27 Replies Latest reply on Jul 15, 2009 12:14 PM by Poovaiah Devenira

# Non-linear Analysis of Bolt Preload

Hello all,

This is my first post on these forums, so I would like to say hello to everyone :-)

So far at our company we have done FEA (Linear Static) of structural assemblies involving bolts & nuts tightened to a specified preload torque - which is converted into tensile contact forces using the thread equation F=T/(0.2*D) and applied at the contact areas between each bolt with the contacting plate & each nut with the contacting plate.

However, I recently read in an internet article that the best way to simulate bolt preload in FEA is to perform a Non-Linear Analysis which takes into account thermal stresses induced in the members due to friction/tightening. I am interested in learning more about this, so could someone please tell me what it exactly means & which books or internet articles would you recommend us to study about non-linear themal analysis in detail?
• ###### you need to be a lot mosre sopecifcNon-linear Analysis of Bolt Preload
You would need tobe alot more specific about exactly what the object of such an analysis would be. here is somehting to think about: The way your post is written it sort of implies that the heat generation of tightening the bolt maye relevant. Hard to really believe that one. Another possibility is that in some codes an NL analysis maybe required to obtain the pre stress state. the fist step in the analysis would be to apply a thermal load to shrink the solid element model of the bolt to obtain the response of the structure due to the bolt tightening. The next step would be to apply the external loads.
Solidworks simulation does all this for you in the static analysis.
• ###### you need to be a lot mosre sopecifcNon-linear Analysis of Bolt Preload
Thanks Bill, yes you're right on the part that a thermal load needs to be applied to shrink the bolt & nut to pre-stress or pre-strain the assembly. I would just like to know how this can be done & basically learn more about thermal loads (as I haven't studied it much in my degree).

The real objective of the analysis is that we're trying to achieve better correlation between the FEA & real-time testing results. The method which we're following at our company (as I mentioned in my first post) does not give us good correlation with real-time stresses & displacements. Hence we're trying to explore the possibility of Non-linear analysis so as to get good correlation with real-time results.

BTW, I would like to know how Solidworks can do this in the Static Analysis itself, as you have mentioned? Because the way we've been doing so far is merely assembling the CAD models of the nuts and botls & applying equal and opposite pressure forces at the contact areas. We aren't applying thermal loads anywhere in our analysis.
• ###### modeling of bolts
You can apply temperature as a load in a static analysis. You should avoid the word thermal as it implies a heat transfer calc. If you apply a temperature as a load the program shrinks or expands the component it is applied to as per the coefficient of linear expansion defined in the material definition. The delta T used is based on the zero strain reference temp defined in the properties of the study (RMB under the top of the analysis tree).
It does nto sound like you are aware of the bolt connector available in the program. You might want to try using it as it automates the procedure outlined in my last post and avoids you have to specify the temperature method to get the pre-load. If your adherent in the bolted connection ae not very stiff the bolted connector can lead to overly conservative pediction of fastener loads. IF you want to to used solid elements to represent the fastener then the temperature option is what you have to use and again a non-linear analysis procedure must be used to get the sequence effects.
• ###### modeling of bolts
Thanks again for your valuable advice, Bill. Just a couple of questions here:

1. Can I use the "bolt connector" option in my Linear Analysis itself, or should I go for Non-linear Analysis for getting the desired accurate results?

2. Where does this temperature load come from? What's the physics behind it? Is it because of the friction between the bolt-head or nut-head with the plates, or is it because of the high temperature that our assembly is subjected to during regular operation?

I apologize beforehand if my questions seem too basic :-) But this is a new area we're venturing into as we haven't had much exposure on it during our educational degrees. We feel this will turn out to be very useful and profitable for our company in the long term.

