Stainless cracking. Metalurgists, puzzel this one

[thinwater]

Most of the bolts I see with damamge are pitted somewhere inside the hull, a distance from both the head and the nut:



[Vyv Cox]



But on Farrier trimarans, a not uncommon failure is for the heads of the bolts holding the lower folding struts to fail right under the head. There are 4-6 bolts in shear, through an anodized aluminum plate about 3/8-inch thick (varies with the model). There is only a split lock nut between the bolt head and the flange. These bolts support the full heeling load of the boat. The plates and struts are conservatively engineered and never fail and the bolts never bend or get loose. The heads fall off. And typically only one will just fail. The others hold without distortion. But typically when they are pulled for replacement, most show serious damamge. The anodized aluminum flange is typically undamaged.





Why would the damamge be right under the bolt heads instead of farther in? Does the anodized aluminum have something to do with this (it seem it must)?



[This strut is in tension. I'd have to run some numbers, but the max working load is probably 3-4 tons spread over 16 bolts (2 on each side of 4 plates).]

[Jammer]

Split lock provides a crack to admit water. 5200 or whatever sealant they use is good enough to preclude migration of the water further down the shoulder of the bolt. Simple answer

I am not a metallurgist however and maybe that's not it.

I no longer use split locks for much of anything. They're awful in so many ways. In that application I doubt if they had any effect at all since the sealant would have a much higher breakaway torque than the lockwasher.

[PippaB]

I'm with Jammer, probably the split washer. Do the hinge bolts ever fail in the same manner? Looks like the installation is slightly different.

Do you have enough data to correlate the direction of the split in the washer on the bolts that failed? For instance, if the split is on the high side, does it introduce a place for salt water to get to the head, and get trapped, and deplete oxygen, and start crevice corrosion? While if the split is on the low side it allows water to drain, and those bolts are the ones that do not fail?

[thinwater]

Originally Posted by Jammer

Split lock provides a crack to admit water. 5200 or whatever sealant they use is good enough to preclude migration of the water further down the shoulder of the bolt. Simple answer

I am not a metallurgist however and maybe that's not it.

I no longer use split locks for much of anything. They're awful in so many ways. In that application I doubt if they had any effect at all since the sealant would have a much higher breakaway torque than the lockwasher.

Correction. I looked at my boat and they are SAE bolting washers. You would never put a lockwasher under a bolt head. No purpose.

[thinwater]

Originally Posted by PippaB

I'm with Jammer, probably the split washer. Do the hinge bolts ever fail in the same manner? Looks like the installation is slightly different.

Do you have enough data to correlate the direction of the split in the washer on the bolts that failed? For instance, if the split is on the high side, does it introduce a place for salt water to get to the head, and get trapped, and deplete oxygen, and start crevice corrosion? While if the split is on the low side it allows water to drain, and those bolts are the ones that do not fail?

The hinge bolts never fail and are never corroded or damaged. Never heard of a case.

No split washer. My bad, please read correction. But more likely if there was free flow of water and oxygen that would actually be better. Pretty sure. Consider the hinge pins. Heck, the tiny circlips on the pins hold up as well as the bolts. Fortunately, they are loads easier to replace; 15 seconds and you carry spares.

All of the failures are as shown, right up under the bolt heads.

I'm puzzled by the location more than the failure due to cracking.


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Could a more noble 316 washer next to a less noble (stronger alloy) bolt be a problem?

[dlj]

Thinwater,

How far south of Annapolis are you? I'd love to come look at your broken bolts. I'll be back down to my boat in a week or two.

If you'd like, send me a PM.

dj

[dlj]

Originally Posted by thinwater

Could a more noble 316 washer next to a less noble (stronger alloy) bolt be a problem?

What alloy are the bolts made from? Do you still have the washers that were next to each broken head? didn't see them in the photos.

dj

[thinwater]

Originally Posted by dlj

What alloy are the bolts made from? Do you still have the washers that were next to each broken head? didn't see them in the photos.

dj

Not my boat. But I just asked those questions.

[dlj]

Originally Posted by thinwater

Not my boat. But I just asked those questions.

The answer would depend upon alloys used. Do you have the bolts? The photos are such it's not possible to give any kind of answer other than a WAG (Wild A$$ Guess). Something the internet is exceptionally prolific at...

dj