Roof Crush Analysis using Radioss and Hypercrash workbench.

AIM:- Roof Crush Analysis using Radioss and Hypercrash workbench.

OBJECTIVES:- 

1)Plot force vs. displacement. Check that the FMVSS 216 target load of 47,000 N (= 3 * GVW) has been met.
2)Plot the energy vs. time curves.

METHODOLOGY

• Import the model in Radioss/Hypercrash workbench. (each platform has there individual PROs and CONs)
• Check the model through Solver(Radioss) or browser (Hypercrash) deck.
• Isolate/ delete the unwanted data from the model.
• Set up different Boundry conditions, Kinematic conditions, interfaces, etc parameters.
• Also, set up the Output request related to the simulation post-process phase.
• Run the model checker (Hypercrash is recommended). and debug the errors and warnings before the simulation.
• RUN the simulation and examine the data collected in the form of graphs and animation.

CRASHWORTHINESS

Crashworthiness is the ability of any given structure to protect its occupants from getting severely injured during an impact or collision in the case of a moving structure. So for that, the Car main Structure also is known as skeleton / BIW should be made up of a material that can easily absorb the impact forces and avoid unwanted injuries on both the passenger and driver side. Material is not the only criterion other than that the structure geometry, connectivity between two parts, Vehicle velocity during impact, Force distribution of the force generated are also important factors.

Crashworthiness analysis can give approximate data regarding the vehicle’s structural ability to plastically deform and yet maintain a sufficient survival space for its occupants involving crashes. 

Old times when actual models are made and push through this sort of harsh condition which we are gone look in short times. What happened is that after each crash-test body got damaged so severely that the prototype cant is used again. So this is a time as well as money consuming process also the data we get from these physical tests is not accurate. Therefore all the companies all over the world adopted the Analysis software to save time and money. 

Crashworthiness helps to Desing Optimization, Material selection, and make the vehicle safer.

ROOF IMPACT TEST

-Stronger the roof better it can protect the occupant, A vehicle roof must be able to handle a weight one and half times to its weight. The Roof Impact type of simulation considered as Quasi-Static Analysis means the Car body is stationary here and the Impactor is in motion. 

-By doing the roof impact test we are actually simulating the strength requirement for a passenger car roof. To withstand the load a vehicle roof as well as its supporting structure must be strong. 

-US Federal Motor Vehicle Safety Standard (FMVSS) 216, are applicable for Roof Crush Resistance of a Passenger Cars. This standard establishes strength requirements for the roofs of passenger cars and is intended to reduce deaths and injuries resulting from the crushing of the roof into the passenger compartment. This standard and procedure are mostly applicable for a vehicle of GVWR 2722 kg or less. for this simulation, we are comparing the GVW x3 value to the max force calculated. Also, the door and windows are kept completely closed during the simulation.

-As per the standard the impactor must be having a fixed size of 1829 X 762 mm and should be impacted on the car body at a specific angle as shown in the below image [we are not getting a full-scale model for this simulation due to the Radioss Student edition ristriction]

PROCEDURE

Unit Check

I had started the Hypercrash in kg, mm, ms, KN unit system, and my starter file not asked me if I want a unit conversion, so I assumed that the starter file comes with the same unit system. Only for confirmation, I had checked the starter file by opening it in the NOTE pad and find the same unit system. It's necessary to check the unit system before went ahead with further simulation because it can cause the wrong result in the future.

Check for Connectivity

-The next important thing is to check if there are any free parts in the assembly or not. Free parts may cause some problems in simulation results therefore get rid of them its important.

We can check the connectivity with the "Check the connectivity of tree section " option which is available in the Quality menu of Hypercrash.

Hypercrash --> Quality -->  Check the connectivity of tree section

After checking I have found out there are three parts that are not connected properly. If I had run the simulation as it is these parts may cause unexpected results. As these parts are not bound together they may fly out of the simulation, which is not a good representation.

-The three unconnected parts I have found out are the strengthening elements, so they have significant importance in our simulation. In the normal circumstance, I may have deleted these parts but because of their importance in our simulation, I had the plan to keep them and deploy necessary connections between them.

-These three parts are free which is not good for our simulation, so I have to fix that as soon as possible. I had used the rigid body connection to define the connection on these free parts.

[NOTE:- as its Quasi-Static Analysis we don't need to connect the part but the connected part may give better result ]

Inappropriate card or the connection data Check

-After the unit and connectivity check, I had checked for any inappropriate card or the connection data that is available. Like in this model the concerned person already deleted Nos. of parts but sometimes what happens is the connection for that part is remains in the system info. and that may cause some error because the parent surface already gets deleted.

 The above two images are one of the remaining files I am talking about. This Time history and contact interface are given to the model when it is a full-scale model and the intension of these parameters are for that simulation, But for our simulation, we don't need all these parameters. All these additional files only increase the computational time and the result of these conditions gives are not that we want either, so the best option is to remove them and set conditions as per our own requirement.

