Week-7 Head Impact
PEDESTRAIN HEAD IMPACT SIMULATION USING LS-DYNA
AIM: To perform the Head Impact Simulation and calculate the Head Impact Criterion (HIC) value for the following cases.
i. Simple head model impacting against rigid wall
ii. Child headform dummy model impacting against rigid wall
iii. Child headform dummy model impacting against hood
Note: The unit system used is kg-mm-ms.
PROCEDURE:
Case (1): Simple head model impacting against rigid wall:
The given LS-Dyna keyword file of simple head FE model is opened in LS-Prepost using option File>Open>LS-Dyna Keyword File.
Fig.2. FE model of simple head
1. Part definition
Section properties:
Fig.3. Section property of simple head.
The section properties of simple head are assigned as shell element with 2.5 mm thickness and ELFORM=2, Belytschko-Tsay element formulation.
Material properties:
Fig. 4. Material property of simple head.
MAT24 (Piece wise linear plasticity) material card is used to assign the steel material properties to the simple head model. The MAT24 represent Piecewise linear isotropic plasticity. With this material model it is possible to consider the effect of the strain rate.
Fig.5. Part details of simple head model.
2. Boundary condition
Initial velocity:
Fig.6. Initial velocity
The simple head model is assumed to be impacting the rigid wall at a velocity of 40 kmph i.e. 11.11 mm/ms in negative Z-direction towards rigid wall.
Rigid wall planar:
Fig.7. Rigid wall planar.
The rigid wall is created at a distance of 15 mm from the bottom of simple head model.
Contact details:
Fig. 8. Contact between simple head model and rigid wall
The contact type selected is AUTOMATIC_SINGLE_SURFACE. It is quite helpful to apply this contact method in the crash models because all the elements are included in one single set and LS-DYNA considers also when a part comes into contact with itself. The FS and FD that are static and dynamic friction coefficient with a value of 0.02 is entered in the contact card. The simple headform model is the slave (SSID=5).
3. Control and Database options:
Control function:
Fig. 9. Control termination
The control termination function is enabled to specify the end time of simulation. The termination time is set for 20 ms.
Database options:
Fig.10. Database binary D3plot.
The time step value of 0.5 ms is given for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT, NODOUT and RWFORC
DATABASE_EXTENT BINARY card with STRFLG =1, is used to compute the elastic strain in the model.
DATABASE_HISTORY_NODE card is used to compute the HIC value of a 688991 node in the model.
The keyword file created is checked for errors using the option keyword manager model check. The keyword file is saved using 'k' extension and is made to run in the solver by getting normal termination message.
Case (2): Child headform dummy model impacting against rigid wall:
1.Part definition
Fig.11. Imported FE model of dummy child headform.
The standard dummy model of child headform was provided with necessary keyword for impact simulation as shown in the fig. 11.
For case (2) impact simulation, the provide standard dummy model of child headform has to be rotated about 50° along Y-axis to replicate the Euro NCAP impact simulation as shown in fig.1, by pulling it into a new keyword file created by LS-PrePost by using *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM as shown in the fig.12 and fig. 13.
Fig.12. Define transformation card to rotate the child headform.
Using DEFINE_TRANSFORMATION card with TRANID=1, the dummy headform model is rotated by OPTION>ROTATE along Y-axis (A2=1) by 50° (A7=50).
Fig.13. Include transform card.
INCLUDE_TRANSFORM card is used to pull the rotated standard dummy headform model file to the main file. The main file is saved with 'k' extension.
Note: Filenames and pathnames are limited to 236 characters spread over up to three 80-character lines.
Fig.14. Child headform rotated at 50° along Y-axis.
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 2. Boundary condition
Initial velocity:
2.Boundary condition
Initial Velocity
Fig.15. Initial velocity.
As per Euro NCAP standards, the initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 500. The vertical and horizontal components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.
Fig.16. Rigid wall planar.
The rigid wall is created at a distance of 15 mm from the bottom of standard dummy headform model.
Contact details:
Fig.17. Contact between child headform model and rigid wall.
CONTACT_AUTOMATIC_SINGLE_SURFACE card is used to define contact between rigid wall and dummy child headform model. The child headform model is taken as slave (SSID=1).
3. Control and Database options: Control function:
The termination time is set for 20 ms.
Database options:
The time step value for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT, NODOUT and RWFORC is given as 0.5 ms.
DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.
DATABASE_HISTORY_NODE card is used to compute the HIC value of a 25679 node in the model.
Fig.18. Keyword saving option.
The keyword file created is checked for errors using the option keyword manager»model check. The keyword file is saved to main file using 'save keyword as' option as shown in fig. 18. The main file is made to run in the solver by getting normal termination message.
Case (3): Child headform dummy model impacting against hood: 1.
Part definition
Fig 19.Imported FE model of hood
Section Property
Fig.20. Section property.
The section properties of hood are assigned as shell element with 1.5 mm thickness and ELFORM=16, Fully integrated shell element (very fast).
Material properties:
Fig.21. Material property of hood
MAT24 (Piece wise linear plasticity) material card is used to assign the Aluminium material properties to the hood. The MAT24 represent Piecewise linear isotropic plasticity. With this material model it is possible to consider the effect of the strain rate.
The keyword file of hood is saved using 'k' extension. The keyword file of hood is pulled into main file using keywords *INCLUDE as shown in fig.22. The child headform is pulled into main file by using keyword *DEFINE_TRASFORMATION and "INCLUDE_TRANSFORM which is similar to the case (2) scenario.
Fig 22 Include Keyword
Fig.23. Main keyword file showing child headform and hood.
The main keyword file is opened in LS-Prepost as shown in fig. 23 to add necessary keywords to complete the impact simulation setup.
2. Boundary condition
Initial velocity:
As per Euro NCAP standards, the initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 50°. The vertical and horizontal components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.
Boundary SPC:
Fig.24. Boundary SPC card.
Using create entity option the nodes on the outer edges of the hood is selected and constrained all the translational motion using option boundary SPC set.
Contact details:
Fig.25. Contact details between hood and child headform.
CONTACT AUTOMATIC SURFACE_TO_SURFACE card is used to define contact between hood and dummy child headform model. The child headform model is taken as slave and hood as master. The FS and FD that are static and dynamic friction coefficient with a value of 0.8 is entered in the contact card.
Note: Increasing the value of FS and FD also increases the value of HIC, hence choose appropriate value for FS and FD.
3. Control and Database options:
The control card and database cards are same as that of case (2) scenario.
Fig.26. Keyword saving option.
The keyword file created is checked for errors using the option keyword manager model check. The keyword file is saved to main file using 'save keyword as' option as shown in fig. 26. The main file is made to run in the solver by getting normal termination message.
RESULTS:
The D3nlot outnut file is onened in IS-Pro Post using option
File>open>LS-Dyna binary plot.
Von-Mises stress and effective plastic strain contour is shown below
v-m stress contour for case (1)
Effective plastic strain contour for case (1)
v-m stress contour for case (2)
Effective plastic strain contour for case (2)
v-m stress contour for case (3)
Effective plastic strain contour for case (3)
2. Stress Plots
Fig 2.9 v-m stress plot for case (3)
In the first case, during the instant of impact the maximum v-m stress developed is 367.2 MPa in the element 15351 which is beyond the yield stress value of 355 MPa.
In the second case, during the instant of impact the maximum v-m stress developed is 3.916 MPa in the element 3314 and reaches a peak value of 12.5 MPa at 3 ms and later the value of v-m stress reduces due to the bouncing back of the headform from rigidwall.
In the third case, during the instant of impact the maximum v-m stress developed is 3.793 MPa in the element 3031 and later it fluctuates steadily until the headform bounces back from the hood.
The v-m stress induced in the element for third case is less compared to second case because the hood gets deformed due the impact and absorbs the energy unlike the rigid wall which does not get deformed during impact
3. Head Injury Criteria (HIC15)
Fig.33 The resultant acceleration plot with HIC15 value for case (1)
Fig.34 The resultant acceleration plot with HIC15 value for case (2)
Fig.35 The resultant acceleration plot with HIC5 value for case (3)
From the plot, the HIC15 value for 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.
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 rigid wall
case (2): Child headform model impacting against rigid wall
case (3): Child headform model impacting against hood.
2. The computational time required for simple case is less compared to real time case scenario but the accuracy of the results is poor for simple case.
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 HIC,5 value obtained for case 3 was within the standard safe HIC,5 value of EURO NCAP protocol.