Roof Crash analysis on a Car using HyperMesh

Aim:

The main aim is to perform a roof crash analysis and to plot the graphs on the given car model.

Objective:

The main objective is to follow the steps given to perform a crash analysis of the car. 

1. To set the location of the impactor as given below.

• A 180° rotation about the global z-axis with the point opposite to C as a base point.
• A 25° rotation about the axis through axis AC.

2. Setting the Interfaces

• Create a Type 7 contact interface between the impactor and the car.
• choose an appropriate stiffness definition.
• choose an appropriate minimum gap.
• change the coefficient of friction to 0.2.
• choose the appropriate friction penalty formulation.
• Save the data from the interface to the time history file.

3. Creating the Boundary condition for the car

• The suspension shock tower nodes should be locked in the Z- direction.

4. Impactor Boundary conditions

• The impactor assembly contains a spring attached for stability. Create a BCS collector to fix the free end of the spring.
• Create a moving SKEW to define the direction normal to the impactor’s.
• Create a BCS collector to guide the master node of the impactor rigid body so that it is free to translate normal to the face of the impactor, but is fixed in all other DOFs.

5. Giving the Imposed Displacement

• Impose the velocity of the impactor starting from 0 mm/s at t = 0. The displacement of the impactor should be 200 mm @ 200 ms.
• Avoid abrupt changes in the acceleration of the impactor.

6. Perform the simulation.
7. Plot the graphs.

Tasks to complete:

The Tasks to be completed are as follows.

1. For the Control cards and the Engine File attachment.
2. Plotting the Force vs Displacement graph.
3. Checking if the FMVSS 216 impactor load is near to 47000N.

Methodology:

The process to be followed is as below.

1. Opening the Car and Impactor Solver File
2. Setting the impactor Location.
3. Creating an Interface.
4. setting the Boundary Conditions
5. Creating the Imposed Displacement
6. Setting up the Control cards in the Engine File
7. Checking for Errors
8. Checking for Penetrations
9. Generating the animation
10. Plotting the graph

Theory:

Impactor:

Impactor acts as the block or some entity that creates an impact on some objects. For the impactor simulation, there are some vehicle Safety Standards that can be used for simulation. During the simulation, a virtual copy of the original standards is taken to make the impact occurring on the object.

Procedure:

1. Opening the Car and Impactor Solver File:

• To Open the file, Go to the File menu in the Standard menu bar >> Click Import >> Click from Solver Deck >> Select the file to open >> click Import.

• The model is displayed in the Graphics Area.
• Now, to import the Impactor file, follow the same steps to open it and the file gets imported in the same graphics area.

2. Setting the impactor Location:

• To set the impactor location, we use the rotate option for rotating the model and the Translate option to move the impactor in the translational direction.
• First, the model needs to be rotated, To rotate the model, go to the Tools panel, click on Rotate >> Select the elements>> select the Z-axis and give a base point at ‘C’ >> Give angle as 180 >> click rotate+.

• Now to make an angular impactor the same method needs to be followed and the nodes N1, N2 are select for the A and C points. >> give an angle as 25 degrees.

• Now to translate the model, go to the Tool Panel >> Select the axis to move>> give the magnitude value >> Click Translate+.

• The impactor spring should be in the same line as the B- pillar.

3. Creating an Interface:

• The Interface for this project is TYPE 7 type of interface. The Details on TYPE 7 is cleared in the Terms used Section.
• To create a new interface the car body. Click on the Groups tab in the Model Browser >> Right-click on it >> Click Create >> Give the Name as TYPE 7 >> In the Entity Editor, Select the Card image as “TYPE 7” >> Select the Slave entities as components and select the component >> Select the Master Elements and Select as components and select the components >> Give the values of Istf (stiffness definition) as 4, Igap (gap/element option) as 2, Idel (Nodes and segment deletion) as 2, Irem_gap (Deactivating slave nodes if element size is lesser than gap value, in case of self-impact contact) as 2, Irem_i2 (deactivating the Slave node, if the same contact pair/nodes have been defined in interface type 2) as 0, stfac (Stiffness or scale factor) as 2, Fric (Coulomb Friction) as 0.2, Gapmin (Gap for impact activation) as 0.5, Inacti (Deactivation of stiffness) as 6, Iform (Stiffness or viscous formulation) as 2.

• The Card is created.
• Another interface should be created for the car and the impactor. For this, the steps are the same as the ones explained previously. It then creates the interface with both the car body and impactor parts in it.

