Simulation of a Crash Tube For Contact Interfaces using Radioss

OBJECTIVE :

To create a mesh for a bumper assembly with mesh size of 6mm and simulate a Crash tube under different conditions

Case {i} : Run Provided crash tube model as is

Case {ii} : Change the value of Inacti to 6 under Type 7 Contact

Case {iii} : Create an additional Type 11 Contact

Case {iv} : Remove both the notches and the boundary condition on the rigid body

Case {v} : Create a new notch in the middle

Case {vi} : Create another notch with nodes only from opposing 2 faces and run.

Case {vii} : Law 36

The above cases are to compared on the basis of :

- RWALL forces ,contact forces,internal energy and Energy variation for all parts.

- Make a note on if and how the notches affect the results

- Plot energies and note any difference.

A report is to be compiled from the data obtained.

THEORY :

Detecting Free Edges :

Free Edges in a model implies that the said edge is connected to only 1 surface which is ideal for the edges at the end of the surfaces, however if free edges are seen between two surfaces it implies that the surfaces are not connected to each other which will result in a poor mesh.

To detect free edges in a component, Simply switch the geometry colour mode to Mixed and switch to wireframe geometry for better view

After Meshing, to check for free edges between the elements, The "Edges" Tool under Tool Radio Button can be used.

Simply select the component/Element and click on find edges to check for free edges. And it is important to click on Delete edges afterwards. Vary the tolerance as well.

The edges can be healed with the equivalence button shown above, select the elements contributing to the free edge and click "Equivalence".

Healing Free edges :

If there are any free edges found, some of the ways to rectify it are :

- Node/Surface edit

- Toggling the edges in the geometry itself

- Remeshing the surface

- Using "Equivalence as mentioned above.

2D Meshing :

To mesh the Component, the "Automesh" tool under 2D radio is used.

Since the ideal element size is 6mm, it can set in the panel :

Now, the surfaces to be meshed are selected and element count is adjusted as required. Furthermore to improve mesh quality, certain elements alone can be picked and remeshed.

Mirroring the Meshes :

In hypermesh UI, it is possible to mirror the meshed elements with respect to an axis or a user defined plane using the Reflect option found in Tools Radio Button.

- Select the elements to mirror, the nodes (N1,N2,N3 and B) which define an imaginary plane to mirror about

- Once the elements and the plane are defined, It is necessary to command the Pre-processor to duplicate the elements and not move them. Hence click on the Highlighted Elems button and click on duplicate before clicking on reflect.

Interfaces / Contact Modeling :

Defines how parts interact when they come into contact with each other.

Type 7 :Variable stiffness : Node to surface contact

- It is a general purpose contact and is used to simulate all conditions (Also self contact)

- It provides a cylindrical gap area around the edges which makes it more accurate.

- Gap is the area defined around the node within which if an entity enters, it affects the node. The gap is created to mimic the actual thickness of the part.

- There are few parameters required to define a TYPE 7 Contact.

- It works on the logic of Impact Penalty method which treats the behaviour between the slave nodes and master segments as springs that generate resistive forces as a function of penetration.

- Inacti : As the above image shows, Inacti comes into action when there are initial penetrations and the option we choose informs the solver on the course of action in the event of an initial penetration in model. The various actions which the solver would take and the option for it are shown below :

= 0/1000 : No action will be taken

= 1 : Stiffness deactivated on Nodes

= 2 : Stiffness deactivated on Elements

= 3 : Changes Node Co.ordinates

= 5 : Gap is variable with Time (int. gap is computed)

= 6 : Gap is variable with Time (int. penetration is computed)

Limitations of Type 7 :

- Since it takes into consideration only the nodes from the slave parts, it can cause discrepancies as the structure is disregarded.

- Time step is reduced significantly in cases of high Impact speeds.

Type 11 :Variable stiffness : Edge to Edge contact

- It is defined by identifying master and slave lines.

- The parameters required to define the Type 11 contact is same as Type 7 with very few differences like the Type 11 taking the input in the form of lines rather than nodes and surfaces.

- It is ideally used in conjunction with Type 7 contacts to overcome the limitation of Type 7 which disregards the structure of the slave parts.

* Energy Error :

- The difference in Energy level in the system between the starting of simulation to the energy level in system at each or final time step is called energy error.

- The value is always computed in percentage and

• If the error is negative, it means that some energy has been dissipated.
• If the error is positive, there is an energy creation with the system.

- The formula for computing Energy error is shown below :

* Mass Error :

- The difference in Mass in the system between the starting of simulation to the energy level in system at each or final time step is called energy error.

* Rigid Wall Forces :

- The force experienced by the Rigid wall due to the effect of the simulation is called Rigid Wall Forces.

- In this case the force experienced as a consequence of the impact of the tube on the wall is computed.

* Internal Energy :

- The energy in a system arising from the relative positions and interactions of its parts.

- To run a defined simulation, Go to Radioss under Analysis Radio Button and set the values as shown below before clicking on Radioss button again.

- The value "-nt 4" is used to limit the number of cores of the computer processor to be used by the solver and can be varied as per requirement

- The input file can also be changed which will make the solver create a duplicate of the file and save the results at the new location.

