Assignment 3-2D Element Formulation Challenge

AIM:

1) Run the simulation with the default property card (CASE 1).

OBJECTIVE:

Comparison of results with base simulation and improved shell element properties.

1.Using the crash beam file provided, 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.

PREREQUISITE:

ELEMENT FORMULATION ..

1D -> Includes SPRING, TRUSSES, BEAMS.

2D -> Includes shell elements like QUADS, TRIAS.

3D -> Includes solid elements such as HEXA, TETRA elements.

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 ->

METHODS TO CONTROL THE HOURGLASS ->

1. Perturbation method (viscous method).
2. Physical stabilization method (stiffness method).

1. Perturbation method(Viscous method) ->

• This method is generally used for the elements with Ishell: 1,2,3,4 or Q4 elements.

• The process how this method work is: When the hourglass deformation occurs---> There will be some damping applied, which is stabilizes the deformation, which results in controlling the speed of deformation.

• But this damping opposes only the velocity, But not the force. So, the force is introduced, which results in adding some stiffness to the elements. This stiffness helps in controlling the hourglass.

• The force is applied externally, by some other means. But not from FEA nature. This force constitutes energy called "Hourglass energy", Which is artificial energy (neither Kinetic nor internal energies). It is a loss of both energy types.

• If the hourglass energy is added along with the kinetic energy & Internal energy, A balance system is obtained.

• Recommended for high velocity or high strain rate problems. Example: Involving high explosives.

• A huge amount of hourglass energy is generated.

2.Physical stabilization method (stiffness method) ->

• In this method, the stiffness is getting added right from the material level. This will have a large effect on our component.

• It is recommended for fewer velocity problems.
• used for QEPH elements.
• Less amount of hourglass energy is needed. Because it is going to be controlled before it arises.

> Types of Reduced and full integrated 3 & 4 noded shell elements. [ 2D Element Formulation.] ->

Q4 element:

• It is an under(or) reduced-integrated element. Because it has only 1 integration point for an element.
• It is represented in the radioss solver by an entity named "Ishell (1,2,3,4)".
• Usually Ishell: 1, Ishell: 4 elements are used, when no more velocities are to be found.
• By default, the solver takes as Ishell: 1.
• As it has only 1 integration point, the time required to solve the problem with this type of element is relatively less.
• The cost required to solve this type of element is relatively low.
• But there will be no accuracy in the results.

QBAT element:

• It is a fully integrated element, which means it comprises 4 integrated points. Each point is near to each node.
• It is represented in the radioss solver by an entity named "Ishell: 12".
• Because of these 4 Gaussian integration points, there will be no Hourglass deformation.
• Using this element is not economical. But the achieved results are more accurate than the remaining Ishell elements of quads.

QEPH element:

• It is an improved under-integrated element with physical hourglass stabilization within itself.
• It is represented in the radioss solver by an entity named "Ishell: 24".
• This type of element is mostly used recently. Because of its best compromise between the cost, quality& accuracy in results.
• It is widely used in the automotive industry.

CO element:

• It is a standard co-rotational triangle, with modification for large rotations.
• Usually, it helps in representing the tria elements with the entity named "Ish3n: 2".
• It consists of 1 integration point.
• No hourglass deformation.
• Low CPU cost.
• Results may not be accurate.

Key parameters in /PROP/SHELL ->

Ishell -> To select the element formulation for 4-noded shell elements.
Ish3n -> To select the element formulation for 3-noded shell elements.
N -> To choose the number of integration points through the thickness.
Ismstr -> To specify whether the strain formulation is small or large.
Iplas -> To choose method for the calculation of post-yield strain.
Ithick -> To determine if the thickness is constant under load or if the element thins.

PROCEDURE:

CASE 1: WITHOUT ASSIGNING THE RECOMMENDED PARAMETER IN THE PROPERTY.

1) Import the model in RADIOSS user interface.

2) Import the file as SOLVER DECK.

3) Now next step is to assign the value of time step in ENG_RUN cards as 55ms as per question.

4) Now we have assigned the number of animation steps. I choose 55ms as number of animation step as per the question.

‘Tfrequency (Tfreq)’ = TIME STEP / NUMBER OF ANIMATION STEP = 55 / 55 = 1.

5) In the property card every parameter property is by default values shown below.

6) Now we will run the simulation, go to ANALYSIS > RADIOSS.

7) In the RUN OPTION panel, it is recommended to write "-nt 4". not stands for number thread it assigned to do the simulation at faster rate.

[Note- choose a seperate folder save as 0000.rad file for store all outputs.]

8) Click RADIOSS and it will show JOB IS COMPLETED in Altair Compute Console Solver View. (CASE_1)

9) Now click on RESULT, it will directly take you to hyperview where we can see the animation of the simulation. Animation file are in name of h3d.

10) Click apply.

11) Now switch to contour mode.

// Click on Contour option you will see the display tab as follows,

// Change the result type as Velocity / Hourglass Energy / Mass Change / Plastic Strain / Specific Energy / Von Misses then the element failure can be seen as followed,

12) After switching choose the RESULT TYPE to see the deformation. I choose VON MISSES / Hourglass Energy and click apply.

> Animation for Von Mises (s)

> Animation for Hourglass Energy (s)

SAFE TOLERANCE IS '-15%' TO '+5%'

14) Graphs for plotted for this various energy. For plotting graph, we have to switch to hypergraph tab and import file T01 which is stands for time history….

[Note- When apply select Units Profile accordingly.]

> Global Variables_ Internal Energy (along with Kinetic / Contact / Hourglass / TE total Energy) [CASE_1]

> Global Variables_ Contact energy - MAG.[CASE_1]

> 4 RIGID WALL 1 - total resultant force [CASE_1]

CONCLUSIONs:

Thus, the simulation has been carried out with both the default element property and with recommended element property and its results like global variables, rigid wall force, hourglass energy, energy errors etc. were compared.