Shroud failure diagnosis

[vyv_cox]

Timmygobang sent me a failed shroud to look at. Here is the report I prepared for him, which he agreed I should post. After I suggested that slackness in the shroud was the cause, Tim told me that that was the reason for the rig inspection. It's something of a change for me, having been used to using microscopes costing in the £1000s to now use one that cost about £20 but I think the image is clear enough.

During a rig inspection by Holman Rigging in 2013 a lower shroud from a sloop rigged Halmatic 30 was found to have two broken strands. The rigging was supposedly replaced in 2006 by the previous owner.
The failure occurred at a swaged joint on a T-ball fitting at the upper end of the shroud. Two strands of the 1 x 19 cable were broken and unravelled from the cable. The swage was straight and appeared to be well made. Projection of the line of the swaged joint passed through the bearing point of the T-ball.

Close examination of the area around the failed strands showed both to have failed deep inside the swage. The ends inside the swage were not visible. Examination of the failed ends of each strand revealed beach marks, clearly on one, with little polishing, less clearly on the other but with considerable polishing.

Beach marks indicate the cause of failure to be fatigue. Their appearance can be likened to ripples on water spreading out when a stone falls into a pond. Quite commonly the two sides of the crack rub together before fracture takes place, polishing away the beach marks.

Fatigue occurs when a crack initiates, usually on some form of stress raiser, on the surface of a component. The crack propagates through the component, driven by fluctuating stress, until the area of metal remaining is insufficient to support the load being applied, upon which it finally fractures. In the case of this shroud there is a small wear scar coincident with the initiation point, probably caused by one wire rubbing over another. Wear scars are a common initiation point.

The swage was well made, with no ‘banana’ shape nor offset that would preferentially load the wires on one side. Both are well known causes of fatigue in rigging. In view of this, it is most likely that fatigue occurred due to undertightening of the rig, probably the most common cause of this type of failure.

 

[30boat]

I'm curious,could this type of failiure happen to a well assembled Stalok terminal?

 

[vyv_cox]

I'm curious,could this type of failiure happen to a well assembled Stalok terminal?

Just about anything can fail in fatigue if the conditions are right A famous cause of forestay fatigue failures occurred because the clevis pin joint at the base of the Stalok was unable to articulate fully, preferentially loading one side. Staloks are well designed and a well-assembled one should be reliable provided there is full articulation with toggles and the rig is tight.

 

[30boat]

Just about anything can fail in fatigue if the conditions are right A famous cause of forestay fatigue failures occurred because the clevis pin joint at the base of the Stalok was unable to articulate fully, preferentially loading one side. Staloks are well designed and a well-assembled one should be reliable provided there is full articulation with toggles and the rig is tight.

I have toggles on all shrouds and at the top and bottom of the forestay so I believe I'm ok.

 

[doug748]

I had a forestay which failed in a similar manner. By good fortune I had the mast down and could see the broken strands, just visible, below the top of the furling extrusion.

The rigger said he often saw similar problems He thought due to owners letting off the backstay too far after sailing. I can credit this, as you often seem to see very loose reefing foils actually flogging in the breeze.

 

[GHA]

Thnx for taking the time to create such an interesting post

 

[GrahamM376]

Looking at the terminal, it doesn't appear to be chamfered. Evidently those are more prone to wire failure if the terminal is slighty offset to the wire. Any thoughts?

 

[vyv_cox]

Looking at the terminal, it doesn't appear to be chamfered. Evidently those are more prone to wire failure if the terminal is slighty offset to the wire. Any thoughts?

Yes, probably a good point as a square edge could create a stress raiser. There is a somewhat similar pic on my website at http://coxengineering.sharepoint.com/Pages/Fatigue.aspx where the failures clearly originate very close to the sharp internal corner. In the one described above the wires have broken inside the terminal, apparently where the swaging stops but a couple of millimetres from the corner, so the stress raiser is perhaps the point at which restraint of the wire stops.

 

[Bosun Higgs]

Not confident of your conclusion vyv. You say that the fatigue failure was deep inside a properly made swage but if this is the case how would the wire get sufficient movement to fatigue? It is unlikely to get lateral movement - the failure would be at the end of the swaged fitting if this were the case. Nor could it let longitudinal cycling - again that would be far less inside a swage than on the outside wire.

Your photo isnt clear enough for me to be sure but it looks to me as if the swage is badly formed with only part of the fitting actually reduced in size. How far inside the fitting was the failure?

 

[vyv_cox]

Not confident of your conclusion vyv. You say that the fatigue failure was deep inside a properly made swage but if this is the case how would the wire get sufficient movement to fatigue? It is unlikely to get lateral movement - the failure would be at the end of the swaged fitting if this were the case. Nor could it let longitudinal cycling - again that would be far less inside a swage than on the outside wire.

Your photo isnt clear enough for me to be sure but it looks to me as if the swage is badly formed with only part of the fitting actually reduced in size. How far inside the fitting was the failure?

As I said previously, fracture was a couple of millimetres inside the fitting. The bore inside the neck is somewhat larger, providing sufficient internal clearance that the wire would flex. This seems to be a consequence of a slightly reduced OD at the neck region of the fitting, hence there is no/less compression during swaging of this part. I assume this to be a measure to avoid the situation that swaging continues to the very end, creating a stress concentration there. I see no evidence whatsoever that the swage has been made badly and the T-ball fitting seems to be quite a clever design.

 

[Martin_J]

Vyv
Out of interest - How were the broken strands found?

It looks like they would not have 'popped' out of the swage unless the wire was bent to release them (as in the photos).. or did they just appear to be slack further down the wire and it was this loose looking two strands that was the give away factor?

 

[vyv_cox]

Vyv
Out of interest - How were the broken strands found?

