Does AutoDesk NASTRAN support linear GAP elements?

[Anonymous]

This is Sam from an Aerospace company (confidential) in Montreal. I have an urgent question about an FEM situation that I am encountering in my work. In our company, we use FEMAP/AutodeskNASTRAN (formerly NEINASTRAN) for our FE analysis, while our principal client [XYZ] uses PATRAN/MSC NASTRAN. We have a special case here in which XYZ has demanded us to send them a particular mesh, built in FEMAP, which then they have added to their existing mesh of several monuments (interior structures) on their side using PATRAN, and sent us back their PATRAN-generated MSC NASTRAN file, for us to analyse. They sent it to us as a .bdf. Unfortunately, this file cannot be run by me (linear static runs) on AutodeskNASTRAN, either directly, or by importing into FEMAP and then generating an AutodeskNASTRAN .nas file. Autodesk-N is trying to run it but then later giving me a fatal error. XYZ has inserted a large number of LINEAR GAP elements on their side. PATRAN/MSC NASTRAN is okay with running linear GAPS, so does NX NASTRAN. My question is, are linear GAPs supported in AutodeskNASTRAN? Or are they the main cause of the failure to run the analyses? If they are not supported, is there an easy alternative in AutodeskNASTRAN? Thanks in advance, - S. Ranjan, Canada.

[John_Holtz]

Hi @Anonymous

I have a few questions to try to understand your situation.

1. What is the fatal error message?
2. How are you running the model that gives the error? I'm not clear whether you are running the file from the customer, or you imported somehow, or what you did.
3. Is the element type "GAP" in the data file from the customer, or something else? If something else, what is the type?

[Anonymous]

Dear Mr. Holtz,

First of all thanks for replying to my question. I am trying to run the model as a .bdf which was directly sent to me by the client. The client has put in linear GAP elements into the file. The linear GAPS are in a separate file, and are called in the main file by command "INCLUDE 'linear_gaps.bdf'". The file "linear_gaps.bdf" actually does not have any CGAP elements. It merely lists some commands like
param,cditer,50
param,cdprt,yes
MPCADD 1000000.....
spoint.....
suport.....
spc....
mpc....

Then when I try to run, it seemingly starts running, giving me lots of warning, then abruptly gives me a fatal error.

FATAL ERROR E5004: STIFFNESS MATRIX SINGULAR OR NON-POSITIVE DEFINITE

Important warnings are:

WARNING T2004: UNRECOGNIZED BULK DATA ENTRY
PARAM CDITER 50

WARNING T2004: UNRECOGNIZED BULK DATA ENTRY
PARAM CDPRT YES

WARNING T2004: UNRECOGNIZED BULK DATA ENTRY
PARAM PRTMAXIM YES

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

WARNING E5051: EXCESSIVE SOLUTION ERROR MEASURE DETECTED

The solution is linear static.

Thanks in advance,

Sam

[John_Holtz · Accepted solution]

Hi Sam,

So it looks like they are bonding the components together using MPC equations. As far as I know, the MPC equations work in "tension" and "compression" which is different than what I think of when talking about "gap" elements (that only work in compression).

I think the key questions are these:

1. The E5004 error indicates the model is not statically stable, or there is a numerical problem in the stiffness matrix. Is the problem caused by the MPCADD statements? If the model should be statically stable without adding in the MPCs from the BDF file (or if you can add some dummy constraints to make the model stable), try running the analysis without the BDF file. This will help indicate if the problem is with the BDF file or something else the customer added to the model.
2. As indicated, the CDITER, CDPRT, and PRTMAXIM commands do not exist in Autodesk Nastran. What do those commands do? (That's a question for your customer.) Is there an equivalent in Autodesk Nastran?

[Anonymous]

Dear Mr Holtz,

The unsupported commands create the linear GAPs. So it means the linear GAPs are not supported in Autodesk Nastran. Thanks a lot for your answer.

Sam