Assignment 3-2D Element Formulation Challenge

AIM

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.

Recommended properties:

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.

MODEL

PROCEDURE

• Import the model in hypermesh software by using import solverdeck tool.
  
• Here we are importing starter file and engine file.
• Change run time to 55ms and  in model tree, under CARDS collector, ENG_RUN card have a Tstop of 60, which is changed to 55.
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• Change the  animation steps to 25 in similar way that above mentioned.
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• The base simulation is run without any modification to element properties using Radioss option under Analysis tab.(Analysis>radios)
• Create a folder to save the rad files in local directory and edit options providing number of threads as " -nt 4 ".
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• After analysis, the .OUT file is viewed to check whether the Energy error and mass error fall within the aceptable range
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• The energy error starts from 0% to 10% which is within acceptable range. Mass error stays constant throughout the simulation at 0.1659e-03 which is also within the acceptable range.
• Since the error values are acceptable,  rigid wall forces, internal energy, hourglass energy, contact energy, total energy of the simulation are plotted with the help of hypergraph 2-D.
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• Hyperview is opened to view the simulation (deformation of the crush tube). model and results (h3d file) are loaded from the newly created folder
• Animation shows how the crush tube deformes (displacement)under the action of force.
• To plot the specific graphs, hyper graph 2-d is selected.
• File named Element_Formulation-Shell-3_assignmentT01 is opened.
• Specified energies are plotted against time on the same plot as shown below:
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• From the plot obtained for base simulation without any modification to element properties, following observations are made
• Internal energy increases throughout the simulation which is due to the force developed on tube due to deformation.
• There is a slight increase in hourglass energy throughout the simulation.
• Total energy of the system has a slight decrease from the start to end of the simulation.
• Contour plot of Displacement is obtained using contour option under hyperview.
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• Do the same procedure by giving property card as per given in the question as below.
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• When recomended properties are changed from the base values, simulation results shows that

  1. Total Energy remains constant rather than decreasing which is clear from the energy error which went till-10% now changed to -0.1%.
  2. Hourglass energy which was increasing changed to zero throughout the simulation.
  3. Contact energy characteristics are same in both cases.
  4. Internal energy is having a gradual increasing characateristics in both the cases.
  5. Rigid wall forces also decreased to a lower value than that of default value rigid wall force value.
  6. Contour plots shows that there is a slight increase in maximum displacement of element with recommended property values.

CONCLUSION

Hourglass energy

• In FEA, the hourglass mode is also known as the "zero energy mode" because deformation energy of elements under hourglassing mode is zero. But the hourglass energy is not equal to zero.
• In a shell element which have a number of integration points, for reduced computation effort, a reduced integration which uses a single integration point is used. In such cases, the deformation mode from the analysis results will be false. This is called hourglass phenomenon. 
• This is just a non physical zero energy mode which leads to excessive element distortions. The more structured and less distorted your elements are, the better the results.