* Post - processing :

Analysing the files through Explorer :

* Once the simulation is finished, it can be noted from explorer that, at the location of input file, lots of new files are created

.out Files

- The File_Name_0000.out and File_Name_0001.out files are the output files from the solver after processing the respective Input files of the same names.

- The .out files can be opened through notepad or any similar softwares.

- The _0000.out file contains inputs to the _0001 file and does not contain any results from the simulation. However the _0001.out file contains the energy datas and solver step datas as shown below :

The acceptable value for energy error is below -15% and the ideal value is -5%.

The first column of the output file shows the number of cycles the solver has iterated and at the end of the simulation, the time taken to finish the analysis is printed.

.h3d and Axxx Files

- The Axxx files (where xxx represent number of the animation steps from 001) and .H3D files contain information from all the Axxx files in one single files.

- Both these file types contain the animation of the simulation and can be opened through HyperView.

- The only difference between them is that each file in Axxx contains only 1 time step. To view the entire animation, all Axxx files must be kept in the same location and the first file should be opened through Hyperview, but in the case of an .h3d file, it is a standalone file which can be opened without any other files.

.T01 Files

- These files contain all numerical datas which are required for plotting purposes.

- They can be opened through Hypergraph 2D

Animating in Hyperview :

- As mentioned above, open the .h3d file or .A001 file in Hyperview

- With the Hyperview UI, the animation can be played, paused, the playback speed can be controlled.

- The Hyperview UI enables us to plot the values of stress, displacement etc while also enabling us to activate dynamic max-min which shows the node locaiton where the maximum and minimum of applied contour is.

The plotting can be done using the "Contour" option and enabling and disabling the Dynamic max-min comes under "Measures" Option as shown below :

Plotting in Hypergraph :

- Similarly, as mentioned above, open the .T01 file in Hypergraph 2D

- Select the parameters along x and y axis respectively and click on apply to generate the graph, the UI is shown below

STUDY OF PROVIDED FILE :

File 1 : Bumper File

- The provided Bumper_system.hm file is opened in Hypermesh

- It can be seen that only one half on the bumper system is provided in the file, hence we need to mirror the elements after meshing.

- The provided model contains 8 components. 5 surfaces to define the face of the bumper, 2 to define the connector and 1 -1D component at the connecting face.

- On closer look, it can be noted that the connector surface are not properly defined (As shown below) and have to be edited before meshing

- Clearance is seen between the surface in the Crash_Box_Upper component while Interference is seen in the Crash_Box_Lower Component.

- Since we need to only mesh the provided files, the properties and the materials defined are irrelevant to the assignment and need not be looked into.

File 2 : Crash Box

- The provided Crash box Radioss solver file can be imported in both Hypermesh UI as well as Hypercrash UI.

- For this assignment, only Hypermesh is used.

- The provided file consisits of 5 components (4 parts and 1 Rigid Body), 2 Property cards ,1 material card and a rigid wall.

* Property Cards : There are two P1_Shell Cards identically defined with the only difference being the thickness of the Shell. One card is defined with the thickness of 2mm and the other 3mm.

* Material Card : There is only one material card, defined by M2_Plastic_Johnson Card with the properties of Steel Material

* Parts :

- PSHELL 1 - The green colour part shown in the above UI with 3mm Thickness Card and Steel material Card

- PSHELL 2 - The red colour part shown in the above UI with 3mm Thickness Card and Steel material Card

- pshell_2mm - The blue colour part shown in the above UI with 2mm Thickness Card and Steel material Card

- pshell2_2mm - The yellow colour part shown in the above UI with 2mm Thickness Card and Steel material Card

- RigidBody7 - A rigid body linking the end nodes of the PSHELL 1 (green) and PSHELL 2 (red) parts.

- Notch Design : It can be seen that two of the components have multiple notches on them.

One of the notches is made around all 4 faces while the other notch is only on two opposing faces stretching barely into the other 2 faces.

- By analyzing the control cards, the following datas are noted :

* Total Simulation Time = 30 ms

* Frequency of Animation Writing = 1ms

- From the Load Collectors, it can be noted that :

* A boundary Condition is applied to lock all degree of freedom except the translational motion along z-axis

* An initial velocity of 13.3 mm/ms is applied on the system in negative z direction (Towards the Rigid wall)

- Rigid wall :

The rigid wall is defined as an infinite plane with the slave node search area as 100mm in z direction.

- A seam weld is defined between the components and a Type 7 contact between the parts as well.

- There are no Inital Penetrations in the Part file provided.

SIMULATION :

Simulation 1 : Performing the meshing of provided Bumper System file :

- As mentioned above, once the file is imported the model is checked for free edges and as shown above, there are free edges in the upper and lower crash boxes which must be rectified before meshing.

- The surfaces are modified using surface edit command as shown below :

- Once the surfaces are required, they are automeshed with element size as 6.00 mm (Recommended value).

- After meshing, using the reflect command, the elements are reflected along a defined plane to create the full bumper system model.

- The model is now saved.

Simulation 2 : Performing the Crash Tube analysis for the Provided Cases.

