clear conclusion can be drwan from the HIC value of the result in case 3 it was found that probability of severe damage to head is quite less