i have posted two simple example cases, one using initial condition type stress and the other is sliding at nut wall surfaces and reversed loads. It seems the latter approach is more generic, surface behavior (contact) can be changed within a step.
Thanks, Xyont. I will keep those options present. (Where could I find them?).
In the meantime, I will try to go deeper in the understanding of the pretension card , its limitations and discarding any possible wrong set up from my side.
A follower force would be nice in case Nonlinear pretension is active.
By other hand, shouldn’t the gap between pretension sections be automatically fixed once the force preload is finished?.
If not, the bolt doesn’t develop additional internal tensile force. In fact it “brakes” as soon as the external tensile load reaches the pretension value. That sounds like a bug.
It behaves as some kind of perfectly plastic contact if you allow me the comparision.
I have made an additional test and it brakes at the exact pretension load.
My pretension is two bolts at 700 N each one.
The clip blows away when I reach 1400 N of reaction force at the base.
Where can I find this model? It’s one @xyont posted somewhere?
Breaking apart at the preload force sounds correct for force pre-tension reacted by contact. Do you have the two steps with displacement applied to pre-tension in the 2nd step?
only some screenshot of example at the thread link provided. The input file and definition can be huge and it need a lot of work, but i have seen the possibility. Below another example case of alternate model of pretension bolt using initial condition type stress.
I have set up the model in Prepomax with two steps but the behaviour is not as expected too.
Redefining the pretension to displacement in the second Step and adding FIXED resets the preexisting tensile load on the bolt.
It’s well known (although somewhat disconcerting) that a bolt under pretension has a flat transition when the operational load is applied .Tensile do not increase in the bolt until the external load induces a tension=pretension. At that point, tensile increasses again but from the previous pretension value.
That drop shown on the graph should be a flat line.
Reaction on the bolts base.
Applied
I would appreciate if someone more experienced in Prepomax can take a look if I have properly set up the model.
actually, i still can not understand the problem from screenshot provided, how purely pre-tension bolt designed to resist tensile load. It’s a friction bolt to be work for shear loads dominant by clamping, some overstrength may exist due to minimum pretension of 70% tensile strength.
however, interesting observation when the solver given report in breaks at bolt reaction equal to pretension loads assigned. So it can be assumed as half value of tensile strength (maximum). But the model may incorrect since low pretension load only produce small friction resistance, the plates begin to slide when exceeded, then contact bearing occurs. Is pretension section approach can maintain compatibility of bolt during deformation? i;m not sure whether it can.
And so on…
the 2 parts of each bolt have to be merged, the pretension section is a section inside the bolt (coincident nodes).
@Disla
I don’t know if I have understood correct, but I believe what cheat, it’s the reaction force, which really isn’t reaction force but actual external force.
At exit of step 1 everything has come into balance, why no external force exists at the beginning of step 2 and for reason RF2 will start from zero again.
Plotting deflection in Y-direction close to the bolt verifies that loads really occurs during the 2 step.
this is an example I developed from abaqus examples problems guide: http://130.149.89.49:2080/v2016/books/exa/default.htm example 1.1.1 that has explanation and link to the original publication.
You can easily add axial and radial (shear) loads to this model in a further step.
Hi Flemming,
It is confirmed that there is an interference between OP=NEW and FIXED card that erase the pretension in Step_2 in Prepomax.
I have been able to obtain the behavior I was talking about with Mecway. One can see the flat section in the reaction while the operational tensile load <Pretension. It’s done with one unique step and the right amplitudes. Still pending to check the finall values agree with input loads.
Adding the axial and radial shear loads in the second step is the problem but thank you anyway. I can see the gasket there. That’s interesting too.
Thank you both and all the previous posters for the replies and support.
Also solve it with custom keywords in Prepomax with the help of xyont and Jackub.
Pending final values check.
Hopefully this can be implemented in a future release.
This is right. If my example from Abaqus examples guide is run in CCX there is evident sign that section does not rotate contrary to Abaqus solution that takes it into account.
if you exceed the pretension in the bolt the joint undergoes separation and then the contact condition in the joint plates changes, the bolt instead of the joint is taking the axial load, under this condition the pre-tension section model is no longer valid probably.
Hi Juan and Thanks,
Yep.That’s the behaviour I was expecting. Big warning cause it’s clearly nonlinear if anyone is interested in loading and looking close to the separation point.
Why do you say is not valid. Do you mean is nonlinear, it’s not valid for service , is not safe?
I am saying that probably the model under this condition will yield wrong results, that is, that will not correlate with real world. So you need a different type of model after separation (no pretension section).
I think I didn’t read this whole thread carefully enough, so I’ll try to fix what I said. It seems I never tried to run a pretension analysis beyond separation or at least didn’t take much care about the results since usually separation is a failure condition, so I updated the boltpipe4 example, this time in prepomax with axial load in a second step: it is necessary to edit the calculix keywords as shown in the picture, do it after creating the pretension load so you know the node id (seems I always did this directly the .inp file).
The results for the bolt load are in the attached picture, same as yours (time 1.0 is full pretension, then at about time 1.5 separation happens, always load a bit bigger that pretension, from then all load is taken by the bolt).
Just a small remark, the “flat” portion of the curve is not completely flat but its slope depends on the relative stiffness bolt/grip as explained here, and the non-linear behaviour can be described as 2 lines with different slope before and after separation. This is the usual “hand calc” analysis.
I provide both the prepomax file and the inp file so anyone can check the results and confirm if appropriate.
Contrary to what I said before the pre-tension section model works well beyond the applied pretension load (if CCX keywords are used properly of course).
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You are absolutely right. It’s not completely flat. And also, the separation happens slightly over the pretension load. In my case the relative stiffness between bolt and plate made the transition almost imperceptible.
Thank you, Juan, I’m sure users will appreciate this valuable applied case.
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