Week-7 Head Impact Simulation in LS Dyna

Crush_Head_Impact_Test

By Enos Leslie

Mechanical Engineer

13th October, 2021

Pedestrain Head Impact 

Pedestrian protection CAE is a part of vehicle safety engineering from a crash perspective. In pedestrian protection, we look for imparting minimum injury to head, upper leg (thigh), and lower leg (including knee) in case of an accident with a human subject. These tests are usually done at a lower velocity from 20-40 kmph as accidents tend to result in more severe circumstances above these velocities and braking in case of an accident often results in collision at low velocities. 

In this project, a children's headform model is provided. Replication of a scenario where this headform will impact on a car bonnet. The bonnet is considered as a rigidwall for the 1st case and while using the meshed hood model a elasto-plastic material card has to be used. From the simulation, the head impact coefficient is calculated manually and with the help of LS-PrePost.

The aim is to perform the Head Impact Simulation and calculate the HIC value in the LS-PrePost. 

•  Organize a multi-include file model using the keywords *INCLUDE, *INCLUDE_TRANSFORM,*DEFINE_TRASFORMATION. 
• The Impact angle of the Pedestrain Head is between 35-75 degrees.
• Create a simple elastic material model for the simple head (meshed) that is provided.

PLAN

The projet is started off with a simple head model and then complexity is added step by step.(head impact with the rigid and the head impact with Hood is expected).

The Rigidwall is then replaced with the Hood and the same simulation is performed.

 INTRODUCTION

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 sport equipment. (1)

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.

 ^[1]

where t₁ and t₂ are the initial and final times (in seconds) chosen to maximize HIC, and acceleration a is measured in gs (standard gravity acceleration). The time duration, t₂ – t₁, is limited to a maximum value of 36 ms,^[1] 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.

SIMPLE HEAD IMPACT

Unit kg/mm/ms

A simple ball is simulated to crash on a rigid wall.

GEOMETRY

Section

The 2D section used is shown below with all indictions specified.

Material

Material properties assigned is shown below. An elastic material is used for the crush ball.

Part

From the part keyword the section,material and all ID's are specified for all the parts.

RIGIDWALL

A rigidwall is created with the normal direction facing normal to the cruch. (Create entity>rigid wall>planar option ).

Boundary conditions

In order to specify the velocity,nodes are specified and these nodes can be seen below.

Initial velocity keyword is created using create entity menu. Velocity of -11.11mm/ms is specified.

Contact Interface

The rigid-wall acts as a contact itself so we have to specify a single self contact to control the self contact possibility of the ball.(Automatic Single Surface). Details are shown below .

Control Cards

Control Cards used here would be Control Termination. Temination time is 5ms.

OUTPUT REQUEST

1. Binary D3 Plot (Definition of the frequwncy the output animation is created-1ms
2. Data HIstory nodes (In order to measure the HiC value , nodes under the ball are picked for acceleration measurement.
3. ASCII ( nodout,Rwforc,elout,glsat)

RESULTS OF BALL 

• EQUIVALENT STRESS

Maximum von mises stress acting on the ball is 0.7862 Gpa at element 15883

• HIC NO

Nodeout is loaded from ascii and a plot of resultant acceleration with time is ploted using the history node set. The number is taken 0.5 and 2.5ms

HIC number is derived using Hic36 and the value was = 4.87e10

RIGIDWALL FORCES

The effect of the ball on the rigidwall is ploted and the forces in the normal direction is shown below. It can be seen that, forces increase upon impact and reduces afer the ball leaves the surface of the wall.

SECOND CASE ( HEADFORM ON RIGID WALL)

The headform is set  up using the AIS 100 standard. The standard requires the headform be position between an angle of 35 to 75 degrees and it should crush the hood with a speed of 45kmph

GEOMETRY

A first keyword to create is Define_Transformation. This is to rotate the headform which would be imported into the deck by 45degrees about the Y- axis .

•  An INCLUDE_TRANSFORM card is used to import the headform and the transformation rotation card is imported
• Rigidwall, section , velocity is the same as in the case one(ball)
• In the headform there are 4 parts. Materials are specified for each. Images can be seen below

 Termination time= 15ms

OUTPUT REQUEST

1. Binary D3 Plot (Definition of the frequwncy the output animation is created-1ms
2. Data HIstory nodes (In order to measure the HiC value , nodes under the ball are picked for acceleration measurement.
3. ASCII ( nodout,Rwforc,elout,glsat)

• EQUIVALENT STRESS

 Maximum von mises stress acting on the ball is 0.005519 Gpa at element 15883

HIC NO:

Nodeout is loaded from ascii and a plot of resultant acceleration with time is ploted using the history node set. The number is taken 0 and 5ms

HIC number is derived using Hic36 and the value was = 689.4

RIGIDWALL FORCES

The effect of the ball on the rigidwall is ploted and the forces in the normal direction is shown below. It can be seen that, forces increase upon impact and reduces afer the ball leaves the surface of the wall.

THIRD CASE ( HEADFORM ON HOOD)

The headform is set  up using the same procedure as the two cases already described above. The hood replaces the rigidwall for the final simulation of headform crush on car hood. Material speciified for the hood is aluminium (Piecewise plasticity Linear) and the ends of the hood is constrained to simulate real-life occurence. An automatic surface to surface is created with the skin as the master and the hood as the slave. Images are placed below.

• EQUIVALENT STRESS

Maximum von mises stress acting on the ball is 0.1823 Gpa .

•  PLASTIC STRAIN

HIC NO

Nodeout is loaded from ascii and a plot of resultant acceleration with time is ploted using the history node set. The number is taken 0 and 2ms

HIC number is derived using Hic36 and the value was = 1.5e4

RESULTS AND DISCUSSIONS

 In the first case, a random material is used (MAT_ELASTIC). The ball is highly rigid compared to the other two cases. The stress developed on the head is highest compared to the 2nd and 3rd case because the ball does not deform after impact so more impact stresss is developed on the head. The HIC number is highest because of rigidity of wall and ball.

The second case, the headorm was used . The material Odgen rubber was used to represent the skin of the head. This was the least stress developed on the head because of the deformation of the elements around the impact of the wall. From the animation it can be seen that very high deformation occurs on the headform and returns back to normal.This is due to the fact that the head being hyperslastic absorbs all the deformation without any being absorbed by the undeformable rigid wall.The least stress is developed on the head compared to the other two because the material is very soft an upon hitting the wall surface absorbs all the energy. The HIC value is the least because of the effect of the head on the rigid wall.

The third case, ican be described as a situation inbetween case 1 and 2. The headform is used with the soft rubber material. But this time an aluminium hood is used . This enables the stresses on the ball to be lower as the hood is not as rigid as the wall. But the stresses are higher than case 2 because the hedform is crushed on a lesser stiff body compared to the case 2.There is some amount of plastic strain but less than case 2. The HIC value is relatively high compared to case 2 but less than case one. The acceleration of the headform is higher in case 3 than case to mainly because in the difference in stiffness of the two impact wall and hood.

 CONCLUSION

The stress/strain analysis was made for all three cases and the HIC number was determined. The Hic no and stress differed due to difference in material properties and rigidity.

files could not be saved on the website to i have attached a google link for access

https://drive.google.com/drive/folders/16i3dbk-ZsW_LzgqzkiFYS4U78HAp9_zH?usp=sharing