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Metallurgists: I need advice on material selection to avoid stress-fracture

Plating introduces a much more complicated level of fatigue resistance, including the possibility of hydrogen embrittlement.
The wind changes, but once resonance gets started, the wind can change a bit and the resonance can keep right on going. Your mast will go through resonance at some point, possibly exacerbated by equipment on the mast, but the question is the material response and whether damage occurs or not.

As someone who used to deal with resonance in wind turbine components (always a problem, a killer in some cases) we started test programs by finding the natural frequencies of the components. Blades and towers, like your mast, were easy to find by hand.

I suggest a start be made by someone near to the installation, perceptive enough to push on the mast and get a regular motion going + another someone to count cycles. Should be able to find a couple different natural frequencies.

Not so useful as it could be, since (if I'm reading correctly) there's no one involved able to calculate natural frequencies and predict changes, but it's more information than you have.

Threadmaking can coldwork/harden depending on method. Could be that the threading process moves the stress concentrator into the solid part of the shaft, where it failed.
These are cut threads, so if anything I'd think they'd be a weak point but the breaks are above the last thread. It'll be interesting to see how the replacements hold up.
so if i imagine this correctly, the upper pole is threaded at the bottom, i guess the pipe is 3.5 mm and the thread 1.5 mm isch? if so i wonder who "engineered" this, because the part is not only weakened by the thread but you get a nasty stress concentration also. a screw will survive this because its in tension (no reversed stress). the transition needs to be stiffened somehow, maybe by pressing in a short tube?

interesstingly, if you were put a circumferential groove above the thread it would reduce the stress concentration a bit.
dian, I'm not following what you're trying to tell me. The pipe wall thickness insome of the parts is about 3.5mm, the thread pitch in some of the parts is somewhat close to 1.5mm but there are no breaks at the thread area.
does the pipe look like a regular screw (threads cut into material) or like a "necked down" screw (portruding threads)? it is common for a regulal screw to break just above the threads as thats where you have the largest concentration of strees flow lines, so that its even weaker than the minor diameter would suggest. the strategy could be to reinforce that area. another hip shot: dont fully tighten the pole in the female thread (pin it or even glue it?) so there is some movement taken up by the threads or use a counter locking ring on the pole, thus separating the areas of max. load and stress concentration (where the thread ends).

what i said about the notch (previous hip shot):


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I understand that there are proprietary considerations here and photos aren't possible, but a sketch would be very helpful. Really tough to give specific advice without specific information.
You have not mentioned what if any failure analysis you have done. If you really want to address the problem you should have a couple of the failed units sectioned to identify where and how the crack started. Then there is also so option of having a dynamics person make a model of the system and run a cumulative damage index calculation (CDI). You could also instrument a good and bad installation to try and measure the differences. Think of three axis accelerometers. None of this is cheap, but depending on your situation it might be worth it. If I remember correctly rklopp here works in FA, or at least used to. That was a while ago. It’s much easier to fix the problem if you understand why it is a problem.

i don’t recall if you mentioned if the failed units were all from the same lot of material. Is it possible that you got a bad batch?
I do appreciate the continued comments, but we're going to let it go for the next year or two with the replacement material and see if that solves the problem. We believe there's a good chance of that, and since no other installation is affected, I doubt there will be any extensive and expensive investigation.
Plating introduces a much more complicated level of fatigue resistance, including the possibility of hydrogen embrittlement.

Gordon, don't ignore this post. There is tons of information on hydrogen embrittlement of 4340, including the specifics of post-plating baking temps/times from companies like boeing.