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Old 21-04-2018, 11:17   #1
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Thread Fatigue?

I was surprised recently when a famous rigger told me that all rigging screws and terminals should be changed often, at least every ten years, due to THREAD FATIGUE.

I never heard this before - what do we think?
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Old 21-04-2018, 11:51   #2
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Re: Thread Fatigue?

The mechanical engineer in me says that a good story, for a rigging business.
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Old 21-04-2018, 11:54   #3
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Re: Thread Fatigue?

Maybe so many millions of cycles.....but 10 years? Hmmmm.
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Old 21-04-2018, 14:00   #4
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Re: Thread Fatigue?

There is a good reason you never heard of this before...

For metal to "fatigue" it has to move, either stretch or bend. Threads do neither, and do not fatigue.

Now... I’ll happily entertain arguments about how it might be a good practice to pull screws at some interval to reapply your favorite corrosion inhibitor, but fatigue? No.
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Old 21-04-2018, 14:48   #5
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Re: Thread Fatigue?

Thread fatigue, or the need to avoid it, is why we pay the extra for rolled threads on the studs (terminal swage studs on shrouds and the studs connected to the chainplates), nicht wahr?

Rolled threads are 5 - 10% less likely to fatigue than cut threads.

The academic who has figured big in the thread fatigue world is Fukuoka Toshimichi at Kobe U. I understand Fukuoka san is now retired.

For one of Fukuoka's classics, read: Finite Element Analysis of the Cyclic Stress Amplitude of Threaded Fasteners Using Helical Thread Models | Journal of Pressure Vessel Technology | ASME DC
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Old 21-04-2018, 14:57   #6
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Re: Thread Fatigue?

So on engines, how often would we need to replace big end, conrod and cylinder head bolts?
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Old 21-04-2018, 15:03   #7
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Re: Thread Fatigue?

Didn't see the subforum and thought this was going to be about forum threads getting old and rehashing the same topics over and over
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Old 21-04-2018, 16:35   #8
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Re: Thread Fatigue?

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Thread fatigue, or the need to avoid it, is why we pay the extra for rolled threads on the studs (terminal swage studs on shrouds and the studs connected to the chainplates), nicht wahr?
Nicht wahr? Nicht! We use rolled threads to avoid stress risers, which leads to stress corrosion cracking.

A good reason to replace parts, but it is not fatigue.

With all due respect to the good professor, bolts holding the lid on an industrial pressure vessel have very little in common with rigging screws.

It is worth mentioning that if your rig is not properly preloaded (I.e., is too loose) ALL the parts will move and be subject to fatigue!
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Old 21-04-2018, 23:12   #9
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Re: Thread Fatigue?

All threads subjected to cyclic stresses are prone to fatigue failure and threads without radiused form thread roots are more prone than those with due to the stress riser effect. The number of cycles to failure depends on the geometry of the thread root and the magnitude of the stress cycles. For any thread root form there is generally a level of stress magnitude below which no fatigue cracks are generated or propagated.

It's real, whether it's significant depends upon the thread form and cyclic loading.
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Old 21-04-2018, 23:26   #10
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Re: Thread Fatigue?

Ok, has anyone ever heard of a rig failing due to this effect?

AND, do people replace otherwise good looking and well functioning rigging screws out of concern over this effect?
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Old 22-04-2018, 02:16   #11
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Re: Thread Fatigue?

I presume its stainless you are talking about, in which case it is likely he means 'crevice corrosion'.
Doesn't need much stress, just salt(chlorine), moisture, lack of oxygen and reasonably warm temperatures. You fail for the last requirement so probably no worries mate.
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Old 22-04-2018, 04:47   #12
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Re: Thread Fatigue?

Quote:
Originally Posted by Dockhead View Post
AND, do people replace otherwise good looking and well functioning rigging screws out of concern over this effect?
Of course they do, because the rigging guy sai to so the insurance company also said to.
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Old 22-04-2018, 05:32   #13
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Re: Thread Fatigue?

Awesome timing, just running fatigue tests now on some ag machinery. All threaded fasteners should be in tension, anything in tension on a boat especially shrouds e.t.c will be cyclical. If any of it is above 50% Ultimate Tensile Strength (rule of thumb) then it is ripe for fatigue issues. A sharp thread root is an ideal starting point for a crack. However:-
1. Most turn buckles or engine hardware is rolled and does not have the sharp cut or discontinues grain flow in the material.
2. Most hardware in these situations are well below the 50% UTS so it shouldn’t be an issue.

Maybe the ‘rigger’ was a ‘medical professional’ in a previous life... going to hide...
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Old 22-04-2018, 15:31   #14
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Re: Thread Fatigue?

This is actually one reason for the the tried-and-true design that uses SS screws and bronze bodies. Way back in 1948 Sopwith determined that using a lower modulus material in the body (in the turnbuckle case) resulted in slight deformation at the interface between the male thread of the harder material and the female thread of the body. That deformation, in turn, created a greater bearing surface in the overall structure and reduced loading (because of the greater bearing surface the load per unit area is lower) which in turn leads to lower fatigue. The lower fatigue value in a straight thread was similar to that encountered in tapered threaded fittings, which is a significant improvement over straight threads.

Guess those old-time designers knew what they were doing even before science painted in the details.
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Old 22-04-2018, 16:14   #15
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Re: Thread Fatigue?

Crevice Corrosion is not fatgue - quote from stainless steel industry

Crevice Corrosion

Crevice corrosion occurs at locations where oxygen cannot freely circulate such as tight joints, under fastener heads and in other circumstances where the pieces of metal are in close contact.

Chloride salts, pollutants and moisture from the environment accumulate in the crevice. The environment inside the crevice becomes depleted of oxygen, enriched in chlorides, and acidified which promotes the breakdown of the passive film and anodic dissolution.

Environmental Factors:

The important environmental factors that favor localized attack are higher chloride content, higher temperatures, lower pH, and more noble corrosion potentials.

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