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[MikeP]

Dear all,

I am trying to model a prestressed bolted connection. The model I have created so far is shown below. I consists of two plates which are connected by one bolt (including the washers and the nut). The thread was modelled as well while the conact between the threaded parts of the nut and the bolt is with friction. The preload is achieved in a first step by a remote rotation of the nut by a certain degree, which is working fine. In a second step, the upper plate is displaced vertically in order to analyse the load bearing capacity of the preloaded bolt (the force reaction of the displacement is therefore considered).

However, in some cases i get strange peaks in the force reaction as shown in the first graph below. In the substeps where these peaks occur, the plastic strain in the bolt stays nearly constant (second graph below). I have tried several things in order to solve this problem. My first guess was a poor mesh, but changing the element size or type did not help. Also, increasing the amount of substeps did not change anything. The problem seems to occur more likely with higher degrees of rotation applied in the first load step. The material model of the bolt was derived from tensile tests and is realised with multi-linear isotropic hardening.

I am running out of ideas what else I could try to solve this issue. The solver output didn't help either, there are no warnings or errors. Has anyone else evere encountered a similar behaviour and/or has an idea for an approach to solve this problem? 

Thanks a lot in advance!

Best regards

[Nanda Veralla]

Hello Mike,

In order to analyse the load bearing capacity of the preloaded bolt, I would model it in much simpler way, reducing my computational effort and also avoiding any potential unwanted behaviours like you're experiencing. You can get rid of those threads, and include a simple cylindrical geometry. You can model the bolt as a line body using beam connections. And apply preload using in-built bolt pretension tool avaiable in mechanical. Have a look at this Ansys Innovation Course, which guides you to model your case more accurately, yet using less computational effort.

Modeling the Bolt and Preload | Ansys Innovation Courses

Connecting Bolts with the Rest of an Assembly | Ansys Courses

Regards,

Nanda.

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[MikeP]

Hello Nanda,

thank you for your reply! I am aware of the much easier ways to model preloaded bolts. However, for my research it is essential to model the application of the preload as exact as possible. For example, it is important for me to consider the torsional stresses resulting of the friction in the thread. Furthermore I am looking at very high over-elastic preload levels beyond necking. Therefore, the simple application of a preload force by the in-built tool would be unrewarding in my case.

Do you have any idea what I could try to improve my results using my current approach?

Thank you again and best regards

[Claudio Pedrazzi]

Hi Mike

>> the contact between the threaded parts of the nut and the bolt is with friction

could you provide all details? coefficient of friction, ans so on?

Have you used the contact tool to study what are the contacts doing when you "pull" ?

 

 

[MikeP]

Hi Claudio,

thanks for your reply. Sure, I can provide more details: the coefficient of friction is 0.12. I haven't changed anything to the default settings of the contact so far. The number of contact elements stays nearly constant while I pull which I interpreted as a sign that the contact behaves as it is supposed to. "Pressure" and "Penetration" show similar peaks, but I assume that this is a consequence and not the reason for the "peaks". Furthermore, I tried to estimate the influence of the contact and used contact step control to change the contact to "bonded" in the last load step - this had no effect on the peaks.

 

[Claudio Pedrazzi]

I was thinking in the direction of stick-slip phenomenon.  Hier an excerpt of the help concerning "frictional" contact

 

"Frictional: In this setting, the two contacting geometries can carry shear stresses up to a certain magnitude across their interface before they start sliding relative to each other. This state is known as "sticking." The model defines an equivalent shear stress at which sliding on the geometry begins as a fraction of the contact pressure. Once the shear stress is exceeded, the two geometries will slide relative to each other. The coefficient of friction can be any nonnegative value. [Not supported for Rigid Dynamics. Forced Frictional Sliding should be used instead.]"