Fatigue cracking in stainless

[AngusMcDoon]

The attached picture is the fracture cross section of a fitting that attaches my mainsheet top block to the boom. It let go yesterday with a loud bang. The fitting is about 10mm diameter, and the fracture started at an inside bend.

It's ages since I studied metallurgy, so please correct if I am wrong, but it looks like the crack has been there for a while, starting on the left, and slowly growing under the varying stress. Yesterday it reached critical crack length, and grew very quickly until the remaining metal left could not take the load, and that gave way in a plastic manner - the shiny bit on the right.

I remember that crack growth like this is accelerated in a corrosive environment, which salt is slightly to stainless steel.

Apart from this easily replaced item, no other damage occurred. But it's a bit worrying. I wonder how many other cracks in more critical places are slowly and silently creeping towards failure point.

 

[choppy]

For what its worth - I had boom fitting (u type) for main sheet attachment go on Dragonfly (If thats what it is) 8m - Again with a very big bang in July - 8yr old boat - lashed it & all fine.
But
A) I thought may go for 1 of those racey web round boom jobs you see (cant ??!! be any more expensive)
& B) It is the sort of thing that is so easily overspeced that shouldnt have broken in such a way.

 

[oldharry]

Isnt stainless lovely stuff? - looks as good as the day it was made right up to the moment it goes bang -with consequences ranging from minor irritation to loss of life and boat.

Why DO we rely on it so much?

 

[Jmolan]

Get rid of it...

I have eliminated SS on my boat. Lashings and "sofites" have replaced every shackle, attachment. The headsail shackles, tack shackles, block shackles, etc. are all Dynex softies now. I have got rid of those turnbuckles and gone to dead eyes etc. Before you think me daft, or that it is an ancient rig have a look. My boat is in the the lower URL below. I saved 40 lbs. off my rig from SS wire and fittings I had before. My Avatar is me holding my 34' entire rig in one hand. 7.5 kgs!
The ideas for the softies came from the racing world and I got mine at Colligomarine.com

 

[halcyon]

Isnt stainless lovely stuff? - looks as good as the day it was made right up to the moment it goes bang -with consequences ranging from minor irritation to loss of life and boat.

Why DO we rely on it so much?

Why do we buy Chinese rubbish, hell it's dirt cheap, when did qaulity come into the equation.

Purhaps if we spent £1,50 on UK made, instead of £1 it would not go bang.


Brian

 

[AngusMcDoon]

Why do we buy Chinese rubbish, hell it's dirt cheap, when did qaulity come into the equation.

Purhaps if we spent £1,50 on UK made, instead of £1 it would not go bang.

Brian

Brian

This part is question was not cheap Chinese rubbish. See my post on PBO about a boom fitting if you are interested.

I've had this discussion before, and got called a fool by someone with a big beard and a boat in Wales. I was in a minority of 1 for advocating not using cheap hardware in load bearing situations. I have no Chinese cheese metal on my boat. My shackles are all Wichard, rigging OS, and this part was made by Z Spars.

But it still went bang.

Angus

 

[AngusMcDoon]

I have eliminated SS on my boat. Lashings and "sofites" have replaced every shackle, attachment. The headsail shackles, tack shackles, block shackles, etc. are all Dynex softies now. I have got rid of those turnbuckles and gone to dead eyes etc. Before you think me daft, or that it is an ancient rig have a look. My boat is in the the lower URL below. I saved 40 lbs. off my rig from SS wire and fittings I had before. My Avatar is me holding my 34' entire rig in one hand. 7.5 kgs!
The ideas for the softies came from the racing world and I got mine at Colligomarine.com

Good idea. I shall look into this, starting off with webbing for boom hangars to attach the mainsheet. Thanks.

 

[maxi77]

Brian

This part is question was not cheap Chinese rubbish. See my post on PBO about a boom fitting if you are interested.

I've had this discussion before, and got called a fool by someone with a big beard and a boat in Wales. I was in a minority of 1 for advocating not using cheap hardware in load bearing situations. I have no Chinese cheese metal on my boat. My shackles are all Wichard, rigging OS, and this part was made by Z Spars.

But it still went bang.

Angus

In reality do you know where the big brand name sourced the component from. A lot of it does come from china today, but that does not mean it is not good quality.

Equally I can remeber when Japanes stuff was considered to be cheap unreliable copies too.

 

[boatmike]

Brian

This part is question was not cheap Chinese rubbish. See my post on PBO about a boom fitting if you are interested.

I've had this discussion before, and got called a fool by someone with a big beard and a boat in Wales. I was in a minority of 1 for advocating not using cheap hardware in load bearing situations. I have no Chinese cheese metal on my boat. My shackles are all Wichard, rigging OS, and this part was made by Z Spars.

But it still went bang.

Angus

Angus you are pretty well correct. The photo is not clear but there has been a crack there for a while which has quickly grown as it reached a critical depth. I think the part is either a forged part or at least the end is "upset" by hot working which is much the same thing so may in fact have suffered work hardening in manufacture. It should have been normalised (similar to annealing) after working but who knows. Fatigue is difficult to predict and defined by the christalline structure being affected by reversals or vibration that is normally well within the elastic limit and/or tensile strength of the part. It can sometimes be caused by rigging being too loose in fact and tension coming on and off as the wind varies and you tack. Increasing the tension can sometimes make the load more constant. There is no easy answer though apart from regular inspection. Thats really why standing rigging is sometimes replaced after 7-10 years hard work even though there appears nothing wrong with it. The only helpful thing to say (if you don't mind the bad news) is that if one has gone perhaps the rest should be checked.....

 

[tommccrorie]

Angus,

I had a simlilar failure of SS but with me it was one of the 2 studs holding my flexible coupling onto the flange on the shaft exiting my TMP gearbox. I've now replaced them both with high tensile ones and covered them in lanolin.

regards,
Tom

 

[Jacana]

I've just had my Malo38 re-rigged after 15 years and there was no sign of any problem. The rigger however said that he can easily feel the difference when he cuts new and old wire with the old stuff much harder (through fatigue). IN retrospect I think 15 years is too long.