Comparison of results with base simulation and improved shell element properties using radioss as solver.

AIM: Comparison of results with base simulation and improved shell element properties using radioss as solver.

INTRODUCTION:

Radioss: Radioss is a multidisciplinary finite element solver developed by altair engineering. It can solve both linear and non linear problems. It is a finite element solver using implicit and explicit integration schemes for the solution of engineering problems, from linear statics and linear dynamics to nonlinear transient dynamics and mechanical systems. This multidisciplinary solver enables designers to maximize performance related to durability, NVH, crash, safety, manufacturability, and fluid-structure interaction, in order to bring products to market faster.

RADIOSS’ analysis capabilities for linear and non-linear finite element analysis, sheet metal stamping, and multi-body dynamics are accessible via two input formats.

Hour glass energy: Consider a 4 noded Rectangular shell element. In hypermes this is called as Shell element. They may have any number of Integration points (depending upon full or reduced integration). Stresses and strains at this element are captured by integrating the values got at these integration points.

It is to be remembered that values are more accurate at the exact location of the Integration point and not so accurate as we move away from it.For sake of lesser computational time, we can use S4R element which means Reduced Integration where in the one Integration point is used and it is ideally at the centre of the element.In the case of these Reduced Integration elements, while post processing results, we can see a false or fake deformation mode (zig zag lines). Such a thing is the Hour Glass phenomenon.The above is an example of a Single Integration point shell element. The values captured at the single integration point at the centre remains same as the dotted lines have not changed in magnitude or in angle. However, the element as a whole has shown a change in configuration. This is because elements with lesser integration points are less stiffer and as there is no stiffness in this mode, these elements have shown this kind of change in Configuration.

In actual physical scenario or in Experiments, this does not happen. But in FEA this happens. This is just a nonphysical zero energy mode which leads to excessive element distortions. The more structured and less distorted your elements are, the better the results.

How to control hour glass energy:

1. By inserting artificial thickness (In static/quasi static analysis).
2. By inserting artificial viscosity (For dynamic and high velocity impact problems).
3. By using fully integrated elements.
4. By refining mesh i.e to improve the quality of mesh.

Shell Elements:

Since the degenerated continuum shell element formulation was introduced by Ahmad et al. it has become dominant in commercial Finite Element codes due to its advantage of being independent of any particular shell theory, versatile and cost effective, and applicable in a reliable manner to both thin and thick shells.

In the standard 4-node shell element, full integration and reduced integration schemes have been used to compute the stiffness matrices and force vectors:

• The full integration scheme is often used in programs for static or dynamic problems with implicit time integration. It presents no problem for stability, but sometimes involves "locking" and computations are often more expensive.
• The reduced integration scheme, especially with one-point quadrature (in the mid-surface), is widely used in programs with explicit time integration such as Radioss and other programs applied essentially in crashworthiness studies. These elements dramatically decrease the computation time, and are very competitive if the hourglass modes (which result from the reduced integration scheme) are "well" stabilized.

Overview Of Shell Elements

• Mass of the element is distributed to the nodes.
• Large rotations with large strains with bilinear shape functions.
• 3-noded shell elements and both reduced and fully integrated 4-noded shell elements are available.
• Geometry defined by a curved mid-surface defined with 3 or 4 nodes and a thickness h—usually small compared to the two other dimensions.
• Shells include membrane, bending and shear deformation while the normal stress (through the thickness) is always zero.

Key Parameters:

1. Ishell: Select element formulation for 4-noded shells.
2. Ish3n: Select element formulation for 3-noded shells .
3. N : Choose number of integration points through the thickness.
4. Ismstr: Specify small or large strain formulation .
5. Iplas : Choose method for calculating post-yield strains .
6. Ithick : Determine if thickness is constant under load, or if element thins.

Shell Elements properties we used in this assignments are:

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.

Objective:

1. To run the basic crash beam simulation with default shell properties, basic run time 55ms and animation steps min 25 to maximum 60.
2. To run the basic crash beam simulation with given shell properties basic run time 55ms and animation steps min 25 to maximum 60.
3. Compare the results of both the simulations and Comment whether there is any change is results and why there is change.
4. Learning out comes and conclusion.

Step 1: To run the crash beam simulation with default shell properties:

1. Import the crash beam file (i.e FIRST_RUN_0000.RAD) into the hyper works using import solver deck option in menu bar.

2.Now we need to change the basic run time to 55ms and animation steps min 25 to maximum 60.

