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Bolted Joint Separation

Bolted Joint Separation

Bolted Joint Separation


I am running a static analysis on an assembly that has some bolted joints. In addition to the stresses in the parts I am interested in the forces in the preloaded bolts to make sure they are not overloaded. All the bolts have a fairly high safety factor and I am not overly concerned about them, however....

I was also asked to determine if there is joint separation between the bolted parts. Is it sufficient to look at the axial force in the bolt and assume that the joint will not separate from the load? The assumption is that because the bolts are not failing that the joints also must not be separating.

Is it that easy or I making some glaring error?

I believe that I need to be able to check the forces between the bolted parts but I'm not sure if there is a way to do that in the simulation. Maybe there is another way that you've used to check this? Run the analysis without preload and then compare forces in the bolt to the preloaded results?


RE: Bolted Joint Separation

If the force on the bolt exceeds the preload, guess what?, it will separate. The idea with preload is that the bolt stays at the preload force while the joint is being pulled apart. When separation occurs the axial force in the bolt should increase.  

www.engtran.com  www.niswug.org
Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."

RE: Bolted Joint Separation

I do not believe that is correct. The bolt does not stay at the preload force when the joint is loaded even if the joints do not separate.

Solidworks calculates the stiffness ratios between the joints and the bolt. When you pull on the plate some of the force is transferred to the bolt (a function of the different stiffness ratios).

I can't just look at the axial force in the bolt and compare it to the preload force. If the parts are loaded it will be higher than the preload by some fraction of the force pulling it apart but that still doesn't tell me if the joint has separated.


RE: Bolted Joint Separation

I suppose you could design a joint in which the bolt carried an appreciable amount of the additional load while loading the joint. But that wouldn't be a conventional joint in which the load path from the bolt head goes directly through the article being clamped to the article being clamped to.

Generally the area under the bolt head is so much stiffer than the bolt that for all intents and purposes it is the part being clamped that unloads without the bolt seeing any appreciable change in tension until separation.

If a joint was designed so that the bolt saw appreciable/significant increase in tension after preloading and the bolt was tensioned to near yield any additional load might begin to fail the bolt or create serious fatigue issues. For example, a bolt on a hold down dog on a milling machine bed.  

www.engtran.com  www.niswug.org
Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."

RE: Bolted Joint Separation

I don't know what your joint design looks like, but in a typical ("standard"?) bolted joint the tension in the bolt should be essentially static until the joint separates - at which point the bolt tension goes up with applied load until the bolt fails.

Depending on preload & bolt strength you could theoretically separate the joint without failing the bolt.  Depending on what you're joining that may or may not be a failure of the joint (but probably is).

RE: Bolted Joint Separation

Look at your displacement. You should be able to see if the parts are pulling away from each other. Also look at your contact pressure plot. If you have contact pressure between the parts, they are still in contact. No contact pressure means they have separated.  

RE: Bolted Joint Separation

   This is similar to my question from a couple years ago.  

   thread1183-236375: Bolted Flange

   My problem was that I wanted to manage the transition where the flange starts to separate, but the bolt holes still are clamped.


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