• ###### modeling of bolts
1) I would try the bolt connector. If your adherents are stiff it will work fine.
2) The temperature load is an articifical way to get the pre load into the fastener by using the CTE to shrink the bolt to induce the pre load if you are using solid elements and contact (or manually placed and bonded beam elements).
• ###### modeling of bolts
Thanks a lot Bill, I'll give it a shot and will let you know how the results are
• ###### modeling of bolts
Bill,
I cannot find the manner to specify in SolidWorks the temperature of the bolt connector. In case of non-uniform temperature of the bolted bodies (thermoelastic static analysis made over a thermal analysis) which assumption makes SolidWorks on the temperature of the bolt connectors?
Thank you for your information
Leonardo
• ###### modeling of bolts
Hi Bill,

I am intrested in knowing more about applying the temperature load for simulation a pre-tension in an anchor bolt. I was'nt sure on how excatly to apply this load. Do i apply the temperature on the cylindrical face of the bolt. Colu u give me more information on how to exactly simulate this. The anchor bolts are 10 ft long and a pretension load of 187.5 kips is to be applied
• ###### modeling of bolts
Guys you need to re-read my original post. The bolt connectrs apply a pre load - so the temperature thing is irrelavent in this context. If you want to have the bolt geometry in the model then you can use a NL analysis ( non-linear ananlysis) and run a sequence of loads to get the job done. First you apply a temperature to the bolts to shrink them and develop the bolted state prestress - then you apply the external loads after that in another load sequence (ie using the existing state of stress as a starting point for the external loads). If the problem is stable you could do it in a static analysis all at the same go assuming the friction forces due to the bolt loads are not required to support other loads.
• ###### modeling of bolts
Hi Bill,

I want to model the bolts as I am more intrested in studying the deflection of the plate to which these bolts are connected. So i plan to use the temperature method.
I wasnt sure if i understood how to do it excatly. Do I apply the temperature on all the faces including the bolt head and nuts or only on the bolt shank. could you also tell me on how to get the required temperature i have to use.

Thank you
• ###### modeling of bolts
Pick the bodies filter and then select the body and it will apply a uniform temperature through out the bolt. Or just pick the body formthe solid bodies folder.
• ###### modeling of bolts
Hi Bill,

a = 6.5 * 10^-6
Strain = alpha * delta(t)
F/A = E*Strain

For A = 3.14 in^2
E = 29,000,000 psi
F= 185 kip

delta(t) = 312 F
Assuming ambient temp to be 82 F ( I set it as 82 in thermal effect, reference temp)
Do I apply (-230) F on the body of the bolt to simulate the pre-tension load
• ###### Non-linear Analysis of Bolt Preload
If you want to model the bolt in a bolted connection, there are two ways to apply a preload.

Apply a temperature Load/Restraint to the BODY of the bolt (not its surfaces). In the Study's properties you can set the reference temperature at zero strain. The temperature Load/Restaint you apply to the bolt will be smaller. The value will depend on the size of the bolt, its coefficient of thermal expansion and the desired preload. Be sure and apply the reference temperature to the other bodies.

An alternative is to model your bolt with an initial interference and use the shrink-fit contact set between the two surfaces. It helps to apply splitlines on the surfaces where your boltheads contacts. The magnitude of the initial interference depends upon the desired preload.

If you want to apply the results from a thermal study then the shrink fit option of applying bolt preload is the way to go.
• ###### Non-linear Analysis of Bolt Preload
Hi Pete,

alpha = 6.5 * 10^-6
Strain = alpha * delta(t)
F/A = E*Strain

For A = 3.14 in^2
E = 29,000,000 psi
F= 185 kip

delta(t) = 312 F
Assuming ambient temp to be 82 F ( I set it as 82 in thermal effect, reference temp)
Do I apply (-230) F on the body of the bolt to simulate the pre-tension load
• ###### Non-linear Analysis of Bolt Preload
Poovaiah,

Your math looks right. Not only do you have to apply -230°F to the BODY of the bolt but you have to apply 82°F to the other BODIES of your assembly (or multi-body part).

