Analysis and Simulation of Crush Beam on Rigid Wall with Base and Modified Properties

Objective:

1. To run the base simulations with the specified run time and animation frequency.
2. To change the properties of the model according to the specifications and run the simulations.

Following questions activities to be performed:

Comparison of results with base simulation and improved shell element properties

1. Using the crash beam file from the previous assignment, change the run time to 55 ms.
2. Change the number of animation steps during simulation to a minimum of 25 and maximum of 60.
3. Run the base simulation without any modification to element properties.
4. At the end of the simulation, do the energy error and mass error checks and determine whether results would be acceptable.
5. If acceptable, Plot rigid wall forces, internal energy, hourglass energy, contact energy, total energy of the simulations.
6. Change all the shell elements properties to recommended properties, save this as separate, rad files and run the simulation.
7. Follow steps 5 and 6.
8. Create a brief report comparing all the results from steps 5 and 6 for both simulations.
9. Comment whether there is any change in the results and why there is any change in the result.

Apart from this, for the second activity of simulation, following properties were provided:

Parameters		Comment
Ishell =24		QEPH 4 nodes shells are best combination of cost and accuracy.
Ismstr=2		Full geometric nonlinearities (default) with possible small strain formulation activation in RADIOSS Engine.
Ish3n=2		Standard 3 noded shell (C0)with modification for large rotation(default)
N=5		Number of integration points set to 5 for accuracy bending.
Ithick=1		Thickness change is taken into account for accuracy.
Iplas=1		Iterative plasticity for good accuracy.

Introduction:

• The crush beam provided in this assignment is having a hat-section connected to the base rail via spot welds of spring type.
• Velocity was given at the end of the beam on a rigid link. A rigid wall was attached in front of the beam on which the beam was crushing.
• The image below shows the different properties of 2D shell elements present in HyperMesh.
  
  
• From the table the ‘QEPH’ element for 4 nodes is most suitable among the others since it is improved under integration, though having only one integration point. This also results in reduced cost of computing.
• Similarly, for the 3 noded elements, CO tria element property is most suitable.
• Hourglass is the zero energy deformation modes. Simply, hourglass phenomenon occur when node is moving but not recording any stress. This is controlled by applying force on the node either during analysis or during the material level.

Procedure:

Case 1 - Base Simulation:

• The starter file provided in the challenge ‘Element_formulation_shell-3_assignment_0000.rad’ was imported in the HyperMesh, using the ‘import solver deck’ option. This imported the crush beam model as seen below.
  
  
• The ‘Cards’ entity was expanded and in the ‘ENG_RUN’ card, the ‘T_stop’ (termination time) was changed from 60.0 to 55 ms as mentioned in the question.
• Considering the animation steps as 50, from the equation,Tfreq=termination time/animation step, the T_freq was calculated as 1.1 ms. This value of T_freq was added in the ‘ENG_ANIM_DT’ card. The image below shows both the T_stop and T_freq .
  
  
  
  
• All the other properties of the model were kept constant and the model was analyzed using the ‘Radioss’ option from the ‘Analysis’ tab. In ‘options’ menu, the value was entered as ‘-nt 4’ which represents the number of processors for the simulation.
• This created the ‘.out’ files, modified engine files, and the animation files at the location that was specified during the operation. A combination file of all the animation files of ‘.h3d’ format was created which had the main animation.
• From the toolbar, the HyperMesh module was changed to ‘HyperView’. This opened the ‘HyperView’ window where the animation can be visualized, and other analyses can be done. Using the ‘import’ option, the main animation file of ‘.h3d’ was selected and the ‘apply’ button was clicked.
• This imported the base simulation file. The ‘rigid wall’ component was hidden and by using the ‘play’ button the animation was played. By using the ‘contour’ option, different types of stresses, displacements, and energies causing the deformation were analyzed from their respective animations. The animation below shows the deformation and the Von-Misses Stress along the model.
  
  
• From the animation, we can easily see that a kink is being formed due to high deformation in the central area of the beam where large amount of stress are being generated.
• Animations for the other results were seen similarly and the observations were:

Maximum Von-Misses Stress=3.499e² N/mm²

Maximum Hourglass Energy=2.234e⁴ Nmm

Maximum Mass Change=7.844e^-1 gm

Maximum Plastic Strain=7.645e^-1

Maximum Specific Energy=3.337e⁴ Nmm

• Now, the energy and mass errors had to be checked to make sure they are within the acceptable range. For this, from the folder where all the files of the simulation were stored, the filw with 0001.out extension was opened with notepad.
• Here the energy and mass errors computed during the simulation were stored for each cycle. The value of the errors at the end of the simulation i.e. at the 54.99^th ms was checked. Energy Error was -10.3% which was well inside the limit of -15%, while the mass error was 1659e^-3 gm. Therefore these values were acceptable and hence the different graphs could be plotted.
  
