Hi Guys,

For Static Stress Analysis of Threads (Internal or External), we do the bearing check and the combined stress check (Tension, Hoop and Shear).

For Fatigue purpose, I would like to know what is the best approach? Well, when I do FEM Fatigue Analysis, I use Signed Von Mises or Signed Tresca (Defining + or - sign, by associating the sign of maximum abs of Max. Principal and Min, Principal to VM Stress).

But, for threads I would go with a classical approach. Now, if I use only shear stress then I cannot associate any sign to shear stress. So, I appreciate to know how do you deal with threads for fatigue purpose?

Thanks,
A.A.Y.  

Are you meshing the internal and external threads and then obtaining stresses within the elements?  There is a lot of research on this subject, try searching scientific journal articles (Google Scholar, Scirus, ASME, SAE).  See also ASTM STP's 1236, 1391 and 1491, Structural Integrity of Fasteners.

There are non-FEM calculation techniques included in VDI 2230 Systematic Calculation of High-Duty Bolted Joints.

Hi CoryPad,

Thank you for your reply. I believe that VDI 2230 is a good tool for advanced thread analysis. But, we are doing a quick fatigue assessment. I usually do detail 3D FEM for strain-life fatigue analysis. But, I don't do FEM for this thread.

I will probably calculate the signed Von Mises stress with correct Kt.

A.A.Y.

the shear you're considering is only from the reacted shear loads (ie not from torque) ?  but then that wouldn't appear at the threads ??

the only stress i'd consider for fatigue is the cyclic axial stress in the bolt.

Guys...

In a pure cyclic-tension joint, the static preload of the fastener system (threaded bolt/nut torque-tension clamp-up) must be exceeded by a cyclic load spike... or a componnent must fail in yield, crushing or wear (IE: fitting or washer deformation relieving preload)... before the fastener system see any cyclic [fatigue] loading.

If cyclic tension+shear loading enters the picture, then hole-to-fastener shank becomes all-important determinant factor: shear slippage will induce wear damage reducing fastener pre-loading. Hence the sloppier the fit then the poorer the joint performance (most likely impossible to model due to ingenious combinations of poor fastener-to-hole fits).

Regards, Wil Taylor

You get some details here
www.fatiguedurability.com

Wil Taylor,

I must respectfully disagree with one statement in your post.  While preload does reduce the cyclic damage in this type of joint, it does not eliminate it.  The joint always feels some of the applied load, no matter how much preload is applied, and this can sum to significant damage even when the applied load never exceeds the preload.

Thanks for your many wise and helpful posts.

Lcubed