Week-7 Head Impact
Objective:
Calculate thr HIC for the Simple head and child headform with the rigid wall and hood by using LS Dyna.
Solver:
Here I am using Explicit solver. Because crash is happening in a smaller interval duration and explicit give accurate result in non linear analysis compared to the implicit solver.
What happens to the Car during a car crash:
The car itself experiences a very large negative acceleration (tens of times larger than g = 9.8 m/s^2 ) due to the almost sudden stop.The absolute (positive) value of this negative acceleration is often referred to as the deceleration of the car.
The impact lasts for only a fraction of a second (typical values of 100 to 250 milliseconds or so). The forces acting on the car during that time are very high and destructive.
What Happens to the Driver (and Passengers) During a Car Crash?
Various parts of the body will move inside the car and impact the inside of the car at various points.
The head, in particular, may also experience very large and short-lived accelerations resulting from impacting the steering wheel, the windshield, the side window, head rest.
These impacts also last for only fractions of a second but can result in serious or deadly head injuries.
So it is important to quantify that it can be prevented. Head Injury Criterion/ co efficient is a quantification of head injury.
A small HIC value does not means that the head injuries will be of low level, similarly High hic does not imply high level injuries. It tells the probability of getting high level head injuries.
The calculation of HIC is based on the acceleration and the event time interval.
HIC = max(t1,t2) {[1/(t2-t1) `int_t1^t2 a(t) dt`]^2.5 (t2-t1)}.
1. Impact Simulation of simple head impact with rigidwall:
The above figure shows the simple head model provided by them. I am assuming this model is in elastic material for simpler analysis and it is a shell model.
Boundary condition:
We need to specify the material and the section of the model. After we define this thing we need to set the velocity of the model. Because the car is impact on the person at a velocity we specified as 40 kmph.
The above figure shows the model with the rigidwall. Here the head model is going to impact on the rigid wall which helps to capture the real event. To capture the result activate the Database keyword (glstat and nodeout to analyze the HIC result).
Specify the contact as automatic single surface and assign both master and slave as head model to run the analysis.
Result:
The above figure shows the equivalent stress value at the crash time. Unit is in g/mm/ms. The maximum value of the stress acting at the crash area is 2.465e-01.
Acceleration:
To calculate the Head impact coefficient, acceleration plays a vital role. The below figure shows the acceleration plot for the node id 69035 (node at the crash area).
HIC Value:
The above figure shows the hic value graph for 36 ms.
Energy:
Stress Plot:
Strain:
The above two plot shows the stress- strain plot for the element 15399 placed in the contact area with the rigid wall.
2. The impact simulation of the Headform provided with the Rigid wall with the Stress / Strain plots and acceleration plot.
For this simulation they are providing the LSTC Headform model. By using * Include keyword we call this model from the another file.
Using the Define transformation and inlcude transform to get the model for our convenience to capture the result.
The above figure shows the headform model transformed by angle of 60 . This model consists of skin, skull, accelerometer block. Create the rigid wall to capture the head impact. This model contains all the necessary keywords to run a simulation. So no need to work within the model. So create a new file and call this model by using include and add the necessary keyword such as initial for the BC, velocity 11.11 mm/ms. Specify the contact and output to capture the result.
Results:
Stress:
Strain:
The above two figure shows the stress and strain value for this head impact analysis. The max and min value of the stress and strain shown.
Acceleration:
HIC 36:
From this head impact analysis by rigid wall, we get the hic value as 166.6.
Stress and Strain Plot:
The above figure shows the stress and strain plot curve for the element placed in the impact or contact area.
Energy Curve:
3. Head Impact simulation with Aluminium Hood :
For this analysis, instead of rigid wall we are using the aluminium hood. Here for material we are using the Piecewise linear plasticity.
We need to give the contact between the hood and the head. Using the include keyword to call the headform and rotate at 60 by using transformation keyword.
Result:
The above figure shows the equivalent stress value for this simulation.
Strain:
Acceleration and HIC:
The above two figure shows the acceleration and hic value respectively.
Stress/strain Plot:
Energy Plot:
Comparision of stress and strain plot of LSTC Pedestrain head form of rigid wall with hood:
The above mentioned the stress/strain plot for the rigid wall and hood impact. There is a huge difference in the analysis. Analysis with hood is like a real time event compare to the rigid wall. In rigid wall, head hit the wall and the we get the result. The area where impact happening have more stress which is clearly shown in the figure. But in hood, the below figure shows the variation.
When the head is impact on the hood, the stress is distributed across the hood which capture the real life simulation helps to predict the result perfectly. That stress carried away and hit to the windshield because of the impact on the headform.
For understanding, below shows the animation result of all the analysis which clearly shows the differences.
Animation:
1st case: simple headform with rigid wall
Pedestrain headform with rigidwall:
With the Hood:
Conclusion:
By using the include, transform keyword, calculating the hic value of the lstc pedestrain headform by using ls dyna and the comparision of hood and rigidwall is carried out. From the simulation, headform with the hood gives the real time simulation result compared to the other.