Head Impact

https://drive.google.com/drive/folders/1fbQenAXBKwnIY3cGvv3N-EiUC_4XFtm3?usp=sharing

(The File is Too large to be Attached)

 

AIM:-

To perform Head impact analysis On child pedestrian head Using LS-Dyna and Calculate Head Injury Criterion (HIC) value in different case setups.

INTRODUCTION:-

Pedestrian safety simulation is a part of vehicle safety engineering. This study focuses on car and pedestrian interaction. The main focus of this is to reduce pedestrian injury in case of an accident (head injury, leg injury, etc). Usually, these tests are performed in the lower velocity range from 20-40KmpH. HIC(Head injury criteria/coefficient) is a measure of the likelihood of the head injury caused due to the impact after an accident. This is basically a measure of average acceleration during a small interval of the crash.

                                                           

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.

 

HEAD INJURY CRITERION:-

                                                                

 

Head foam impactors are used to test the behavior of 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 head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact. The HIC can be used to assess safety-related to vehicles, personal protective gear, and sports equipment.

Normally the variable is derived from the measurements of an accelerometer mounted at the center of mass of a crash test Dummy's head when the dummy is exposed to crash forces.

 

where t₁ and t₂ are the initial and final times (in seconds) chosen to maximize HIC and Acceleration is measured in gs (standard gravity acceleration). The time duration, t₂ – t₁, is limited to a maximum value of 36 ms, usually 15 ms.

This means that the HIC includes the effects of head acceleration and the duration of the acceleration. Large accelerations may be tolerated for very short times.

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.

 

CASE-1: SIMPLE HEAD IMPACT ON RIGID WALL:-

Geometry:

 

1. A simple head Geometry Keyword file is Imported and the Model is verified using Model info

2. The GiVen Simple head Model has meshed with Shell elements.

                           

3. The Unit Kg/mm/ms is followed throughout the Analysis

Rigid Wall:-

1. A Rigid wall is created Normal to the Z-axis by Create entity >Rigid wall > Planar option.

2. The Rigid wall is Displaced 15mm normal to the Z-axis

 

 Section Creation:-

1. For Defining Section Go to Keyword manager > All > Section > Shell

2. For the Simple head Section_SHELL  is created because it has meshed with simple 2D elements.

3.The element Formulation is Given as 2 And the thickness of 2.5mm is Assigned.

Material Model:-

1. For Defining materials go to  Keyword Manager > All> MAT > Select the Material card and define the material properties like density, Young’s modulus, Poisson’s ratio.

2. For the Simple Head MAT_Elastic Material Card is assigned with the Properties Of Silicone

 

Massdensity(RO)= `2.300e^-06`kg/`mm^3`

Young'sModus(E)=50Gpa

Poisson'sratio(PR)=0.49

Specifying the Material and Section ID to the respective part IDs. 

1. In parts, we basically assign material and sections with their proper ID for every part individually.

2.Assign the Section-Id and Materials Individually as Part-1.

3. For Creating part Go to Keyword manager> Model > Part and assign the Materials and Sections of the Simple head

 

 

Boundary Condition:

1. Initial velocity Is defined to the Simple head By Keyword manager > New > Initial > Velocity. select the Nodes By part Apply 

2. An velocity of -11.11 mm/ms (40kmph) Is assigned to all the Nodes in a Simple head.

  

CONTACT INTERFACE:-

1.As the wall Created is Rigid Here, it acts as Master in Nature, So  Automatic Single surface Contact is created defining Head Model as a slave.

2. The slave segment type (SSTYP) is defined by the Part IDs, select the simple head as Slave.

3.All other Parameters are given as default 

 

 

Control Cards:

1. Control card *CONTROL_TERMINATION is specified

2.Termination time defined by using Control_Termination card, The termination time is set to 1.2ms

Database Creation

1. BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.01

2. DATABASE_HISTORY_NODE- A set of nodes created Below the Head where It will impact to measure the acceleration And HIC value

OUTPUT REQUEST:-

1. To view the results of Simulations Following Steps are Specified.

2.The time interval between the outputs is 0.05 is defined in D3plot

3. Go to Keyword Manager > Database > ASCII select GLSTAT, RCFORC with DT 0.01

4. ELOUT, MATSUM, GLSTAT, RCFORC, RWFORC, SWFORC, NOD OUT this required option assigned into the ASCII option.

 

Run The Simulation.

