Rigging fatigue failure - what causes it?

[snowleopard]

A question for the engineers among us. I'm working on the problem of using a wire forestay on a flexible rig. The forestay failed through fatigue after 10,000 sea miles, caused by the continual slackening and tightening of the stay as the rig flexed.

My question is what pattern of stresses makes a wire stay fatigue and what is different in my circumstances from a conventional rig where the loads on the forestay are also cyclical but with a higher base load. Is there more problem with tensions going from say 0-5000 Newtons as opposed to say 5000-1000 Newtons?

 

[graham]

I am not an engineer but could it be as simple as the slacker the rigging the more sag so the bigger amount of bending and straightening of the wires?

Did your stay fail next too an end fitting as they usually do?I dont think I have ever heard of a stay breaking in the middle.

 

[snowleopard]

It failed in two places- at the exit from the top of the furler foil and the toggle below the reefing drum cracked across.

 

[Vara]

Also not an engineer but would it not be possible to replace metal fore stay with one of the new(in my terms) exotic materials like dyneema(sp) as it is alleged that they are much more fatigue resistant.
Generally speaking no stainless material will endure happily the sort of work load you are putting on it.

As a side note I am surprised that more rigs don't fall down in any given year as IMO most rigging is set up too slack.

Just struck me that "Parafil" might be a solution,it could be useful to talk to them. Parafil

 

[capt_courageous]

Fatigue failure is caused by the load ( called stress in the material ) cycling from a low value to a high value for many cycles. The higher the max load in the cycle the quicker ( less cycles ) the material will fail. Think how you can break quite a thick wire by bending it back an forth so long as you bend it a lot. Until the max load in the cycle reaches a certain level fatigue failure will not occur no matter how many cycles.
I think it is likely that if your rig is too slack 'snatching' leading to high instantaneous loads and hence high stress will occur. This will result in fewer cycles being required to damage the stay. Remember that it is the cycling of the load that causes the fatigue damage and that the max load in the cycle can be a lot less than the steady load the stay can take. Hope this helps - I was a sort of engineer in my youth.

 

[MacW]

Agree with Capt.courageous as regards the engineering,but what you`ve got to ask here ,is the forestay the design weak link in the rig? And will a tighter forestay move the loading somewhere else, for instance ,to the carbon fibre mast / swing beam neck? 10000 miles doesn`t really seem too bad, when compared to most boat`s rigs which fall apart from doing nothing !

 

[boatmike]

I am a chartered mechanical engineer but also incredibly stupid sometimes and don't know everything! On a thread last year I was properly corrected by those with more rigging experience than I when I said most failures are probably not due to fatigue but overloading and corrosion. They were right and I was wrong. There is indeed a high incidence of true fatigue failures in conventional rigging. Since that time, I have talked to riggers and looked at a number of failures and agree they are indeed classic fatigue failures and my only excuse for my previous statement is that I have never examined such failures under a microscope before.

So here is what I know about fatigue.
1. It is not so much a function of loading (amplitude) as the number and frequency of reversals of stress. A fatigue breakage can happen were the loading is a fraction of the UTS of the component.
2. The weakest point as far as tensile' compressive, or flexural strength is concerned is not necessarily the point where fatigue breakage occurs
3. Any small crack, or even a scratch in a component can cause a "stress riser" which fatigue failures will proliferate from.
4. When viewed under a microscope fatigue failures exhibit a classic chrystalline structure that is quite unlike tensile fracture.
Therefore it is logical that fatigue tends to cause breakages wherever there are frequent reversals of stress and that it will probably (but not necessarily) happen wherever there is a stress riser. One such common failing is at the top of forestays where the roller reefing top bearing oscillates with the wind loading on the sail causing the stay to bend at the point it exits the bearing, and again just as the stay joins the terminal fitting. Same happens elsewhere but this is a common one. This is why as others have said, a slack forestay (for instance) will flex more that a tight one and may suffer higher fatigue, where if it were tighter the static load is higher but the flexing is reduced.
Things that help then are.
1. Keep rigging tensioned properly
2. Avoid rapid changes of section.
3. Radius all corners as big as you can
4. Taper the top exit hole of bearings that run on forestays to spread the load.
5. Keep rigging clean and polish components to as high a finish as possible.
6. Go to church on Sundays and pray it doesn't happen to you!

Hope this helps....... /forums/images/graemlins/wink.gif

 

[Benbow]

Two observations tht may help...