  
• Later for the graphical analysis, the screen or the window was split into two from the ‘page window layout’ option from the toolbar. In the second window, ‘HyperGraph 2D’ module was opened by changing the ‘HyperView’ to ‘HyperGraph 2D’.
• This opened a graphical area having X and Y axis. Here, from the import option, the ‘T01’ file was selected and applied. The type of analysis was selected as ‘Rigid wall forces’ and in that sub type was set as ‘NF- Resultant Normal Force’ and ‘TF- Resultant Tangential Force’ and all the settings were applied.
• Two graphs of both the analysis types were plotted on the second screen. The Y- axis was showing the rigid wall force and the X- axis was representing time in ms. The normal force started beyond 100000 N while the tangential force started from origin. The following image shows the graphs.
  
  
• Apart from this, 4 more type of analyses had to be plotted. For this, both the windows were cleared and a new session was started. In the first window, the ‘internal energy’ was plotted from the ‘global variables’ list. And in the remaining windows, ‘hourglass energy’, ‘contact energy’ and ‘total energy’ was plotted respectively. The image below shows all the four graphs.
  
  
• As seen from the graphs, the internal energy keeps on increasing as the body keeps on deforming. While the total energy decreases over the period of time. The hourglass energy is also increasing slightly but, on the other hand contact energy is 0.

Case 2 – Modified Properties:

• For this case, the model remains same just the properties of the model and its material card have to be changed. The properties to be changed are mentioned above in the objective.
• The model is imported in the similar manner using the ‘import solver deck’ option. Upon importing all the values are changed from the ‘property’ tab for each property.
• The following image shows the changed properties of the model.
  
  
• After changing all the properties, the T_stop and T_freq values are also changed just like the previous case. The T_stop and T_freq are kept as 55ms and 1.1ms respectively.
• ‘Radioss’ operation is done on the model from the ‘analysis’ tab entering the ‘options’ value as –nt 4. This created many other files in the destination folder path specified.
• Later, ‘HyperView’ was opened similar to the previous operation and the ‘.h3d’ file was imported.
• Upon playing the animation, it was quite evident that the deformation of this model is not similar to that of the previous one since the property card and other properties have been changed. The deformation was abrupt at the end and was quite more than before.
• From the ‘contours’ option, the ‘Von-Misses stress’ was selected and applied. The averaging method was set on ‘simple’. The gif below shows the animation for the Von-Misses stress
  
  
• As seen, large amount of stresses are being developed at the kinks. The maximum amount of Von-Misses stress developed in the body was 491e² N/mm².
• Similarly, the animations for all the other types of analyses can be seen by selecting that specific analysis from the ‘contour’ option.
• Maximum Hourglass energy=0

Maximum mass change=7.844e^-1gm

Maximum Plastic Strain=1.140e⁰

Maximum Specific Energy=5.144e⁴ Nmm.

• The energy and the mass errors were checked for this case also by following the previous steps i.e. by opening the 0001.out file format in the notepad.
• The maximum amount of energy error recorded at the end of the analysis at the 54.99^th was -0.1% which is way below the boundary value of 15%. This was due to the QEPH type element selected in the material card while the mass error remained same i.e. 0.1659e^-3 gm. The image below shows both the errors from the 0001.out file of the model.
  
  
• Now, the screen was again split into two parts, and by opening the ‘HyperGraph 2D’ the graph for ‘rigid wall forces’ was plot. The two graphs were of ‘NF-Resultant Normal Force’ and ‘TF- Resultant Tangential force’.
• The image below shows the graph and the simulation of the Von-Misses stress.
  
  
• From the graphs, it can be easily seen that the normal forces and the tangential forces are slightly different from the previous case. Apart from this, 4 more graphs were plotted in the similar manner, by starting a new session.
• Graphs for internal energy, hourglass energy, contact energy, and the total energy were selected.
  
  
• Here, unlike the Case 1, the total energy is constant over the period of time, and the hourglass energy remains 0. While there is slight change in Internal energy.
• The reason for the hourglass energy to become 0 is the type of material card property selected. In the 2^nd case, the value of Ishell is selected as 24 which is QEPH element type. In this type, the hourglass stabilization is at the material level and hence becomes 0.

Results and Conclusion:

• If the results of both the Case1 and Case2 are compared, it can easily be figured out that there is a big difference in the values of hourglass energy, plastic strain, and maximum specific energy.
• Also, the amount and nature of deformation of the 2^nd case is much higher and different than the first case. This deformation is more practical than the base simulation. All of these changes are due to the material card and its element properties.
• The QEPH elements (Ishell=24) are improved under integration, despite of having single integration point at the center only, and thus their relative cost as compared to the other types is low.
• Therefore, the /DEF elements set is much more practical in real life. The assignment was thus completed successfully along with the analysis of the model.

GDrive link for files : Base and Modded Sim files