Save the edited keyword in a separate folder and Run the Simulation in LS Dyna Manager.

                                                                        

 

The Maximum Stress acting On the Head is 1.287Gpa at element 16626

The Minimum Stress acting On the Head is 0.000 at element 15142

                                                            

Max effective Strain is 0.0460

 

                    

From The graph, we can see that Total energy Is In a straight line as it satisfies the Law of Conservation of energy, as the Internal Energy produced from the wall is equal to the Kinetic energy of the Head impacting On the wall.

The Kinetic energy is constant till 0.38ms And after the impact decreasing Gradually.

Similarly, the Internal energy rises at the same moment when the impact has happened

                             

As there is No accelerometer Placed An Random node selected Based on the Impact and HIC value is Plotted

HIC=9.167e+04: average calculated between 0.45ms to 1 ms 

 

 CASE-2: HEADFORM IMPACT ON THE RIGID WALL:-

1. As per the Automotive Industry standards AIS 100 The Headform should be Set Up.

                                                                                  

 

2. The Angle of Impact of the Headform Must be within 35-75 degrees and the Speed of the Impact Should be 40kmph (11.11mm/ms)

3. The Pedestrian Headform is Imported 

4. As we can see  the pedestrian Head form is in the Angle 90deg with respect to the z-axis considering x and y as base 

5. Since the Transformation of original Geometry Is necessary To replicate the Solution Like real-time Impact *DEFINE_TRANSFORMATION Card is used.

6. A new keyword file Is created for the Rotation of the Headform Using the *DEFINE_TRANSFORMATION card.

7. Give TRANID as 1 and Choose Rotation as Option

8. The head form is rotated Along Y-axis at an angle of 45deg

9. *INCLUDE_TRANSFORM card is used to Complete the Rotation Process.

10. While defining *INCLUDE_TRANSFORM card Original Geometry which is Pedestrian head form is called and TRANID is Applied.

11. The Keyword is saved in the same Folder Where the Original Geometry Is Saved.

12. Both *INCLUDE_TRANSFORM and  *DEFINE_TRANSFORMATION cards are included with the original geometry

    

13. Import the New keyword which contains Both *INCLUDE_TRANSFORM and  *DEFINE_TRANSFORMATION cards

                                                

14.As we can see the Headform is Inclined at an angle of 45deg from the Global Axis

The Pedestrian Child Headform Consists of the following parts

                                                              

 

Rigid wall creation:-

1. A Rigid wall is created Normal to the Z-axis by Create entity >Rigid wall > Planar option like same as the previous process

                                                                

2. The Rigid wall is Displaced 15mm normal to the Z-axis

 Section Creation:-

1.Here The *SECTION_SOLID is predefined for all the parts in the Headform with Element Formulation 1

MATERIAL:-

The material is predefined in the Keyword, for Skull, Accel block, plate MAT_020_Rigid IS defined and for the Skin, MAT_O_077_Odgen rubber is Used

    

The accelerometer was Placed Inside the Inner sphere, which is used to record the Impact 

Contact Automatic Surface To surface Is predefined with Skull as master and Skin as Slave.

Other predefined Cards Such as:-

Constrained: Extra_nodes_Set, Rigid_bodies

Control: Bulk_viscosity, Time_step, Energy, Output..

are Noted for their parameters.

Boundary condition:-

As the velocity of impact is 11.11mm/ms  is defined in the previous case as its is acting in a single direction before rotation is Induced

Since the Model is rotated to a certain angle the velocity should be resolved according to the axis 

In this case, The Headform is rotated 45 deg

`V_x=11.11cos45`

`V_z=11.11 sin45`

 

         

 

Control Cards:

Control_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 5ms

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|.)

