Anatomy of a Roller Swaged Fitting

[TradewindSailor]

I've often wondered how well a roller swage fitting integrates with the wire.

I thought you might be interested in these photos of the eye terminal (top) of a 3 yr old shroud.

Points:

The wire is 12mm

The hole is (16.5mm) stretched to 17mm toward the end. Standard 12mm eye terminals are 19 mm diameter.

The crack? In the end of the terminal .... Impossible to see whilst rigged due to the toggle

The roller swage was one pass rather than using a second pass to roll out the shoulders as is this riggers usual practice. You can see that the swaging process was complete with good forming of the swage.

Despite this terminal being the top terminal, dirt has penetrated 30 mm into the swage. If this was a lower terminal it could be more. This would support the crevice corrosion cause of failure for these fittings in general. ( This fitting is 3 years old).

The fitting is slightly bananaed .... but not enough to worry me

One strand of the 316 1 x 19 wire was broken at the internal shoulder of the swage entrance. This could be due to accentricity during swaging. Broken strands like this are much more visible when the wire is loaded ... The tend to contract back into the fitting when not.

Any thoughts?

 

[vyv_cox]

Interesting photos. Can you manage a very close-up shot of the fracture face of the broken wire?

 

[TradewindSailor]

Hi Vyv ..... Unfortunately the tablet won't focus on the tip of the wire. I'll try to describe it instead:

Firstly the break was at the inner shoulder of the swage, 2mm from the end of the swage.

The end of the broken wire shows 3 semicircular ridges evenly spaced across the face of the break, generated from where the wire met the shoulder. The first ridge is 20% in from the edge and is more prominent than the other two. I can see no marks at all before the first ridge.

Interestingly the marks around the wire bundle at the shoulder are difficult to remove even with emery paper. This would suggest to me that it could be the start of crevice corrosion ... micro-pitting

I cannot identify any plastic necking.


I suspect the first 20 % crack generated from high-cycle, low-stress fatigue .... followed by low-cycle, high-stress fatigue.

The high stress was certainly due to the slack lee shroud snatching whilst working through 2.5m head seas.

Both ends of the shroud were well toggled so there should be no reason for one of the 1x19 strands to be higher stressed than the others unless there was accentricity in the swaging process. This wire was on the aft edge of the swage fitting.

I have 3 interesting photos but I can't upload them because apparently there is no space ??

 

[TradewindSailor]

Just boosting this in case Vyv hasn't seen it

 

[vyv_cox]

There are photos of a fatigued swaged fitting at http://coxengineering.sharepoint.com/Pages/Fatigue.aspx I suspect that the location of the failed wire was dictated by the banana that you have mentioned but the cause seems to have been slackness.

What you describe as the onset of crevice corrosion looks to me that it might be fretting. There is a noticeable red deposit extending down into the swage that has the appearance of alpha Fe2O3. Difficult to tell at this magnification. Quite possibly some corrosion as well, which would occur readily on the nascent surfaces exposed by fretting.

 

[TradewindSailor]

Thanks Vyv.

Very clear illustrations on you site ...

You have a photo showing the fatigued ends of 2 broken strands. My broken strand shows similar markings. The bright area on the edge that may be the initiator is interesting. It shows up on both your wires and mine ..... I wasn't sure whether this was the cause of the damage or caused post damage by movement of the loose end at the entrance of the swage. As there was no sign of a similar mark on the mating side, it suggested to me that it was post break .... Now I'm not so sure ......

As for fretting ..... I am sure you are correct as the brown deposits are only on the surface of the wires where they are in contact with the shoulder ..... i.e. it doesn't go any further around the strand. Looking at the end of the swage fitting: the discolouration extends to at least 12mm into the swage. Interestingly this discolouration is on the valleys in the swage fitting, whereas the peaks are shiny, the peaks being where the swage is pressed in between the wire strands. This would suggest to me that the wire does work within the swage for at least this distance. The obvious question is why the permanent marks on the wire are only in way of the shoulder?

 

[TradewindSailor]

Vyv .... Have you come across similar breaks with swageless fittings?

 

[vyv_cox]

No but there are far fewer of them of course.

Following your example I have cut open the fatigued fitting referenced earlier. I have added the new photos and comments to that page. http://coxengineering.sharepoint.com/Pages/Fatigue.aspx Two more broken strands found inside the fitting, possibly more but I don't want to cut any further. A lucky escape for the owner.

 

[TradewindSailor]

My shrouds are now Dynex Dux so I won't be getting the same problem with the wires, but of course the snatch loads caused by a slack rig will still be transmitted through the rest of the rig. At least the end terminal factor of safety is at least double that of the old cables. Moral of this story is to ensure that the Lee shroud never goes slack. ...... either more pretension, or reefing earlier!