Not apply bolt preload, is it conservative? 1

[marulanda]

Hi, I've a question for you.
Is in FEM static analysis conservative not to apply a preload on the bolts?
I have heard many different opinions on this argument, so now I have confused ideas.
Someone can help me, please.

Regards.

 

[SWComposites]

It is a completely unnecessary complication. Fasteners should be modelled as simple springs to get joint loads, for use in hand joint strength checks.

 

[NRP99]

marulanda
Are you modelling bolted connection with bolt, nut and other clamping components?

If yes then it is required to apply preload in the bolts to simulate initial condition of the bolted connection where initial condition means bolts are in tension and holding together the desired components which are in compression under bolt preload before applying any load. Modelling of bolts is recommended if you are interested to see overall behaviour of the connection.

If no then you can apply bolt preload/clamping force directly on the clamped components along with other loads. This is required if you are not interested in bolt connection behaviour.

No confusion. As simple as that.

 

[marulanda]

I have to verify a steel structure for railway. The bolts are modeled with bar which are joined to plates with RBE.

Actually the only way to apply a bolt preload is apply an appropriate temperature load to the bar elements. In this case I have to run a multi-step analysis:
1 setp applay bolt preload
2 step apply the load to verify

Now, as SWComposites said, I think that the first step with the bolt preload is an unnecessary complication, and the the verification of the bolts with the hand checks is good.

But I'm not sure, because with the second step only there aren't the stresses in the plates induced by bolt preload.
So the question is:
This stress induced by bolt preload is primary to verify the plates considering the von mises stress coming from the fem analiysis or not?

for NRP99
What do you mean with "bolt preload/clamping force directly on.." ?

Thanks all,
regards

 

[NRP99]

marulanda

Bolt preload/clamping force directly on... means apply the clamping compressive force which must be equal to bolt preload to the components which are being clamped. Example- for 2 plates clamped with 1 bolt connection under plate in-plane loads can be analysed by following ways
1) Modelling bolt, nut and plate and applying bolt preload first step and then preload + in-plane loads second step
2) Modelling plates only with in-plane loading along with bolt preload as compressive force on plate in one step only.

 

[Mustaine3]

Without knowing the purpose of the simulation it is difficult to to say what is necessary.

Another limitation is the software. If the tool is not able to apply bolt load, maintain a resulting shortening in the bolt in the next steps and is generally unable to transfer the result of the bolt load step to the next one with the service load, then it makes no sense to recommend something like that.

 

[marulanda]

II have to verify a structure:
1 The joints according to EN_1993-1-8
2 The items according to Von Mises method

I have done a test with single lap joint model, and with method of fake temperature load (for bolt preload) is impossible to have a double step analysis, there is the problem introduced by Mustaine3.

So the options are:
1 Apply bolt preload and load in one step
2 Apply the load only (considering the bolt preload only in the hand calculations)

Obviously I would prefer the second option, but as I said I'm not sure if it is good for the verification of the items.

I think that I will try to follow the advice of SWComposites.

What do you think about?

Thanks,

regards

 

[Mustaine3]

Again, it depends on the problem if the second method is valid. When your bolt load is high compared to the material stiffness of the connected parts, then the bolt load already brings significant stresses into the structure. Also the behavior under the service load might be different when the bolt load is (not) applied. You have to validate your method and decide.

 

[marulanda]

I'll do some ran, in first and second condition to evalute the difference before to ran all load cases.

Thanks

 

[MGZmechanical]

Maybe not the case but for pressure vessels one of the conditions for the calculations is the bolt preload at assembly to check that the flanges have not problem.

 

[aerostress82]

If you were checking the fatigue life on your bolts somehow for a selection of fasteners from a group of vendors, then you might have needed it.

If you are only checking the static analysis for a "limit" loading, then you might want to have it again if those areas are expected to be critical.

This bothers me somehow:
Why don't people make a workbench of their bolt pretension load analysis for each different combination of "thickness+bolt type+material" and superpose those stress results (in r-theta domain around the bolt hole) into their global detail FEM results around the bolt areas?????? Or better: just simply integrate an easy hand calculation of pretension load stress distribution?

I know automotive people aren't that handy with hand calcs. And aerospace aren't with advanced FEA applications.

Transient nonlinear analysis isn't that common at all. Why don't we have these results to superpose into our original linear transient/static/whatever linear analysis? It would be perfectly accurate for those problematic stress concentration areas (where applicable). Just an idea.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer