Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

Status
Not open for further replies.

Francesco Bro

Aerospace
Jan 31, 2021
24
Hello,

We are modeling a FEM model for an aerospace company. This structural system is built of several components, attached via jointed bolts (screws).

What would be the difference of using these techniques for FEM modeling the jointed bolts in NX / MSC Nastran?
1- CFAST elements
2- CBEAM(CBAR)+RBE2 elements

Can both formulations support bolt-preload? I found these bulk cards in the docs (maybe for Technique # 2-). Is this also supported for Technique #1-?
a- BOLTFOR or BOLTFRC (NX Nastran)
b- BOLT (MSC Nastran)


Thank you in advance.

Regards,
Francesco
 
Replies continue below

Recommended for you

I would analyze the screws by hand. I would model the structure so I could easily extract the load that the fasteners experience. There are different ways to do this, depending on how many fasteners you're talking about ... <10, >100.

another day in paradise, or is paradise one day closer ?
 
Hi SWComposites, bolt preload would be important to get pre-stiffening effects in the structural attaching zone, after bolt preloading.
 
Essentially every airplane designed to date has not included bolt preload in the stress analysis.

If you want to investigate preload effects you will need to use a very detailed 3D FEM, and even that is likely to be only an approximation of little practical value.

What exactly are to trying to analyze?
 
If you are modelling individual fasteners and are worried about preload effects, a hugely significant factor is going to be preload scatter ... which will lead to an "infinite" number of models. Of course you could say ...
1) assume all fasteners nominal preload,
2) assume all fasteners minimum preload, or
3) assume all fasteners maximum preload ...
but these are so coarse that the assumption will swamp any significant effect (which is IMHO insignificant).

From a hundred years of designing preloaded bolts, I think we understand how to design safely. And how "sharpening the pencil" will usually equate to maintenance costs (inspection or replacement). Preload is generally only significant in tension joints under fatigue loads.

Now, of course, you could be in a field where money spent on analysis is not viewed as a cost, and extracting the last "psi" (stress optimised) or the last "gram" (weight optimised) from the structure is more important. In that case "fill your boots" ... pick any appraoch and analyze away. Remember you'll need a full ground test program to validate this analysis.

another day in paradise, or is paradise one day closer ?
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor