Head Impact simulation in LS-DYNA

Objective: To perform the Head Impact Simulation and calculate the Head Impact Criterion (HIC) value for the following cases:

1. Simple head model impacting against a rigid wall.
2. Child headform dummy model impacting against a rigid wall.
3. Child headform dummy model impacting against the hood.

Procedure:

Case (1): Simple head model impacting against the rigid wall:

• The given LS-Dyna keyword file of the simple head FE model is opened in LS-PrePost.

Fig 1: FE model of the simple head.

• Section properties:

Fig 2: Section property of simple head.

• The section properties of the simple head are assigned as shell element with 2.5 mm thickness and ELFORM=2, Belytschko-Tsay element formulation.
• Material properties:

Fig 3: The material property of simple head.

• MAT24 (Piecewise linear plasticity) material card is used to assign the steel material properties to the simple head model. The MAT24 represents Piecewise linear isotropic plasticity. With this material model, it is possible to consider the effect of the strain rate.

Fig 4: Part details of the simple head model.

• Initial velocity:

Fig 5: 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 the rigid wall.
• Rigid wall planar:

Fig 6: Rigid wall planar.

• The rigid wall is created at a distance of 15 mm from the bottom of the simple head model.
• Contact details:

Fig 7: Contact between simple head model and rigid wall.

• The contact type selected is AUTOMATIC_SINGLE_SURFACE.
• It is helpful to apply this contact method in the crash models because all the elements are included in one set, and LS-DYNA also considers when a part comes into contact with itself.
• The FS and FD that are static and dynamic friction coefficients with a value of 0.02 are entered in the contact card.
• Control termination:

Fig 8: Control termination card.

• The control termination function is enabled to specify the end time of the simulation.
• The termination time is set for 20 ms.
• Database options:

Fig 9: Database binary D3plot.

• The time step value of 0.5 ms is given for the BINARY_D3PLOT and DATABASE_ASCII options 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 the ‘.k’ extension and is made to run in the solver by getting a normal termination message.
• The D3plot output file is opened in LS-PrePost.

• The animation of Von-Mises stress and Effective plastic strain contour is as shown below.

• v-m stress plot for an element:

Case (2): Child headform dummy model impacting against the rigid wall:

• Part definition:

Fig 10: Imported FE model of dummy child headform.

• The standard dummy model of child headform was provided with the necessary keyword for impact simulation, as shown in fig 11.
• For case (2) impact simulation, the provided standard dummy model of child headform has to be rotated about 50 degrees along the 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 fig 11 and fig 12.

Fig 11: 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 50⁰ (A7=50).

Fig 12: 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 the ‘.k’ extension.

Fig 13: Child headform rotated at 50 degrees along Y-axis.

• The main file is opened in LS-PrePost, as shown in fig. 13, to add necessary keywords to complete the simulation setup.
• Initial velocity:

Fig 14: Initial velocity.

• As per Euro NCAP standards, the initial velocity is taken as 40 kmph, i.e., 11.11 mm/ms at an angle of 50⁰. The vertical and horizontal components of the velocities are -7.14 mm/ms and -8.51 mm/ms, respectively.

Fig 15: Rigid wall planar.

• The rigid wall is created at a distance of 15 mm from the bottom of the standard dummy headform model.
• Contact details:

Fig 16: Contact between child headform model and rigid wall.

• CONTACT_AUTOMATIC_SINGLE_SURFACE card is used to define contact between the rigid wall and dummy child headform model.
• The termination time is set for 20 ms.
• 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 17: Keyword saving option.

The keyword file created is checked for errors using the option keyword manager>model check. The keyword file is saved to the main file using the ‘save keyword as’ option, as shown in fig. 18. The main file is made to run in the solver by getting a normal termination message.

Case (3): Child headform dummy model impacting against hood:

• Part definition:

Fig 18: Imported FE model of the hood.

• Section properties:

Fig 19: Section property.

• The section properties of the hood are assigned as shell element with 1.5 mm thickness and ELFORM=16, Fully integrated shell element (high-speed).
• Material properties:

Fig 20: The material property of hood.

• MAT24 (Piecewise linear plasticity) material card is used to assign the Aluminium material properties to the hood. The MAT24 represents Piecewise linear isotropic plasticity. With this material model, it is possible to consider the effect of the strain rate.
• The keyword file of the hood is saved using the ‘.k’ extension.
• The keyword file of the hood is pulled into the main file using keywords *INCLUDE.
• The child headform is pulled into the main file using keywords *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM, which is similar to the case (2) scenario.

Fig 21: Include keyword.

Fig 22: 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.
• Initial velocity: As per Euro NCAP standards, the initial velocity is 40 kmph, i.e., 11.11 mm/ms at an angle of 50⁰. The vertical and horizontal components of the velocities are -7.14 mm/ms and -8.51 mm/ms, respectively.

Fig 23: Initial velocity is defined.

• Boundary SPC:

Fig 24: Boundary SPC card.

• Using create entity option, the nodes on the outer edges of the hood are selected and constrained by all the translational motion using the 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 coefficients with a value of 0.1 are entered in the contact card.
• The control card and database cards are the same as that of the 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 the main file using the ‘save keyword as’ option, as shown in fig 26. The main file is made to run in the solver by getting a normal termination message.

From the plot, the HIC₁₅ value for the first and second case is beyond the safe value because the head model is impacting a non-deformable rigid wall. The HIC₁₅ value obtained from LS-PrePost for the third case is 198.1 and HIC_(d) value is 315.8 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,

`HIC= underset(t1,t2)(max){[1/(underset(2)(t)-underset(1)(t))int_(t1)^(t2)a(t)dt]^2.5(t2-t1)}`

`int_(t1)^(t2)a(t)dt=198.1`

The average value of acceleration for the time interval of t₁=2.5 ms and t₂=16 ms is, `[1/(underset(2)(t)-underset(1)(t))int_(t1)^(t2)a(t)dt]=198.1`

Therefore, `HIC=underset(t1,t2)(max){[46.4]^2.5(16-2.5)}=197.9`

`HIC(d)=0.75446*(Free Motion Head HIC)+166.4`

HIC(d)= 0.75446*(197.9)+166.4

HIC(d)= 315.7

The HIC₁₅ value obtained from LS-PrePost for the third case is 198.1 and HIC_(d) value is 315.8. The HIC₁₅ value obtained from the manual calculation for the third case is 197.9 and HIC_(d) value is 315.7. There is a small variation in the HIC value obtained from LS-PrePost and manual calculation which is acceptable.

Conclusions:

1. The impact simulation of pedestrian headform is created for a simple case scenario to a complex real-time scenario i.e.,
   1. case (1): Simple head model impacting against a rigid wall
   2. case (2): Child headform model impacting against a rigid wall
   3. case (3): Child headform model impacting against the hood.
2. The computational time required for the simple case is less compared to the real-time case scenario but the accuracy of the results is poor for the 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 HIC₁₅ value obtained for case 3 was within the standard safe HIC₁₅ value of the EURO NCAP protocol.