VALIDATE RESULTS NASTRAN AND ABAQUS 2

[Giang123]

Hi all,
Currently, I am validating between MSC NASTRAN software and ABAQUS software. I import a BDF file into ABAQUS; however, the ABAQUS results differ by 19% compared to MSC NASTRAN. So, I'm not sure if there's anything specific to consider when importing a BDF file into ABAQUS. I have attached the BDF file below.

 

[centondollar]

That's an excellent point as the stiffness of some elements can vary from program to program. And good luck on trying to get shape functions and so forth out of the developers of these programs. They guard them like trade secrets (especially the more complicated elements).....which I guess they are.

To my knowledge, a 10 year old version of the abaqus manual (found through google) contains the details of the shape functions used by the program, so I would assume that the newer manuals include it too. MSC Nastran probably also has an extensive technical manual, considering that it is very popular in the US aeronautical industry.

I would not place much faith in a software developer that refuses to reveal element shape functions, since knowing only the shape function is of almost no use to someone who wishes to code a FEM solver.

 

[centondollar]

I posted on the forum because I'm new, and I don't have much time to read the manuals for both software packages.

I would advise not to attempt what you are doing if you are new to FEM and don't have time to figure out the basics of each software package.

Have you been hired to engineer and simultaneously verify the correctness of market-leading software? The latter aspect is more of an academic exercise best left to someone with more experience.

So, I posted to see if experienced users could suggest what I should do. Your suggestions could be very helpful to me.

Your model is quite complicated (e.g., beams offset from shells, which you have not mentioned even though I brought it up) and you state that you "do not have time to read manuals", so I very much doubt that suggestions given here will be helpful.

 

[ThomasH]

My task requires converting from MSC NASTRAN software to ABAQUS, so I'm using a BDF file for importing. The material information is correct and is in millimeter unit system, provided by a company.
I only use static analysis

So part of the task is to convert files. Then I would start with a simpler model so you can check the results better. I would also read the manual for ABAQUS/CAE. I was involved in a project several year ago where som Nastran-files I had made were translated to ABAQUS. The guys that did the translation said that it worked fine. But there were a few things they checked to ensure that it was fine.

Regarding the units:

If you Google "FEMCI Book" and click "Unit consistency" you will get the table you see below. So for consistent unit you need to follow line 2.

Below the table you can see the data from when I import your bdf-file into Femap. Young's modulus should be in MPa and assuming that you are working with steel that means 210 000, not 21 000. If the material isn't steel, my comment is of course wrong.

And for steel the mass is approximately 8000 kg/m^3 (7850 is probably more accurate). And converting that to tonne/mm^3 gives 7.85e-9.

Perhaps you are working with a material that is close to steel in some respects but not in others. But if it is steel I would check the numbers .

 

[Giang123]

Hi ThomasH,
I am using a unit as you described in the table below, and it is not made of steel.

 

[WARose]

I would not place much faith in a software developer that refuses to reveal element shape functions, since knowing only the shape function is of almost no use to someone who wishes to code a FEM solver.

The last time I asked for shape functions was some associated with solid elements in STAAD (some years ago). And they refused (at first by avoiding the question).

But I still remain a big fan of STAAD. (As much as I miss the v8i version.)

 

[ThomasH]

May I ask what type of material it is? It seems a bit odd that it is "softer" than steel and the density is much higher.

 

[Giang123]

Hi all,
After rechecking with the professor, I realized that the issue was related to the unit system.

The problem was that the problem should be using the unit of kgf instead of kN, as I mentioned earlier. When I switched to kgf, the results matched. However, I still encountered the following issue:
In MSC NASTRAN, in the BDF file format, there is a card named ASET1 (it is a reduction to the ASET1 DOFs). When I added it, it eliminated the local panel modes, and the model only displayed global modes (I tested this in MSC NASTRAN by using both ASET1 and constraints, but they are not equivalent). So, when converting to ABAQUS, how should I understand this ASET1?
Best,

 

[Giang123]

Hi all,

I have tested, and ASET is used to reduce the computational effort; however, it seems that ASET1 is different from the constraint. For example:
- With format: ASET1 123 1 34 mean node 1, node 34 has DOFs (Degrees of Freedom) in the translational x, y, z directions but no rotation.
So, is it equivalent to not using ASET1 and instead constraint the rotation in the x, y, z directions?
(I have tested both cases using ASET1 and constraint the rotation in x, y, z, and the results are different)
Do you know why they differ like that? I am currently trying to convert ASET1 from MSC NASTRAN to another software, for example, ABAQUS, and I do not know how ASET1 functions at the moment.

Best,