• Now, to call the interface, go to the Output Blocks in the Model Browser >> Right-click on the Output blocks >> click Create >> In the Entity Editor, Give a name to it >> In the Entity ID, select the interface which have both the car and impactor.

• The interface is called. 

4. Setting the Boundary Conditions (BC):

• The Boundary condition needs to be given for the Shock absorbers, impactors, spring, and rigid bodies.
• The local coordinates should be created before creating the boundary conditions, To create the local coordinates, go to the Solver browser >> Right Click on the Tab area >> Click Create >> Click SKEW >> Click MOV >> select three nodes to create the local coordinates.
• The skew should be created at the locations of the components for which the local coordinates are given.
• To create Boundary conditions for the Shock absorbers, Go to the Solver Browser >> Right-click on the Tab area >> click create >> click Boundary Condition >> Click BCs >> Give a name to it >> In the entity ID, click on the rigid body nodes and click proceed >> In the Skew ID, Click the skew system created >> The Degrees of Freedom (DOF) should be given in the z-direction, select DOF3, and DOF6 box, constraining the Z-axis.

• The process is similar for the impactor, spring, and rigid bodies.
• The DOF for Impactor is DOF1, DOF2, DOF4, DOF5, and DOF6.

• The DOF for the rigid body and the spring is constrained from all directions. Therefore DOF1, DOF2, DOF3, DOF4, DOF5, DOF6 boxes are checked.

• The boundary conditions are given to the shock absorbers, Impactor, Spring, and the Rigid bodies.

5. Creating the Imposed Displacement:

• Initially, a curve needs to be created for the displacement.
• To create a curve, go to the Model Browser >> right-click on the Tab area >> click Create >> Click Curve >> a new dialog box appears >> Click New >> give a name to it >> Give X and Y coordinates as (0, 0) and (200,200) [Note: Here X and Y values have 0 and 200 values each].

• To create the Imposed displacement card, Go to the Solver Browser >> Right-Click on the Tab Area >> click Boundary Conditions >> Click IMPDISP >> Create a name for it >> In the Ground ID, select the node at the spring >> Select the function ID curve, which we created before and click proceed >> In Dir, select the Coordinate as “Z”.

• The Imposed Displacement is created.

6. Setting up the Control cards in the Engine File:

• Initially, we get only a starter file. To run the file to its fullest the Engine file should be created but to create the engine file, we don’t have the prerequisite data to create a new one.
• The engine file can be copied from any previously created engine file. The file name should be renamed with the same name as the Starter file.
• Now, open the Engine file and change the name to the same name as the File name and save the file.
• Next, Import the model again and check that all the data cards are installed in it.

7. Checking for Errors:

•  To check for the errors, go to the Tools menu >> click on the Model Checker >> Select Radioss Block >> In the Tab area, a new Browser opens >> on Errors, Right-click and Click on run all >> Check for the Errors (Errors are generally noted as a cross mark) >> clear the errors.

8. Checking for Penetrations:

• To check for penetrations, go to the Tools menu >> Click on the Penetration check >> In the Entity Editor, click on Selection and click on group >> give the group. In the Tab area, the penetrations are checked.
• There are no penetrations in the model.

9. Generating the animation:

• To create an animation, go to the Hyper View in the Client Selector. >> Hyper view is opened>> Select the File to view the animation (Note: File is in .h3d format in the respective case folder) >> Click Apply.

• The animation is opened and it can be seen by clicking play in the Toolbar.
• In the Tools panel, select Contour >> In Result Tab, Select Nodal Mass (s) >> Click apply.

• The process is similar for the Plastic Strain and the Von-Mises Stress.
• The animation changes with the input for different types.

10. Plotting the graph:

• The plotting of the graph is done in the Hypergraph 2D, To open the HyperGraph 2D, Go to the Client Selector >> Click HyperGraph 2D >> The Hypergraph is opened >> Browse the file to plot (Note: The file is a ‘T01’ type of file).
• To plot the Energies, Click Global Variable in the Y Type >> Select the Energy you want to plot >> Select MAG in the Y component >> Click Apply.

• To Plot the interface card >> Select the Interface (Note: The name which we created to save the interface) >> Select Total Resultant Force >> Click Apply.
• Now to Create a Force Vs Displacement Curve, the following process is followed.
• Go to the Hyper View >> Click Build Plot >> Select the Result Type as Displacement >> Click MAG >> Now, go to the Request, and select the node >>Click Apply.
• A new plot is created for the displacement.
• Now, open HyperGraph 2D >> give the Interface graph in a new split area.
• Go to the File Menu >> Click Load >> Click Preference File >> A new dialog box appears >> Click Vehicle Safety Tools under the Registered Preferences >> Click Load.
• MATH option will be displayed in the Menu Bar. Now, Click on the MATH option >> Click Two Curves >> Click Cross Plot >> The tools panel is changed.
• Next, the plots are kept beside each other. In the CurveX, Select the Displacement curve by clicking Shift+ Left-click on the mouse >> In the CurveY, Select the Total resultant force curve >> Click on the Create new pages >> Click Apply.
• The graph is created. Now to rename the axis >> click on the axis >> In the label, give the name you want to assign in it >> Click Apply.
• The process for the cross plot is explained in detail in the link provided below.