Case {i} : Basic Type 7 Contact

File name : Crush_tube

Changes made to Provided File : There is no need for any change to be done on the Provided file as it satisfies the requirement for the current case.

The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 83,691

Energy Error : -3.8%

Mass Error : 0.0

Simulation Time : 90.38s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

Case {ii} : Type 7 Contact with Modified Inacti Value

File name : Crush_tube_Inacti_modified

Changes made to Provided File :

- The provided file is copied and placed in a new location with file name modified.

- After importing the new file, the value of Inacti of the Type 7 Contact is changed to 6 from 0 as shown below

- The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 83,691

Energy Error : -3.8%

Mass Error : 0.0

Simulation Time : 86.13s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

Case {iii} : Type 7 Contact with Modified Inacti Value Along with Type 11

File name : Crush_tube_Type11

Changes made to Provided File :

- The file from the previous case is copied and placed in a new location with file name changed.

- After importing the new file, an additional Contact interface (Type 11) is defined as shown below :

- The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 97,494

Energy Error : -3.9%

Mass Error : 0.0

Simulation Time : 129.30s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

Case {iv} : Combination of Type 7 and 11 between parts and without any notches

File name : Crush_tube_Without_Notch

Changes made to Provided File :

- The file from the previous case is copied and placed in a new location with file name changed.

- Using Node Edit tool available in Hypermesh the notches on the surface of all components are removed.

- The assembly file without notches is shown below :

- The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 87,194

Energy Error : -3.2%

Mass Error : 0.0

Simulation Time : 108.81s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

Case {v} : Combination of Type 7 and 11 between parts and without A notch created in the middle

File name : Crush_tube_Middle_Notch

Changes made to Provided File :

- The file from the previous case is copied and placed in a new location with file name changed.

- Using Node Edit tool available in Hypermesh a new notch is created on all the components at the middle as shown below :

- The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 91,777

Energy Error : -1.9%

Mass Error : 0.0

Simulation Time : 114.65s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

Case {vi} : Combination of Type 7 and 11 between parts and without A notch created in the middle and an additional notch on two opposing faces.

File name : Crush_tube_2_Notch

Changes made to Provided File :

- The file from the previous case is copied and placed in a new location with file name changed.

- Using Node Edit tool available in Hypermesh a new notch is created above the current notch on the parts such that the notch is only present on two opposing faces, as shown below :

- The simulation is run as shown above.

Output file : Once the simulation has been completed, the output File_Name_0001.out file is opened from explorer using notepad where the necessary values can be noted. (As discussed above). The entire output file can be found in the file attached at the end. The ending of the output file alone which has the parameters to be noted is shown below.

From the above output file, the following Datas are noted down :

Number of Cycles : 93,386

Energy Error : -2.3%

Mass Error : 0.0

Simulation Time : 116.49s

Animation :

The animation obtained through Hyperview is shown below :

Graphs :

The following Graphs are plotted:

1. Time vs Rigid-Wall Forces

2. Time vs Contact Forces

3. Time vs Internal Energy (System)

4. Time vs Internal Energy (For Individual Parts)

5. Time vs Total Energy / Kinetic Energy / Hourglass Energy

COMPARISON REPORT :

- Comparison of Output Files :

Case	No. Of Cycles	Energy Error	Mass Error	Simulation Time	Observation
Case 1	83,691	-3.8%	0.0	90.38s	- Across all the cases, there is no variation in the mass error. - The number of cycles is found directly proportional to the complexity of the input with case 3 being the most complex as the position of notches in this case creates more complexity adding to additional Type 11 contact. - The energy error variation is seen significantly in Case 5 and Case 6 which has a middle notch and an Additional notch respectively. - This shows that the notches when placed strategically help to reduce energy errors as well. - Simulation time is directly propotional to the number of cycles
Case 2	83,691	-3.8%	0.0	86.13s
Case 3	97,494	-3.9%	0.0	129.30s
Case 4	87,194	-3.2%	0.0	108.81s
Case 5	91,777	-1.9%	0.0	114.65s
Case 6	93,386	-2.3%	0.0	116.49s

- Comparison of Simulation :

The Datas were compiled onto a presentation and is attached at the end. The comparison slides are shown below :

Animation Comparison :

Rigid wall Forces Comparison :

Contact Energy Comparison :

Internal Energy Comparison :

Part Internal Energy Comparison :

Other Energies Comparison :

Inference from the Simulation :

- How does the notch affect the results?

It can be clearly seen that the notches play a significant role in the results obtained. The notches are the first to collapse (if present) in the event of a crash and by controlling the positioning of the notches, the Rigid wall forces, contact energy, internal energy flow can be controlled by the designer.

CONCLUSION :

- Thus the mesh for a bumper assembly with mesh size of 6mm was created and a simulation to analyze a Crash tube under different conditions was conducted.

- Almost no differences were noted between Cases 1 and 2 because there were no initial penetrations in the model for the Inacti to have an effect.

All relevant datas are attached in the link shared below :

https://drive.google.com/file/d/1AcRND39a1nrr3rucaD7pEGateGrldvt8/view?usp=sharing