Assignment 6

ASSIGNMENT-6

OBJECTIVE: To perform frontal crashworthiness analysis of NEON BIW model under the given conditions.

PROCEDURE:

Case setup

1. Initially check the unit system from the radios engine file provided. The unit system can be also checked by opening the radioss file in Hypercrash and observing the mass of the model from mass balancing tab and also by observing the Young’s modulus and density values in materials tab. It should be set to [kg mm ms kN].
2. The initial interfaces are deleted and new single global interface is created involving all the elements as the master elements with all the nodes selected as the slaves.. During a frontal crash if there are no sub-systems available in the model as in this case, but a single frame then it is recommended to have a single interface. A TYPE-7 general contact interface is created with the recommended parameters.

Global contact interface and recommended parameters

3. Do a penetrations and intersections check by going into quality tab in hypercrash and selecting check for penetrations options. If intersections arise, then manually shift the nodes out. Initial penetrations (low penetrations) are removed initially by using the recommended parameters (Inacti=6) from contact specifications.

4. Check for rigid body connections in the model manually. Delete all the unneccesary and free rigid bodies from the model by intuition. Also, move in to quality tab in Hypercrash and click on check for connectivity, pick all the parts from the graphic window and find any free bodies if they exist. By intuition, if required delete them.

5.Create a rigid wall barrier in the front of the vehicle. In Hypercrash->loadcase-> Rigid walls-> create new->plane. Enter the nodes for the normal direction. Give the slave search distance of 1000mm, enter the sliding friction coefficient as 01 and hit save to create the wall.

6. The target mass of the model is approximately 700kg. The mass initially provided was 188kg approximately. In Hypercrash-> mass->balancing-> check for the COG and the mass of the model. The COG has to be shifted to the center rail behind the driver’s seat. This can be achieved by adding mass (type 1 mass evenly distributed on the nodes). By intuition, mass of 200 kg for both passenger seats and 75kg for a single driver is given at the appropriate places. Also, some of the already provided masses at the rear end are increased by 7kg at four locations to meet the target mass criteria and also move the COG to appropriate position.

7.In Hypercrash->load case-> INIVEL. Initial velocity is provided to the entire model by picking the entire model from the graphics. An initial velocity of 35 mph (15.6464 mm/ms) is given as translational x-axis velocity.

8. In Hypercrash->data history-> set nodal, elemental time steps. The time steps given were 0.5microseconds (0.0005 ms). A scale factor of 0.67 for nodal time step and 0.9 for elemental time step.

9. The end time is set to 80ms.

Output Requests

1. Sections were defined at the desired node locations provided. Plane sections were created by choosing the part, and then by picking the nodes in appropriate manner to create the section plane. Then systems (movable frames) were defined by appropriately picking the nodes on the same places where sections were desired. The sections were then assigned to the corresponding frames. A total of 8 sections were created. Time history outputs of the sections was included by turning the data history option of the sections to YES.

The above figure shows the 8 sections created.

Red-Bumper bracket

Dark blue- A-pillar

Sky blue –Rail

Black - Shotgun

2. An accelerometer sensor was applied at appropriate location at the bottom of the B-pillar, its time history output was also turned to YES.

3. Footpedal and Dashwall intrusions are measured with reference to BIW frame floor. This is achieved by creating 2 noded spring elements connecting appropriate nodes (according to intended parts displacements). Spring property card is also made, with low stiffness and target mass and then assigned to the spring elements. Time history output of springs is requested under default condition (to get all the data) and TH SPRINGS card is created.

Springs

Quality check

1.In Hypercrash->Quality, check for connectivity of parts, penetrations and intersections and debug them manually.

2. Run the model checker, before starting to solve and check for errors and warnings. Debug them manually.

3. The model can be saved and imported to radioss for solving.

RESULTS:

Energy Balance

As is evident internal energy increases as the deformation increases with time, this is achieved by conversion from the kinetic energy which is maximum initially, before the crash.

Contact energy shows an increasing trend, as the BIW frame buckles and frictional contact forces increase among the various parts in contact within the frame. However, contact energy variations are very small compared to internal, K.E and T.E variations.

Initially, hourglass energy increases initially as hourglass modes occur, however after a certain time through hourglass control, the energy dissipated through hourglass control stabilizes. The small variation in hourglass energy and contact energy leads to slight negative energy error.

RWALL Forces

A peak RWALL force of about 118kN is observed at 15ms. This is observed at the onset of buckling of BIW frame.

Section Forces

The above diagram shows the left-side section forces of the rail, shotgun, bumper bracket and A- pillar.

There are few interpretations from this graph. It is observed that maximum of RWALL forces get transferred through the rails while the bumper bracket and shotgun experience similar forces. The A-pillar experiences very less force. This is essential at it implies that the driver cabin is relatively subjected to less deformation keeping the driver safe, which is the requirement of good crashworthiness. It is also observed that the sectional forces of different components peak at different times. This is due to the distance of the components from the initial impact site. The greater the distance from the initial impact site, greater the time it takes to achieve a peak. We can see that the bumper bracket experiences a peak first, followed by the rail and then the shotgun. While, the A-pillar experiences the peak a lot later. This shows that not only less forces are transmitted to the driver cage, but it is transferred relatively slowly, thus reducing the impact a driver faces making it safer for him/her.

Right sections forces

It is seen that right side sections experience greater forces than the left section. This is perhaps due to greater buckling experienced on the right side. This occurs at the shotgun on the right side is provided with an additional hole, thus reducing its stiffness compared to the left side. This helps in protecting the driver (sits on left side abroad).

Foot pedal and Dash wall intrusions

Dash wall intrusions

Foot pedal intrusions

It is observed that intrusions increases with time, over the foot pedal and dash wall. However, the left dash wall experiences a springback after some time. Dash wall intrusions reach up to a maximum of 35mm and foot pedal intrusion reach a maximum of 42.5 mm by the end of the crash. The intrusions are low, and thus is suitable for occupant safety.

Acceleration

As can be seen, during a crash scenario the frame experiences several jerks. Peak acceleration of about 1.4 mm/ ms² is observed. This is followed by several small jerks.

Mass Balance

As seen previously, the initial mass of the system was 690.8 kg. There is a small percentage increase by the end if the simulation. Final mass at the end of simulation is 691.7 kg. This is attributed to the constant time step (CST) algorithm used for time step control. Deformation of the elements decreases the critical length of the element, which in turn reduces the time step below the critical time step. To control the time step, the speed of the elastic wave is reduced by increasing the density by adding mass.

Mass balance

CONCLUSION:

Thus, through the project I learned about the setup of the frontal crash on preprocessing tools such as Hypermesh and Hypercrash. Understood, the concept of contact modelling, creating load cases, time step control and generating output requests such as section time history plots, acceleration plots, intrusions etc. Finally, learnt how to debug different kinds of errors in a case.Various crashworthiness parameters were also understood, to interpret the results effectively.