-I had removed these set conditions along with some others like Unsupported card, Engine card that we won't require for this simulation, etc.. For that, we can directly delete the part if those parts don't have any important role in the simulation. Another way is to correct that by deploying the appropriate connections.

 Contact Interface Deployation

-After that, I had moved forward for a Penetration check, But it's not valid to do a penetration check without defining the appropriate Interface connection, AS we Know that during a crash scenario the parts collide with a rigid wall as well as themselves. For that, it's mandatory to define an appropriate interface between them.

For the next step, I had decided to go with the TYPE7 interface type which is recommended and more frequently used to defined ELasto-Plastic material interface connections. For that, I had open the Interface browser in the Hypercrash and select the TYPE 7 interface [Load Case --> Contact Interface] and go with the all ideal values that are suitable for our simulation purpose.

-I had defined two Type 7 contact interface, One over the entire car body where the nodes act as slaves and the surfaces act as a Master. The second Interface is between Impactor and the CAR body, where nodes over the car body defined as slave nodes and the Impactor body defined as the Master as you can see in the above image.

Penetration Check

After that, I had checked for Initial Penetration in my model and removed them. To check the penetration using Hypercrash is simple and we get a shortcut button inside the Contact interface BOX as you can see in the above image extreme left. By clicking on that button we can easily check for Penetration. 

In Hypercrash Steps are as follows Menu Bar --> Tool --> Penetration check

After this, I had Set up My own condition related to our simulation like Imposed displacement, Timestep, simulation time, animation time, and frequency,  etc.

Model Checker

-Now is time to RUN the Model checker, so we can identify any hidden error in this huge model. For that in

Hypercrash --> Quality --> Model checker-->RUN

Radioss --> Tools --> Model Checker --> Radioss Block

-I don't find any errors in this model but some warnings are there. All the Warnings are not harmful but the Error should be taken care of before proceeding with the Analysis. Out of All the warnings I had got The last two may cause some issue so I had deleted that Unused GRNOD and Surfs. The others are acceptable and we don't have to worry about them as they won't affect my simulation. 

- I had set up the RUN time to 200ms and time step 0.001ms. and print time history 0.1ms (/TFILE)

-After done with all the changes and condition definition I had Exported the file and open it in Radioss. go through all the parameters again and RUN the simulation for 200ms.

ENGINE OUTPUT FILE 

CASE 1:- Free parts In the Assembly

CASE 2:- All possible free parts are CONNECTED with Rigid connections.

-As per Standard Energy Error should be between +5 to -15 %. and the -ve sign indicated energy dissipation, and the +ve sign indicates the energy

-As you can see the Energy error we are getting at the end of the simulation is -15.4% which is not good and acceptable. We also have to understand that this simulation is of Quasi-static type also there is no full-scale model is available here. So, this -15.4% energy error is due to the parts that are flying away from the car body during testing and this is an acceptable Energy error for this test.

-If you compare with the Case1 output file where the Energy error is -22.5% at the end of simulation you will get the idea that the Increase in energy error is due to the parts which are not connected and getting separated during the simulation due to the load case.

-The Imposed time step which was 0.001 is larger than the nodal time step inside the Engine output a file to compensate that the mass is added on the nodes and registered as MAss error. As per the standard mass, the error must be less than 1% and during our simulation, it's 4.5% which is a not good indicator of stable simulation.

-Mass error globally should be less than 5%

-The Total simulation time I had noticed was 3hr 41min and 29 sec.

ANIMATION OUTPUT FILE 

CASE1:- Animation results with free parts in the Assembly.

-As you can see in the simulation as soon as the impactor hit the model deformation happened in the respective components but the noticeable thing here is because some parts are not connected to the there respective parts the impact absorption is not valid and not representing the real-life scenario.

-As the impactor moved forward these free parts are getting thrown away from the main body, meaning they are not providing any support at the start, not during the simulation too. So the conclusion is the result of the overall simulation is not acceptable due to this.

CASE 2:- All possible free parts are CONNECTED with Rigid connections.

-You have also noted the free parts in this assembly also, But these free parts are not completely free they are fixed at any one side or edge therefore Hypercrash won't able to identify as a free part.

-By applying rigid body connection we minimized the Energy error.

GRAPH PLOTS

Force vs Displacement Graph

-So as we don't have a full-scale model we can only compare the Force value to the Gross Vehicle weight x 3 [GVW x 3 X 9.81= 23544] which is 23544 N = 23KN and the value we have obtained from CASE 2 is 4.52KN which is very low. This means as per the FMVSS216 standard the roof can easily withstand the Load apply by the impactor.

Energy Graph

CONCLUSION

-From the Obtained animation and plot result I can say that the Vehicle roof test is Not successful and the roof can not able to handle the load up to 5.4KN as per the FMVS216.

-Energy is also linear after the impact and behaves as per the law of conservation of energy.

-The result is not the perfect one as we don't have a full-scale model. With the full-scale model, the result may be different. but for now, the roof is compatible for apply load.

-For the roof impact test the parameters defined under FMVSS216 must be followed for proper results.

-In the case of failure the material and the strengthening element should be taken care of.