If you want to perform thermal studies then this artificial method of applying bolt preload will interfere with the thermal results. In that case use the other method I suggested in my previous post to apply bolt preload.
• ###### Non-linear Analysis of Bolt Preload
Hi Pete,

Thanks for the confirmation, I am using this pre-load on the bolt ,only to determine the deflection in the bolt base plate, however when i do a thermal analysis i will try the second method proposed by you.
• ###### Non-linear Analysis of Bolt Preload
Yes. You need a -312F delta T from the zero strain temp.
• ###### Non-linear Analysis of Bolt Preload
Hi Bill,

Thanks for the information.
• ###### Non-linear Analysis of Bolt Preload
Hi, Poovaiah,

I am wondering which areas were included in your A calculations above.

Also, have you compared this temperature method vs simply applying normal forces on the bolt head and the nut?

Pei
• ###### Non-linear Analysis of Bolt Preload
Hi, Poovaiah,

I am wondering if you were able to run the simulation successfully.

I setup a non-linear statics case: Part A and Part B are the two pieces to be bolted together. Also the bolt was modeled as a dumpbell with the length exactly the same as the combined length of Part A + Part B.

Setup a contact pair, with the faces on the bolt in one set and the faces on Part A and Part B on another set - selected face to face contact. Also setup a contact set between Part A and Part B - face to face contact.

Applied a small force on one face on Part A.

In "External Loads"/thermal effect, set 82F for reference temperature at zero strain.

Set the body of the bolt(s) to 40F (just testing)
Set the bodies of Part A and Part B to 82 F.

But, the case failed to converge.

Did I do anything wrong?

Any suggestion?

Pei
• ###### Non-linear Analysis of Bolt Preload
You failed to fully constrain the part from moving. If you apply a force but do not constrain it enough the part keeps moving and you will fail to converge.
• ###### Non-linear Analysis of Bolt Preload
Hi, Pete,

I forgot to mention the fixed constrains. I actually did have a fixed hinge and two fixed faces (in all DOF) in the model.

Pei
• ###### Non-linear Analysis of Bolt Preload
Enable the "Soft Spring" option for the study & re-run. It should allow it to converge. You may see unanticipated deflection if you missed constraining all DOFs.
• ###### Re: Non-linear Analysis of Bolt Preload

Hi,

I did manage to simulate the Pre load and the results looked acceptable.

1. I used Static analysis.
2. I modeled the bolts and set the bolts in the bolt holes, the bolt holes were slightly bigger than the bolts.
3. I set the nut end of the bolts as a fixed constraint.
4. Well try not to set up a contact constraint between the bolt head and the plate in contact.
5. Apply the temperatures for the bolt and other components.
6. make sure the entire model is constraint

Let me know if it works

• ###### modeling of bolts
dear Bill,
this is not my point. Let me try to explain better. The usage of the bolt connector on an isotherm structure is clear to me. But I want to use the same method to deal with a non isotherm structure, in which the bolts have a temperature different from the compressed members. Or even a case in which the temperatures are all uniform, but different from the zero strain temperature. Now my point: it is possible to specify for the bolt material a coefficient for thermal expansion, but, seemengly, not a bolt temperature: this is illogic. The program should not have any difficulty to keep in consideration the thermal inelastic strain of the bolt, when appling the preload (which is for sure modeled in turn as another inelastic strain).
Insted, the software gives incorrect results even in the simple case of temperature uniform throughout the model but different from the zero starin temp. Do you avail an explanation?
Thank you
Leonardo
• ###### modeling of bolts
Hi Leonardo,
I do not have a good understanding of what you are trying to do. Good luck.
• ###### Non-linear Analysis of Bolt Preload
This doesn't sound like a non-linear analysis. This is more of a non-linear dynamic analysis. The heat is being caused by friction with time? You have to know more about how fast the pre-load happens. You have to consider velocity and acceleration of the turn at a given time to generate pre-load. It's this acceleration that generates the heat (power). You probably need to consider things that are well beyond the scope of this discussion.