Cards>ENG_RUN>Tstop = 55.

Cards> ENG_ANIM_DT>Tstart = 25 >Tfreq = 60.

3. Simulation Run:

• In order to start the simulation we need to get into analysis>Radioss from panels.
• Then we need to save the file in an specified location and set the options to –np 4 or –np 6 depending on the cores on computer and the click on radioss option to get the output file(i.e 1RAD FILE).
• Since we got the output file we now run the simulation by importing the out file i.e .h3dfile into the HyperView.
• Now in hyperview we can run the simulation and also we can view how the changes happens in stresses , displacements etc. using contour option.
• The following are images while running simulation.

• Now we need to check energy and mass errors to verify our simulation. The energy error should not be greater than -15% and mass error should not exceed 5%.Now we need to check in output file where our mass error is 0.1659E-03 and energy error is -10.3%,which are acceptable.

• Now we need to plot the graphs for our simulations using hypergraph 2d.for that we need to open hypergraph and click on build plots then open , T01 file from the browser .

graph1: Internal energy ,hour glass energy ,total energy vs time plot.

graph2: Contact energy vs time plot.

graph3: resultant forces vs time plot.

Step 2: To run the crash beam simulation with recommended shell properties:

1. Import the crash beam file (i.e FIRST_RUN_0000.RAD) into the hyper works using import solver deck option in menu bar.
2. Now we need to change the basic run time to 55ms and animation steps min 25 to maximum 60.

Cards>ENG_RUN>Tstop = 55.

Cards> ENG_ANIM_DT>Tstart = 25 >Tfreq = 60.

3. Now go to property id and change the recommended shell properties as :

Ishell = 24, Ismstr = 2, Ish3n = 2, N = 5, Ithick = 1, Iplas = 1

4. Simulation Run:

• In order to start the simulation we need to get into analysis>Radioss from panels.
• Then we need to save the file in an specified location and set the options to –np 4 or –np 6 depending on the cores on computer and the click on radioss option to get the output file(i.e 1RAD FILE).
• Since we got the output file we now run the simulation by importing the out file i.e .h3dfile into the HyperView.
• Now in hyperview we can run the simulation and also we can view how the changes happens in stresses , displacements etc. using contour option.
• The following are images while running simulation.

• Now we need to check energy and mass errors to verify our simulation. The energy error should not be greater than -15% and mass error should not exceed 5%.Now we need to check in output file where our mass error is 0.1659E-03 and energy error is -0.1%, which are acceptable.

• Now we need to plot the graphs for our simulations using hypergraph 2d.for that we need to open hypergraph and click on build plots then open , T01 file from the browser .

graph1: Internal energy ,hour glass energy ,kinetic energy vs time plot.

graph2: Contact energy vs time plot.

graph3: resultant forces vs time plot.

Step 3: Comparison and commenting of results of both simulation:

Property	Case 1	Case 1	Comment
Displacement	The displacement observed is 761mm.	The displacement observed is 777mm.	The displacement in case 2 increases because there is change in Ismstr =2 as a result strain increases.
Von mises stress	Max stress observed is 346.3Mpa.	Max stress observed is 348.9Mpa.	There is a very small difference.
Hour glass energy	It increases gradually with time.	It stays at zero.	It is because of QEPH shell formulation.
Internal energy	It increases gradually with time.	It increases gradually with time.	Stays same in both cases because there is no change in material properties.
Kinetic energy	It decreases from 6000000J to 3500000J.	It decreases from 6000000J to 4000000J.	There is change in k.e at the end of simulation because of there is change in energy errors.
Energy error	-10.3%	0.1%	There is change in energy error because of QEPH shell formulation.
Mass error	-1.65	-1.65	It is almost same.

Learning outcomes:

• In this assignment we have learnt how to run the basic simulation using Hyperview.
• We learn how to change the shell formulation properties and hour glass effect.
• We also learnt how check mass and energy errors from the 0001_OUT.RAD FILE.
• We learn how to plot the various graphs using Hypergraph 2d.

Conclusion: We successfully completed the basic simulation run by using default shell properties and recommended shell properties.

LINK FOR MODEL FILES:

https://drive.google.com/open?id=1DfChZfs8wjVFiaAs4yZI--rDUx01T-iF

Conclusion: We successfully completed the basic simulation run by using default shell properties and recommended shell properties.