Shroud failure diagnosis

[boatmike]

Common experience there then Vyv! I retired at 60 and am now 71 but worked on the development of carbon fibre blades on RB211 all that time ago.... First stage blade failure can make a royal mess can't it!!

Mike

 

[lw395]

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.

Thanks for the clarification.

Should we expect a certain number of early failures anyway?
If 10 years is the design life and 99.9% last comfortably longer than that, will .001% fail in half that time? Just by chance?
That would still be a few yachts a year.

One of my dinghy failures was at the bottom of the shroud where it was not possible for it to be an alignment issue.

 

[boatmike]

Thanks for the clarification.

Should we expect a certain number of early failures anyway?
If 10 years is the design life and 99.9% last comfortably longer than that, will .001% fail in half that time? Just by chance?
That would still be a few yachts a year.

One of my dinghy failures was at the bottom of the shroud where it was not possible for it to be an alignment issue.

One of the problems with fatigue life prediction is that the magnitude and frequency of reversals (vibration if you like) has to be constant. In identical or very similar conditions like a turbine blade this is possible to predict to a reasonable level of certainty but on boats it is such a variable that only "guesstimates" can be made. The idea that all rigging has a 10 year design life is therefore rather tenuous. All that can be said is that it is prudent to inspect regularly and replace within 10 years or so to be as safe as possible. No two boat installations will be the same

 

[vyv_cox]

Over and above what boatmike says, it is the 'initiation' period that is impossible to predict. Experts have discussed this for many years, with no outcome. A single corrosion pit or scratch could compromise any calculated figure. Crack propagation may then continue in a regular manner but it is the nature of machines (including boats) that use is irregular, with highly loaded periods possibly followed by lower loads or no use at all. I suspect that your .001% figure is quite a likely scenario, given that the odd rig will not be assembled perfectly.

 

[Bosun Higgs]

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.

Sorry Vyv but you did say " the failed strands showed both to have failed deep inside the swage. The ends inside the swage were not visible" and did not mention 2mm. Deep is not 2mm. If this is where they failed and the fitting at that point provided internal clearance to allow flex then thats different. The shaft of the T ball after the forged in bend should be straight. The curve suggests poor swaging to me. Given the flat surface of the T, this could result in there having been a bend where the wire came out of the swage. All depends on whether the T had some sort of female mast fitting which allowed it to articulate. Either way its more than likely to have been slack rigging at issue

 

[vyv_cox]

Sorry Vyv but you did say " the failed strands showed both to have failed deep inside the swage. The ends inside the swage were not visible" and did not mention 2mm. Deep is not 2mm. If this is where they failed and the fitting at that point provided internal clearance to allow flex then thats different. The shaft of the T ball after the forged in bend should be straight. The curve suggests poor swaging to me. Given the flat surface of the T, this could result in there having been a bend where the wire came out of the swage. All depends on whether the T had some sort of female mast fitting which allowed it to articulate. Either way its more than likely to have been slack rigging at issue

Well, too deep for me to see the failed ends. Might be 3 or 4 mm, impossible to tell. See the photo.
There is absolutely no curve in the swaged section, perfectly straight against an edge. Refer to my photo in post #1 relative to the line I have added. The first bend in the fitting is around 25 mm beyond the extremity of swaging.

 

[timmygobang]

For anyone interested, vyv has kindly written up a detailed analysis of the failed shroud

http://todayiboughtaboat.eu/wp-content/uploads/2013/10/Failed-shroud.pdf

Thansk vyv for taking the time to look at it.