Traditionally rigged boats, with wooden spars often use galvanised rather than stainless rigging because it has much more 'give'. You don't wind the whole system in bar tight like you do on a modern boat. It maybe more suitable for your purpose for the same reason.

Also stainless is notorious for fracturing under shock loads - at least that why I was always told never to make my climbing gear out of stainless wire.

 

[boatmike]

It's perfectly true that galvanised carbon steel will be far less prone to fatigue than stainless.

 

[Sea Devil]

I had two shrouds fail in the South Pacific - Lowers on each side - had to sail some 45O miles with the mast partly held up by lots of rope standing rigging!

I had renewed all the rigging in Gibraltar a couple of years before at Shepard's - I made the Sta-lock connections and Shepard's did the swages for he ball ends that drop into slots in the mast.

Like most failures it was on the end of the swages - a few then more and more single wire strands started to break at the end of the swage - on one side there were only perhaps 2 or 3 left!!!!

According to the rigger in Tahiti the swages had been made with a press designed to swage galvanized wire for power cables hanging off towers - it produced a swage with sides to it - swages for marine - stainless purposes should be round.... This may well be true - my own feeling was that the wire had not been exactly central in the swage when it was pressed - or the angle of the 'ball' on the end of the swage was a bit off -

Hard to tell - no problem at all with the uppers - The previous rigging had been up for 18 years and showed no sign of wear but the insurance required I re-newed it - the new swages on the shrouds put up in Tahiti show no sign of failing - I am happy to say!

The next time I was in Gib I made a point of telling Shepherds about the problem and looked at their swage machine - They had a new one - a proper yacht one!

 

[Birdseye]

[ QUOTE ]
It's perfectly true that galvanised carbon steel will be far less prone to fatigue than stainless.

[/ QUOTE ]

That might well have been true in the old days when stainless was invariably made from scrap steel using small furnaces, even induction furnaces, but it certainly isnt generally true now. The fatigue life of steel relates to the cleanness of the steel - freedom from inclusions / dirt etc - as well as to other factors. So dirty carbon steel from 3rd world countries could cause lots of fatigue problems.

The disadvantage of stainless relative to drawn mild steel wire is its work hardening.

 

[boatmike]

I actually don't agree with you on this birdseye. I think all material from doubtful sources will be suspect. We must assume both are of reasonable quality to make a comparison. I also believe there is a correlation between work hardening and fatigue. Although it is not the same thing, a material subject to one will be more likely prone to the other although this is perhaps too sweeping a statement to make in general. Perhaps this is another subject that we need the input of a metallurgist on? Which is more likely to exhibit fatigue failure? Stainless or carbon steel rigging wire?

 

[Sea Devil]

In the case of my rigging failure I am pretty sure it was to do with the angle the SS wire came out of the swage down to the bottle screw - because of the design of the ball at the end of the swage it can only rotate a small amount - The wire failed because there was more pressure on the wire at the top of the swage than at the bottom - so the strands at the top were taking all the weight - and failed.

This was due to either the angle of the swage end into the mast or the wire being pressed into the swage not quite central.

My feeling is that it is more to do with mechanical problems rather than the quality of the wire....

 

[tonyran]

Interesting thread! There was an article about swaging in one of the sailing mags a year or two ago which emphasised the importance of using the correct materials and equipment for swages - with some pics of catastrophic failures caused by faulty rigging work.

 

[boatmike]

Thats where I was coming from originally because the majority of failures I had seen were caused by bad swageing, fittings not being provided with 2 way swivels, undersize rigging and a host of other things butr regardless of this I now believe there are many instances of fatigue, especially associated with roller reefing headsails that concentrate the stress on one place on the stay. That is exactly what has happened to snowleopard and his failure is classic of the syndrome.

 

[tcm]

i agree with all this.

I wd also say that the exact manufacturing method and the circumstances of the individual items thereafter will come into play in any real-world fatigue failure. So, gear that was carefuly carefully carried to the boat, and never ever slightly whacked will last longer than otherwise, even in the same batch. The things that seem to happen to masts/rigging in yards is somewhat frightening - but the effects will happen a long time later.

Manufacturingwise, the way in which the tensioners etc are made is important so i imagine that really fab forged items are the biz with a grain structure that follows the line of the eventual worked piece, altho most mass-produced seem to be simple machined items. I would again imagine wide variations in (not very mass produced) items to give similarly wide variations in mean time between failure (MTBF) a common engineering term but not one very much quoted it seems...