Assignment 3-2D Element Formulation Challenge

REPORT

Questions

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.

Aim

1) Run the simulation with the default property card

2) Run the simulation by changing the property card

3) Comparing both the cases

THEORY

The following properties are changed for case 2.

IshellIshell: This is the element formulation for 4-noded shells

IsmstrIsmstr: This property is to specify small or large strain formulation

Ish3nIsh3n: This is the element formulation for 3-noded shells

N: The number of integration points through the thickness

IplasIplas: Method for calculating post-yield strains

IthickIthick: Property that decides if the thickness is constant or to consider if element thins.

Hourglass:

"Hourglass is a deformation, that is noticed when an element is deformed due to the applied load. But the strain energy due to this deformation is not recorded for that element". It is also known as "Zero energy deformation modes" where an element is seen deformed.

• It indicates instability.
• It is also said to be as, non-physical mode of deformation, that occurs in reduced or under integrated elements & produces no stress.
• Usually, it can be seen in the post-processing stage, where the elements will have zig-zag formation.

BEFORE HOURGLASS EFFECT

 AFTER HOURGLASS EFFECT

How to reduce hourglass?

There are two ways to control hourglass formation. One is via Perturbation method and the other is through Physical Stabilization. It is important to deploy the right method based on the conditions of the simulation.

Perturbation is deployed by Q4 elements (IshellIshell= 1, 2, 3, 4) and involves introducing an opposing force to stabilize whichever node it detects potential hourglass formations. This opposing force produces energy and that's known as 'hourglass energy'. As mentioned before, this method fails for large rotation and elastic loading-unloading cycles. In addition to that, it uses a simple under integrated method and this takes lesser time and computational power but that comes at the cost of accuracy.

This is why the Physical Stabilization method is the best method since it can be used in any case. This method is deployed by QEPH elements (IshellIshell = 24) and involves introducing an artificial stiffness. As a result, shell elements are controlled from a material level and in addition to that, they use an improved under-integrated method that provides the best compromise between computational cost and accuracy.

PROCEDURE

CASE 1:--

                  WITHOUT ASSIGNING THE RECOMMENDED PARAMETER IN THE PROPERTY

1. We shall be changing a couple of simulation properties by accessing them through Hyperworks. This first question requires us to change the run time to 55 ms. For that, we first need to import the solver deck file in question to RADIOSS.
   
   For that, we need to go to File > Import > Solver Deck. This brings up the import menu. Here, we can select the file we need to import. The file would be the starter input deckfile (which is the file that ends with '0000.rad').
   

With the model imported, we can go ahead and change the run time to 55 ms (as required). To do that, we need to proceed to the solver tab in the sidebar. Proceeding to ENGINE_KEYWORDS > RUN > ENG_RUN, we can now edit properties of ENG_RUN in the small window below the tab.

Here, we can change the TstopTstop value to 55.

2. The next question requires us to change the number of simulation steps to a minimum of 25 and a maximum of 60. For this, we need to go to the same solver tab and proceed to ENGINE_KEYWORDS > RUN >ANIM > ANIM/DT. We shall be changing this particular attribute's properties.
   
   
   
   In the property section, we can change the Tstart value to 0 and to accommodate a maximum of 55animation steps, Tfreq can be set at 1.
3. Next requirement is to run the base simulation. For that, we need to save the current file with edited parameters. This can be carried out via the analysis process.

For this, we can go to the 'Analysis' section from the bottom panel.

There, we can select RADIOSS.

In the next section, we are given the option to save the file before running it in the solver. We can go ahead and give it a different name from the base file to avoid confusion. Clicking 'save as' lets us do that.

After saving, before running it into the solver, we can activate 'include connectors' and in the options bar, we need to enter "-nt 4" to increase the processing power of the solver (as in to use all 4 cores). After that, we can click 'Radioss' to activate the solver and it processes t the file. We get a solver window that tells us what's going on in the process. It runs for several seconds before finishing and letting us know about completion.

The simulation is complete, now we just need to visualize it. For that, we need to switch to the Hyperview utility. This can be done by clicking the client selector option in the toolbar (might need to be activated from View > Toolbars > Hyperworks > Client Selector). In that menu, we can select Hyperview to switch to it.



After accessing Hyperview, we are asked to input the simulation file. For this, we shall be using the h3d file that was generated when RADIOSS processed the starter file. It should be generated in the same folder. After selecting it, we can click 'apply'.

Doing so generates the simulation animation. We can change what is being represented by selecting the 'contour' option in the toolbar.

The following animation depicts Von Mises stress in the component during the simulation:

4. Next step is to carry out energy error and mass error checks and this is done by analyzing the RADIOSS engine output file. This can be accessed from the same directory as the starter and engine files and is denoted by the '.out' extension. We need to check the file that contains '_0001.out' by using any text editor.

On opening the file and scrolling down to the end, we can see the final energy and mass error values.

The final energy error is -10.3%, which is less than the threshold of -15% and is acceptable. Acceptable energy error values are supposed to be between -15% and 10%. The closer to 0 it is, the better.

5. With the energy and mass errors deemed acceptable, we can go ahead and plot the graphs. For this, we shall be using the Hypergraph client. We can switch to it using the same client selector option in the toolbar.
6. 

In the Hypergraph section, we are met with a graph and in the panel below, we are to input a file. For this, we will be using the result file generated by the RADIOSS solver (also known as the 'T01 file'). This file is available in the same directory as the starter, engine and animation files. The file name ends with 'T01'.

After adding it, we can select the variables that would go with time (x axis).

First, we can take a look at the energy graphs (internal energy, hourglass energy, contact energy, kinetic energy and total energy)

Now to generate the graph for rigid wall forces (Rigid Wall/Rwall Force - 4 RIGID WALL 1 - total resultant force).

 

Case:2

            Changing the shell elements properties to recommended properties

The shell elements properties are modified to recommended property values:

Running the simulation with the newly defined recommended properties using Radioss option under Analysis tab.Create a folder to save the rad files in local directory.

The energy error starts from 0% to -0.10% which is within acceptable range.Mass error stays constant throughout the simulation at 0.1664e-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-DFile named Element_Formulation-Shell-3_assignmentT01 is opened.Specified energies are plotted against time on the same plot as shown below:

 

From the plot obtained for simulation after 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.

Total energy remains same throughout the simulation.

Hourglass energy and contact energy remains zero and follows the origin throughout the simulation. A new plot is created to plot the rigid wall forces.

Total resultant force and Resultant normal rigid wall forces are plotted against time as shown above. The rigid wall force curve is of a non-linear behaviour, which is due to the variation in the force exerted by the crush tube on the wall throughout the simulation.

Conclution

Two simulations were carried out on the same crush beam using different shell element formulation properties.

Comparing the results of both cases, it can easily be figured out that there is a big difference in the values of hourglass energy, internal energy and rigid wall forces.

Also, the amount and nature of deformation of the 2^nd case are much higher and different than the first case. This deformation is more practical than the base simulation. This improvement is definitely due to changes in the material card and its element properties.

It was established that using the recommended settings helped reduced hourglass formation drastically due to the introduction of the physical stabilization method that is deployed by QEPH elements.