Assignment 7
Objective : To perform the simulation according to the given parameters and get the desired output requests.
Procedure :
PRE-PROCESSING :
- Make sure the unit system is in Kg mm ms by checking in the starter file.
- Create type 7 interface on the whole vehicle body as follows.
- Go to tools-->penetration check-->select group and click check.
- Next create a cylindrical pole as rigid wall and give diameter 254mm and give suitable Dsearch value. The pole should be positioned just opposite the b-pillar of the vehicle.
- Add solver masses to bring the BIW weight upto 700kg while maintaining a good position for cog of the vehicle. In this case the right side of the vehicle must have more heavier masses attached to it than the left side to compensate for the lack of structures on the right side of the vehicle. The following masses have been added to the structure.
As we can see in the below pic, the cog of the vehicle is now in the centre position and situated where we want it at the drivers level.
The masses have been added at the following locations of the car body(highlighted in green).
- Initial velocity is given to the car body in the y-direction to crash against the rigid pole at a speed of 15.64m/s.
- Timestep is kept at 0.5 microseconds in the DT/NODA card in the solver deck choosing CST option and scale factor as follows.
- Keep runtime as 80 ms.
OUTPUT REQUESTS :
- Sectional forces: 2 sections are created on both cross members of the BIW by using create-->cross section option in order to study the forces acting on these sections as the simulation progresses.
Front_section
Mid_section
- Intrusions: Intrusions at 3 locations are to be studied , at the hinge pillar, at the base of b-pillar and at the location of the fuel tank. A moving skew is created on the opposite side of the vehicle body to accurately measure the displacement of these nodes once the car is moving.
An output request is created for TH/NODE with card DEF and skew id is mentioned of the above created skew as follows.
This process is done for other 2 nodal positions as well to get the 3 nodal displacements.
- Peak velocity : The node on the inner door panel is selected for peak velocity measurement.
This velocity is measured with respect to the moving skew which we just created for measurement of intrusions.
POST-PROCESSING :
Crash Mechanism : The shell of the car folds inwards on itself after hitting the pole and does so till the end of the simulation with peak forces being developed near the fold.
Sectional forces in cross members :
Given below are the resultant forces acting in the cross members of the vehicle body.
Front section
Mid section
Both together
As we can see the forces experienced in the mid section are greater than the forces experienced in the front section. This is due to the location of the pole which comes directly in line with compressing the mid section of the vehicle.
Intrusions :
The following are the intrusions at the desired locations as follows.
- Hinge pillar
- B-pillar
- Fuel-tank
- All intrusions
As we can see the displacments of all the 3 nodes are same until 10ms mark at which point the pole comes in contact with the side of the vehicle body and they all get displaced by varying amounts as shown above.
Peak velocity of inner door node:
The velocity distribution curve is given above and after the contact with the rigid pole the peak velocity of the door is around 7.5m/s as shown in the graph.
Von misses Stress:
The peak stress value is found to be 0.3874 Gpa developing near the contact point with the pole rigid. The cross members are the members that resist the bending and twisting of the car body upon side impact. Hence they are crucial for the structural rigidity of the vehicle in a side impact scenario and increase its stiffness across the width of the car.
Contact energy:
In this simulation we see that the contact energy is not much, only around 2300 J. This is good which means that the amount of internal energy lost to contact energy is very less and hence even the energy error will be minimum.
As we can see the contact energy curve is somewhat like a mirror image of total energy curve. Here the contact energy is less than 5% of total energy which is our tolerance limit. It is around 2.7% of the total energy hence amount of energy lost is minimum and the simulation is more accurate.
Hourglass Energy :
Even the hourglass energy is bare minimum and almost negligeble. This means the simulation is accurate in terms of energy conservation laws.
Energy Error and Mass Error:
Here we see the peak energy error is -2.9% which was lost to contact energy. This is well within the tolerable limit of -(10-15)%. Also the mass error by the end of the simulation is 0.004552% which is well within the 1% tolerance limit we have for good FEA practices.
Timestep:
We see that throughout the simulation the minimum timestep of 0.5 microseconds has been maintained. This allowed the simulation to complete within a reasonable amount of time of 1hour 23 minutes.
The graph for timestep shows us a small spike in the beginning due to the initialization of mass addition, but after that the timestep stabilizes and remains at a more or less stationary value with very small deviations.
IE v/s KE :
As this is an energy absorbtion impact we see that almost all the internal energy has been converted to kinetic energy by the end of the simulation indicating that the energy balance of the simulation has been maintained properly.
Total translational Energy:
The total tranlational energy remains fairly constant throughout the simulation as we can see in the graph comparing it with hourglass and total energy.
Conclusion : In this way we have studied the crash of a vehicle body by a rigid pole and successfully studied the side impact behaviour of the vehicle.
Result :