Frontal Crash Simulation – BIW

Frontal Crash Simulation – BIW  

Objective:

The objective of this project is to simulate frontal crash of a car’s BIW model and obtain the requested forces, acceleration and deformations experienced by the model.

Case Setup: Model setup

1. The model checker is run in hypercrash immediately after importing and the errors and warnings are then checked to find any unassigned properties or materials for the components.

2. The errors in the given model are depicted in picture below

3. Since there are no unassigned materials or properties, the thickness intersection is checked and if any intersections are present, they are removed by manipulating the nodes

4. After these preliminary checks are done, the load case is set up.

Setting up Load case: The unit system followed is kN, kg, mm, ms.

1. Rigid Wall (RWALL)

1. A rigid wall is created right next to the bumper of the car and the search distance for slave nodes is set to 1200 mm

2. The search distance is used to make sure that the slave nodes within the search distance do not penetrate the rigid wall during collision

3. Care should be taken not to have too many slave nodes for rigid wall as it increases computational load.

4. It should be made sure that the arrow depicting the normal to the rigid wall is opposite to the direction of initial velocity

2. Gravity (GRAV)

1. Gravity is applied to all the nodes in the model

2. To apply gravity, a function/graph is first defined.

3. In this model, as the gravity is acting in -Z direction, the direction in translation option is set to Z.

4. To make sure that the gravity is acting in the -Z direction, either the Y- Scale factor is set to -1 or while defining the graph, Y value is entered as -0.00981.

5. A yellow arrow in the model indicates the direction of the gravitational load.

  

3. Initial Velocity (INIVEL)

1. Initial velocity is also applied to all the nodes.

2. The initial velocity is 15.64 mm/ms in the +X direction

3. The green arrow in the model depicts the direction of initial velocity.

Setting up interface:

Interfaces:

1. Global

1. A global contact interface is used here to take into account self-contact or penetration of the BIW parts during impact with the rigid wall.

2. Hence a multi-purpose contact (TYPE7) is defined in the model.

3. In the contact interface tab, a new contact interface is created, all the parts in the model are selected and self-impact checkbox is checked.

4. The self-impact option automatically calculates the surfaces and nodes in the model and creates the contact interface

5. The optimum parameters for type 7 are then set.

2. Rail Axial left and right

1. To measure the force transmitted, from one part to another part, a contact interface is needed.

2. In this project, to obtain the magnitude of axial forces transmitted from the bumper to the rails on both sides, a contact interface is created by choosing the master and slave entities from both the bumper and rails.

3. These newly created interfaces on either side of the rails provide the forces transmitted from the bumper to the front rails.

Setting up Cross sections:

Cross sections:

Cross sections are used to obtain the physical quantities like normal force, moments, work of forces and moments, etc. in certain area/region/cross section within a component.

Rail, Shotgun and A Pilllar:

1. To create a cross section, in the time history, select cross section (Circular)

2. Select 1^st node to define the tail of local x axis, 2^nd node to define the head of local axis, 3^rd node to define the local xy plane, pick a node for the origin of the circular cross section and select N1-N2 option to choose the cross section plane normal.

3. The diameter of the circular plane in rail is 140 mm

    The diameter of the circular plane in shotgun is 110 mm

    The diameter of the circular plane in A Pillar is 70 mm

4.Then select the required elements for which the cross sectional forces are required

 

Accelerometer

1. An accelerometer is used to measure the accelerations experienced by the body about all the axes.

2. Acceleration values obtained from the accelerometers help calculate the g-Forces experienced by the frame during impact

3. Accelerometers are placed on the rocker of B Pillar on both the sides of the frame

4. To place an accelerometer, go to Hypercrash-> Data History-> Accelerometer. Pick the node where accelerometer must be placed and select the local coordinate system if any.

5. And a default cut off value of 1.65 is applied.

 

Mass Balance (ADMAS)

1. As only the front portion of the BIW frame involved in absorption of most impact force is used here in this project to reduce computational cost, additional mass must be added to the frame to simulate or imitate a full car model with a balanced center of gravity.

2. The frame used in the model weighs approximately 189 Kgs. Hence mass is added strategically to make sure that the final weight is 700 Kgs and the center of gravity lies right next to the driver

3. The ADMAS types used in this model are type 1 and type 3.

4. When type 1 is used, the user defined mass is distributed within a group of selected nodes.

5. When type 3 is used, the user defined mass is distributed in the selected part or component according to the volume weighted distribution

6. The additional masses are added in the region depicted below.

 

Output requests:

Data History:

1. Data history is used to record some very important physical quantities like force, displacement etc. The physical quantities are recorded along with the simulation time and save in T01 file.