It looks like they would not have 'popped' out of the swage unless the wire was bent to release them (as in the photos).. or did they just appear to be slack further down the wire and it was this loose looking two strands that was the give away factor?

Sorry, can't answer that. Timmygobang sent the shroud to me after the inspection discovered it. No more than 5 cm along the wire there is no evidence of the broken strands.

 

[lw395]

It looks curved to me.
The failed strand is on the outside, a classic for the t-ball not pivoting enough and making a less acute angle with mast than the shroud as a whole?
You don't have to look far to find boats with that.

 

[vyv_cox]

It looks curved to me./QUOTE]

Should have gone to specsavers! It is perfectly straight.

 

[lw395]

It looks curved to me./QUOTE]

Should have gone to specsavers! It is perfectly straight.

Not the swage, but the wire leaving it.
I know it's been bent to show off the broke strands.
But is it supposed to be just coincidence it's failed on the outside?

BTW, why is it always two broken strands?
I've caught two dinghy rigs in the nick of time, both with two strands gone.
That's 2 out of 7 in 3mm Dyform.

 

[vyv_cox]

Not the swage, but the wire leaving it.
I know it's been bent to show off the broke strands.
But is it supposed to be just coincidence it's failed on the outside?

BTW, why is it always two broken strands?
I've caught two dinghy rigs in the nick of time, both with two strands gone.
That's 2 out of 7 in 3mm Dyform.

I have to assume that the wire was bent to release any fractured strands, it arrived with me like that.

I repeated the checks I carried out earlier. The swage is perfectly straight when lined up with a 6 inch rule in both rotations at 90 degrees to each other. The inner faces of the 'T' are evenly polished with little fretting, suggesting that they have been loaded correctly. The scar at the inside of the point where the T-ball bears on the mast socket looks pretty much symmetrical. An older design of T-ball fitting did cause asymmetric loading, which is why I drew the projected line on the photo, showing pretty clearly that the loading was fair. I know of a couple of cases in which the old design led to cracking in the socket, or even in the mast, but this has not been seen as far as I know.

I cannot say why the two outer strands failed, but note that they are not adjacent, there is a fixed strand between them. I may try to cut the fitting open to investigate further but I don't really have the equipment for this kind of accurate work nowadays.

 

[maxi]

For me, the prime suspect would be a misalignment in the shroud, almost certainly caused by the 'T' bar not sitting completely in it's socket.

The position of the failure suggests that, as a result of the dog leg effect of the misalignment the outer 2 or 3 strands of wire have been doing all the work with little assistance from the remaining circa 16.

It is depressingly common to see this type of misalignment, but very easy to correct.

 

[vyv_cox]

For me, the prime suspect would be a misalignment in the shroud, almost certainly caused by the 'T' bar not sitting completely in it's socket.

Not what the observation says. The T-ball appears to have been seated accurately with equal contact on each leg and across each face.

 

[boatmike]

Very interesting post Vyv. I too have done some limited work on the analysis of fatigue and it might help to explain to those who obviously don't understand your point that you don't need to excessively bend or unequally load a component to cause it. In fact the classic definition of fatigue is continuous reversal of stress well below the elastic limit of the component (you could probably elaborate in more detail on that). It is interesting that even a small but continuous vibration at some stress riser point, which can sometimes be just a scratch in the surface originally, can cause eventual failure and you are of course right that slack rigging rattling around in the wind might well be a cause. The problem with fatigue, as was first defined on the old Comet window in the early 60s when the phenomenon was first discovered, is that it is counter intuitive. Who could have thought that sometimes, a design could be increased in fatigue life by several times by just polishing it?
I am sure that many people think that over tightening rigging puts exessive stresses in to their boat structure, and of course in extreme cases they could be right. If the rigging itself is considered however slack rigging could be very bad news from a fatigue failure standpoint, which may be counter intuitive, but very true.

 

[vyv_cox]

Very interesting post Vyv. I too have done some limited work on the analysis of fatigue and it might help to explain to those who obviously don't understand your point that you don't need to excessively bend or unequally load a component to cause it. In fact the classic definition of fatigue is continuous reversal of stress well below the elastic limit of the component (you could probably elaborate in more detail on that). It is interesting that even a small but continuous vibration at some stress riser point, which can sometimes be just a scratch in the surface originally, can cause eventual failure and you are of course right that slack rigging rattling around in the wind might well be a cause. The problem with fatigue, as was first defined on the old Comet window in the early 60s when the phenomenon was first discovered, is that it is counter intuitive. Who could have thought that sometimes, a design could be increased in fatigue life by several times by just polishing it?
I am sure that many people think that over tightening rigging puts exessive stresses in to their boat structure, and of course in extreme cases they could be right. If the rigging itself is considered however slack rigging could be very bad news from a fatigue failure standpoint, which may be counter intuitive, but very true.

Absolutely right Mike. The example shown, pic 4 on my website http://coxengineering.sharepoint.com/Pages/Fatigue.aspx is on a boat of which there are hundreds of examples that suffer no failures. On this one the mast fell down twice (!!!) simply because the owner slackened the rig off before leaving the boat. It lived on a relatively exposed mooring, so suffered almost constant wave motion in addition to vibration caused by the wind.

Some of the most dramatic fatigue failures I have worked on were in the compressor section of industrial gas turbines. The 1st and 2nd stage compressor blades on a big GT are meaty pieces of engineering with a root around 1 x 4 inches and the blade nearly a foot long in some cases. A small change in the angle of one stator blade can send a tiny pulse of pressure the wrong way, ultimately resulting in failure of one of these blades. Sod's law then determines that the blade goes right through the machine, wrecking the compressor stage and sometimes even the turbine. £10M to fix.