Shear Fatigue in Threads of a Fastener

[ks11]

Hello, I'm looking for resources on how to determine fatigue life of the threads in a joint under cyclical loading. I have a grade 8 1 1/4-7 cap screw clamping two pieces of 304L, with the second piece of 304L being tapped to hold the cap screw. The cap screw has a preload of about 24% of its yield strength, which for the size of material and depth of threads I have calculated the internal (304L) threads to have a safety factor of 1.12 under the maximum load they will see as a static case, but the loading is cyclical and I am having a hard time finding information about how to look at such a case. Most of what I have seen fatigue wise assumes the bolt and clamped material are the same material or looks only at the bolt as a failure point. I have a seen it mentioned a few places that fatigue failure in shear is very rare, though I'm not sure why that would be the case or if I misunderstood what that statement was being applied to. I also understand the first thread in the joint is typically carrying the largest portion of the load, some 30-40% which decreases somewhat when the internal thread is a softer material and the threads yield to more evenly distribute the stress across multiple threads. Can anybody point me in the right direction or clear up some of the things I am misunderstanding? Thank you in advance.

 

[azcats]

Curious to see the responses.

Is the clamping force not adequate to transfer the shear between the two pieces via friction? If there's shear in the bolt, there's also bending, no?

 

[ks11]

Is the clamping force not adequate to transfer the shear between the two pieces via friction? If there's shear in the bolt, there's also bending, no?

Yes it is, perhaps I wasn't clear. I am trying to look at the potential for stripping the threads due to fatigue of the weaker 304L threads from the combined axial load on the screw, not the screw itself shearing.

 

[azcats]

Well that's a much more interesting question than I envisioned. Good luck.

 

Shear is not fatigue, and shear fatigue is not a thing, so you need to clarify your concerns.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."