Database Creation

1. BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 0.1ms 

2. DATABASE_HISTORY_NODE  nodes on Accel Block of the head were defined to Record the acceleration and calculated HIC value

OUTPUT REQUEST:-

1. To view the results of Simulations Following Steps are Specified.

2.The time interval between the outputs is 0.05 is defined in D3plot

3. Go to Keyword Manager > Database > ASCII select GLSTAT, RCFORC with DT 0.1

4. ELOUT, MATSUM, GLSTAT, RCFORC, RWFORC, SWFORC, NOD OUT this required option assigned into the ASCII option.

                                          

The Maximum Stress acting On the Head is 0.0189Gpa at element 2953

The Minimum Stress acting On the Head is 0.000 at element 4033

 

The Stress is a maximum of 0.0189 GPA at 1.70ms at the element 2953

              

The Maximum strain is 0.01103 at element 

Min 0 at element 4033 

                   

max effective plastic strain:0.0110 at 1.70ms

In this case, the head foam is impacting the rigid wall with is undeformable, so all the stress will be acting on the head foam which eventually will have more deformation and we can see the total strain increases gradually during impact. and drops due to the hyperelastic properties of the materials and stays constant.

 

Head Injury Criteria(HIC):-

                                                    

The HIC value recorded in the Accelerometer Is plotted and from the Graph, we can see that the Acceleration is Increased and decreased At 1.70ms As the head Bounces back after rhe Impact.

HIC=5.861e+04

The HIC average is calculated Between the time interval 1.1ms to 2ms 

ENERGY PLOT:-

                                           

From The graph, we can see that Total energy Is In a straight line as it satisfies the Law of Conservation of energy, as the Internal Energy produced from the wall is equal to the Kinetic energy of the Head impacting On the wall.

The Kinetic energy is constant till 0.6ms And after the impact decreasing Gradually.

 

CASE-3:  IMPACT Of HEADFORM in THE HOOD:-

1. The Meshed Hood is Imported As per the Automotive Industry standards AIS 100, in a new keyword File

2. The head form is rotated about 50deg release angle using *DEFINE_TRANSFORMATION card So like the previous case

3. The HeadForm Model is called Using *INCLUDE_TRANSFORM to the Hood Keyword file

4. The Keyword is saved in the same Folder With the Hood

                  

                                    

 

Section Creation:-

1. The sections of the Headform is predefined, so the sections of Hood are Created

2. For the Hood, Section Shell is Assigned, as it has meshed with The 2D element

3. SECTION_SHELL card with ELFORM-2 And Thickness of 2 is Assigned

Material:-

1. Since the Impact on the rigid wall is replaced By a Hood, proper Material should be assigned for the Hood Too.

2. In this case, The Hood is assigned with the Material card *MAT_024 or *MAT_PIECEWISE_LINEAR_PLASTICITY, With the Properties of Aluminium.

3. MAT_24 is an elastoplastic material card since the Hood has chances of regaining its shape to its original form after impact and at the same time Hood maybe Deformable if the load is High.

Contact Creation:-

An AUTOMATIC_SURFACE_TO_SURFACE contact has been defined between Hood and headfoamHood as master and the Headform as slave. 

Boundary Condition:-

Initial velocity

1. The velocity of -7.5mm/ms Is applied normal to the Z-axis and X-axis for looking at exact Impact.

2. As the velocity of impact is 11.11mm/ms  is defined in the previous case as its is acting in a single direction before rotation is Induced

3. Since the Model is rotated to a certain angle the velocity should be resolved according to the axis 

4. In this case, The Headform is rotated 50 deg

`V_x=11.11cos50`

`V_z=11.11 sin50`

The resolved velocity is 7.5mm/ms since its acting in a negative direction -7.50mm/ms is defined

  

Boundary-SPC:-

The Corner nodes in the Hood are Constrained in All degrees of Freedom using the Boundary SPC option.

 

Control Cards:-

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

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|.)