2. This file can be used in hypergraph to generate different plots to better understand the simulation.

3. Radioss usually saves the global variables like Internal energy, Kinetic energy, hourglass energy, contact energy, etc by default.

4. Other quantities like forces can only be saved if data history is requested for the specific type of force or moment or work.

5. For this project, the data history contains cross sectional forces, accelerations and displacements.

6. Whenever a cross section is created, it is automatically added to data history. All that has to be done is to choose the required physical quantities to be recorded.

7. For cross sections, the local forces, Normal forces and tangential forces are chosen as output variables

8. Add new TH of Accelerometer, choose the newly created accelerometer and select the quantities to be recorded. Here, accelerations along X, Y, Z is selected.

9. For displacements, usually the relative displacement is recorded.

10. Hence to establish a local frame of reference to measure the relative displacement, a moving frame is created on the b-pillar cross rail on the floor board.

11. Go to Time history, add new TH of Nodes, choose the nodes to be monitored, select the newly created moving frame as frame of reference and select the physical quantities to be recorded which in this case is displacement.

Animations:

1. Animations are also created to visually assess the deisplacement, stress, strain, etc

2. To request animation, in the control cards tab, under engine keywords, toggle ANIM_DT and set the time interval to 1ms.

3. As the software mainly uses keyword, the keywords for animation i.e /ANIM can be used from the control cards tab to request different animations

4 . Next, the required animations like element stress, strain for shell elements, brick elements, displacement, acceleration etc are requested in the same way.

5 . Another method to request animation is to edit the _0001.rad and adding the necessary keywords after the model is exported.

 

Observations/Findings:

Energy Balance.

1. After the simulation is done, energy balance is checked by plotting internal energy, contact energy, hourglass energy, kinetic energy, spring internal energy and total energy in a graph.

2. The total energy is more or less constant without any abrupt change.

3. The kinetic energy is maximum and gradually decreases.

4. The internal energy is zero at the start of the simulation and gradually increases.

5. The engine output file is also checked to make sure that the energy error and mass error are within acceptable limits

6. The energy error is -2.2 % and mass error is 0.13 % and within acceptable limits which indicates the simulation results are valid and acceptable

Animation:

 

Axial force received on the rails from bumper.

1. The total axial forces transferred to the rails from the bumper to the left and right rails in the direction of impact (X-X Axis) are 10 kN and 8.27 kN respectively.

2. And the total resultant force transferred to the left and right rails is 10.73 kN and 8.51 kN respectively

 

3. The maximum axial force that can be transferred through the contact surface area between the bumper and rails is 10.37 kN on the right side and 8.51 kN on the left side

Sectional forces received on the rail next to node 174247.

1. The sections are used to find the magnitude of the physical quantities within a particular section of a part or a component.

2. From the graph, it can be concluded that the sectional forces (Total resultant forces) in both the right and left rails are 28.536 kN and 22.21 kN respectively.

3. However, it should be noted that only a part of the force propagating through the section of the rail is transmitted from the bumper.

4. Rest of the forces are transmitted by impact of the other components attached to the rail with the rigid wall

5 . It should also be noted that mesh of the model is not symmetrical and hence there will be a difference in forces propagated through the left and right sections of the rail

 

Shotgun sectional forces.

1. The shotgun cross sectional forces (Total resultant forces) in the right and left are 13.31 kN and 10.20 kN respectively.

2. The forces in this section are the forces transferred from the rails.

3. From the graphs, it can be found that the forces in the shotgun are lesser than the forces in the rails. 

4. This proves that the force of impact with the random wall is distributed to all parts in the front. The absorption of forces in the frame is evident from the decrease in forces in the individual components.

 

A pillar cross sectional forces.

1. The A pillar cross sectional forces (Total resultant forces) on the left and right sides are 3.368 kN and 2.936 kN respectively.

2. The A pillar cross sectional forces are lower than that of shotgun cross sectional forces and this shows that the forces are decreasing gradually from the front.

 

Acceleration curve obtained by the accelerometer on the base of B- Pillar.

1. The BIW frame moves along the X-axis before it is made to hit the rigid wall.

2. The acceleration values obtained when the frame impacts with the rigid wall are depicted in the graphs.

3. An important thing to be considered here is that there will be high accelerations about Y Axis and Z axis as well.

4. In this model, the maximum acceleration value recorded among both the sensors on the left and right rocker is -1.601 mm/ms² along Y-axis.

