Assignment 3-2D Element Formulation Challenge

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:

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.

Solution:

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)

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.

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.

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	o 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.

CASE 1:--

WITHOUT ASSIGNING THE RECOMMENDED PARAMETER IN THE PROPERTY

Steps:

Import the model in RADIOSS user interface

Import the file as SOLVER DECK

click import.

assign the value of time step in ENG_RUN cars as 55ms as per question

i given as 55ms as per the qustion

Tfrequency= TIME STEP/NUMBER OF ANIMATION STEP = 55/55 = 1

Now we will run the simulation ANALYSIS--RADIOSSfrom the bottom analysis tab then click Radios that will show as the job is compleated

Result Opened in Notepad

Now click on RESULT Shown in previous tab, it will directly take you to hyperview where we can see the animation of the simulation. Animation file formate is in name of .h3d

In the RUN OPTION panel it is recommended to write -nt 8. not stands for number thread it assigned to do the simulation at faster rate.

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

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:

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.

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.

Observations:-

The simulation findings demonstrate that Total Energy does not decrease when recommended attributes are modified from the base values, as evidenced by the energy error, which was formerly -10% and is now -0.1%. Throughout the experiment, the growing hourglass energy decreased to zero. In both circumstances, the characteristics of contact energy are the same. Both situations include internal energy that gradually increases in characteristics. Additionally, rigid wall forces fell to a lesser extent than their default rigid wall force value. With recommended property values, there is a modest increase in the maximum displacement of the element, according to contour plots.