Database Creation

1. BINARY_D3PLOT -  It defines the frequency at which the animation file is to be created and is set to 1ms 

2. DATABASE_HISTORY_NODE  nodes on Accel Block of the head were defined to Record the acceleration and calculated HIC value

3.DATABASE_EXTENT_BINARY- This is used to write Strain Tensor data  To the D3Plot when STRFLG is set to 1

 

OUTPUT REQUEST:-

1. To view the results of Simulations Following Steps are Specified.

2.The time interval between the outputs is 1 is defined in D3plot

3. Go to Keyword Manager > Database > ASCII select GLSTAT, RCFORC with DT 1

4. ELOUT, MATSUM, GLSTAT, RCFORC, RWFORC, SWFORC, NOD OUT this required option assigned into the ASCII option.

                           

Max Effective Stress =0.305Gpa

 

 

                     

Effective Plastic Strain Values:-

Since the Headform is made up of Hyperelastic Material we cant see the stress-strain acting on the head so an element in the Head where it experiences Greater deformation is selected and stress-strain value is Plotted for that

   

 

 

                                                                            

Since the head is rotating during the Impact we can see the rotation in the Axis there 

Displacement of Hood in Z-Axis:-

 

Some elements which experience maximum impact is Plotted, The deformation Starts at 2.5ms and Displaces 51.7mm in the Z direction at 15ms, and Tries Regain to its original shape due to its elastic properties, so that we can see the Increase in curve after 15ms.

 

Energy Graph:-

 

Because of hood deformation, we can see kinetic energy decreases and internal energy increases gradually but in another case because of head foam impacting on the rigid wall there was an abrupt change in energy at impact 

Head Injury Criterion:-

 

The HIC-36 value for head form impact is measured from an accelerometer place in the geometry. From the graph it can be seen, the resultant acceleration is increased and during the initial impact stage decreased after the 15ms  because the hood deforms gradually and slows down and its again increasing to peak around 135mm at 20.5ms due to the Rebound

 The average starts from 2.5ms to 30ms

HIC:463.7 and HIC(d)=516.3

HIC=463.7 is less than HIC Standard protocols  HIC value 650

 

RESULTS:-

Case-1:Simple head Impact:-

1. In the first case, the Headform is defined with the MAT_ELASTIC material card so that we can see the stress acting on the head only during the Time of Impact and Rebounds to its original shape as soon as after the Impact.

2. The head form only consists of shell elements, as there is No accelerometer Placed to measure the Resultant Acceleration or HIC value.

3. So even the HIC value Could be plotted by taking a node at a point of Impact and Plot the HIC values The value Obtained might Not be realistic in Nature.

 

Case-2:Child Headform and Rigid wall:-

1. In case -2  the Head is Set up as per AIS100 regulations and Impacted On the Same Rigid wall again. The Child Headform is Assigned with Proper Materials, and an accelerometer Is placed for recording the Impact. 

2. Since the HEadform is Impacting the rigid wall Which is Undeformable, we can see a Drastic amount of Stress acting on the head and it is failed to rebound to its original shape 

3. The Hic value obtained here is Less than the Case-1, It IS because Of the change in MAT Cards related to the real-life experiment in case-2, As the Rigid wall is the same in Both cases.

4. Since Hyperelastic Material Is used It absorbs More More stress and Strain, So the HIC value is Comparatively Less with Case-1

 

Case-3:Child Headform And Hood:-

1.In case 3 The Head Model Is set up the same as Case 2 But with The Hood Instead of the rigid wall.

2.The Hood is Assigned with the Material card MAT_024 or MAT_PEICEWISE_LINEAR PLASTICITY  To Study the Real-life Impact6 experiment         

3. The Hood Is Considerably deformable during the Impact, So the hood deforms significantly While the head from impacting and Absorbs Relative stress and strain more comparatively with the Rigid wall.     

4.As The Headform is Hyperelastic with the deformable Hood the HIC value is less Compared with Case-2   

 

Conclusion:-                                  

 Thus the HIC value of the pedestrian Head is Solved by the Standard methods, the Value might be differ based on the Material of the Hood, different angles, and Different release angles

 The Severity Index for The Head is 2.5 And has different values for different parts Of the Body, HIC value Can be calculated for different parts of the body and COmpared with the standard values.