Observations:

Animation:

• The animation shows that the Nodal mass impacts at the few nodes near the A-pillar where the impactor is falling and near the C pillar where the component is bending.

• The Plastic strain can be seen at the location where the impactor is acting. The car body gets deformed and the A, B, and C pillars are the main area of the impact strain happening deforming the car structure and leading to the vibrations of the car.

• The Von-Mises Stress animation shows that the Impact Stress is maintained all over the roof area and side door part.

Graphs:

• The Force Vs Displacement graph shows that the force is getting fluctuated concerning the displacement as the simulation is going on. The force is seen increasing after the impact and keeps on increasing with a fluctuating force. It is to be expected as the vibrations increase in the car due to the impact, the fluctuation is seen.

• The Energy Graphs shows that the Hourglass is maintained constant but there is a minute increase in the hourglass about 0.025`(kg-mm^2)/(ms^2)`. The increase is minute and therefore it is acceptable. The Contact energy has a slight increase concerning time.
• It is seen that the Total energy increases and is the total of the kinetic and internal energy. The kinetic energy is seen as less than 100 kg-mm/ms and Internal energy is seen to be increasing with an increase in time.

PARITH Card:

• The PARITH card is checked and made sure it is kept ON.
• To check the card, go to the Model Browser >> Click on the Cards >> check ENG_PARITH >>Check if the Keyword 2 is ON.

Results:

The Animation is created and the Stresses are calculated.

The Plots are taken for the energies and the Force vs Displacement curve is also plotted.

The Time History is printed for 0.0001s which is similar to 100 ms. A negative sign is given as the iteration should reduce till it reaches 0. 

The Frequency is added to the model. It is also noted that frequency can also be termed as a time step in Hypermesh. 

The run time is given to the model. The run time is given as 200ms 

To calculate the target load of the FMVSS 216, check the initial vehicle mass, it is around 159.744 kg. To convert it into the weight, the gravitational force should be considered. Therefore, the GVW value is 1567.08N and the target load is 3 times the GVW.

 Load = `3 xx GVW`

 Load = `3 xx 1567.08`

 Load = 4701.26 N or 4.701 KN

The target load given is 47000 N but the load attained is 10% of the target load. The mass needs to be added as the given model is a trimmed version of the original car model. As the student version is being used, the limits of the elements it can save are 100,000 only. Therefore, the material, components of the car are reduced to 100,000 elements by deleting some of them. But if the complete car model is simulated, it may reach the target load.

Terms used:

Von-Mises stress:

Von Mises stress is used to determine if a given material will fracture or yield. It is mostly used for ductile materials, such as metals. The von Mises yield criterion states that if the von Mises stress of a material under load is equal or greater than the yield limit of the same material under simple tension then the material will yield.

Hourglass Energy:

Hourglass Energy is the work done by the forces that take away from the physical energy of the system. It is essentially a spurious deformation mode of a Finite Element Mesh, resulting from the excitation of zero-energy degrees of freedom. It typically manifests as a patchwork of zig-zag or hourglass-like element shapes, where individual elements are severely deformed, while the overall mesh section is not deformed. This happens on hexahedral 3D solid reduced integration elements and the respective tetrahedral 3D shell elements and 2D solid elements.

GVW (Gross Vehicle Weight):

Gross Vehicle Weight is the maximum vehicle weight without considering the seats, interiors, and all the additional parts. It takes only car body weight.

Parallel Arithmetic:

The parallel Arithmetic should be kept ON. The same numerical results will be obtained whatever the number of processors. This result is not guaranteed in the incompatible kinematic conditions in the model.

FMVSS (Federal Motor Vehicle Safety Standards):

FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks, and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS 208.

Cross Plot:

Cross plot is a plot created to compare multiple measurements made at a single time or location along two or more axes.

Learning Outcomes:

• Learned to create imposed displacement.
• Learned to create different types of animations.
• Learned to create Cross plots.

Links:

FMVSS: FMVSS 216

Cross Plot: Cross Plot procedure