5. In order to reduce the computational efforts, the model used here is only the very essential BIW frame in the front part and has no structural members from the rear part

6. The structural members on the rear part will help reduce certain motions like twisting of frame and rotation of frame along Y axis and Z axis thus keeping the rotations of the frame in check. 

7. As the rear structural members reduces the bending and twisting of frame along Y and Z axes like shown in the animation to an extent, the accelerations along Y and Z will be controlled within acceptable limits in real life scenario.

Left sensor:

8. The maximum acceleration along X axis (AX) is -0.204 mm/ms². The negative symbol indicates that the BIW frame is decelerating.

9. Dividing the value by acceleration due to gravity gives the acceleration in G units.

10. Thus the maximum G-force experienced during the impact along the X axis is, 20.79 G’s which is within acceptable limits as typical car crash involves G-forces of about 30 G’s

Right sensor:

11. The maximum acceleration along X axis (AX) is -0.12 mm/ms². The negative symbol indicates that the BIW frame is decelerating.

12. Thus the maximum G-force experienced during the impact along the X axis is, 12.244 G’s which is within acceptable limits as typical car crash involves G-forces of about 30 G’s

Intrusions on the Dashwall.

1. The intrusions are calculated to ascertain whether the crash causes injuries to the feet of the driver due to the crumbling of the front portion of the vehicle.

2. To measure intrusions, a moving frame was created on the B pillar cross rail for a frame of reference during model setup.

3. The intrusions are calculated with reference to the moving frame.

4. From the graph, it can be found that the marked nodes (Node no 66244, 66695) where the foot pedals are, move 179.08 mm and 201.83 mm respectively towards the B pillar cross rail

5. The negative sign indicates that the nodes are moving towards the frame representing the driver's position

6. From the animation, it can be observed that the deformation does not happen in a way to entrap the feet of the driver.

7. As the feet are pushed towards the driver and not entrapped in the crumbled frame during the crash, it can be said that the injuries to the driver’s feet will be very low.

Result:

Frontal crash of the BIW model was simulated and the findings are presented

 

Output Requests		Magnitude
Sectional force in rails (kN)	Left	22.21
	Right	28.536
Axial force received on rail from bumper (kN)	Left	10.73
	Right	8.51
Shortgun cross sectional forces (kN)	Left	10.20
	Right	13.31
A pillar Cross section  (kN)	Left	3.368
	Right	2.936
Acceleration along X Axis (G units) or (mm/ms2)	Left	20.79 G's or -0.204 mm/ms2
	Right	12.244 G's or -0.12 mm/ms2
Intrusions on the dash wall (mm)	Node - 66695	201.83
	Node - 66244	179.08

 

Learning outcome:

1. Checking the newly created FE model using model checker is absolutely vital because it can help pinpoint any modelling errors

2. Interference and penetration checking should be done very carefully because even one tiny penetration of node can result in high contact forces leading to significant drop in time step

3. Hence choosing the best gapmin value for contact interface is important.

4. The gap min value must not be too high or too low as both results in very high contact forces.

5. Unchecked initial penetrations can cause very low time steps, high contact forces and high energy errors.

6. This will reduce the stability of the simulation.

7. If time step control is enabled and if initial penetrations are present, the energy error will reach more than 99% and mass error will reach more than 90% rapidly.

8. For example, using this model, another experimental case was simulated where there was a penetration in the left rail. When the simulation was executed, as the front rail is the first to make contact with the rigid wall, within 200 cycles, the energy error jumped to 99% and mass error jumped to more than 124% suddenly.

9. As the penetration was in the front of the model, the simulation was terminated early by the solver and the appropriate corrections were made to de-penetrate the nodes.

10. If the penetration had been at the rear part of the frame, the solver would have solved the model till the force reached the rear part and terminated the simulation.

11. This would have wasted time and computational resources.

12. This shows the importance of penetration checking.

13. When creating rigid wall, the normal direction of the rigid wall must be opposite to the direction of motion of the car and direction of gravity must be checked well.

14. The direction of the velocity and gravity must also be checked prior to running the simulation.

15. After model setup is done, model checker must be run again to make sure that the errors and warnings do not affect the simulation.

16. Some errors might not cause termination of simulation while some errors won’t.

17. Also, care should be taken while deleting unused sets because sometimes, accidentally deleting sets can lead to problems while solving.

18. Incompatible kinematic conditions must also be checked thoroughly as they can cause very high energy errors.

19. While this model might have been enough for understanding the loads transferred by the parts of the frame, to study more about the accelerations, a full model will be most helpful.

Conclusion:

The given frame’s crashworthiness was tested using the front impact simulation scenario, the forces transferred through components in the load path, accelerations involved in the frame along the axis of impact were studied in detail