Compression of UHMWPE eg Dyneema

[Seaworthy Lass]

Over the last few years I have posted images and commented on the glassy, rock hard appearance that can occur when UHMWPE line (eg Dyneema, Spectra, Stealth-12, Acera….) is subject to high loads. I have frequently seen this at the base of stopper knots on soft shackles and where lines pass around low friction rings.

I have referred to this as “melting”, but a while back a member (I think it was Thinwater, I can’t find the post) suggested it was due to compression, not melting. Closer inspection has shown me that this is correct. The fibres have not melted at all.

When the area is bent back and forth and scrunched up (particularly after being washed) the appearance reverts to normal, at least to the naked eye.

I am currently replacing some of the UHMWPE ropework on board and a Dyneema strop with a loop cow hitched on was one of these items. The cow hitched portion of the attached loop (and the underlying Dyneema had taken taken on a glassy appearance. The cow hitch could not be undone and had to be cut off, but the underlying eye could be examine easily once this was done.

I have attached a few photos:

1 The cow hitch.

2 The super hard, glassy appearance of the underlying eye. The faint red mark was used during construction to note the peak of the eye. It serves as a reference point now.

3 The same region following being bent back and forth and scrunched up.

4 A zoomed in photo showing the individual fibres can still be seen. No melting has occurred.

Note the general “fluffy” appearance of the strop. One huge advantage of Acera and Stealth-12 is that this does not occur as easily, even after several years of use.

Does anyone know if permanent damage to the line has occurred following compression such as this?

SWL

[Breaking Waves]

Originally Posted by Seaworthy Lass

Does anyone know if permanent damage to the line has occurred following compression such as this?

possibly . . . probably at least a little.

The fluff indicates broken filaments which indicate reduced strength. Dyneema does 'flow' so some of the glass look could also be from that. There is also fuzz outside the compression area, so some of this is probably UV damage.

There is a lot of research on the compressive failure of Dyneema. Samson has a paper somewhere exactly on the compression within a cow hitch - looking at actual real-world samples - the strength losses were highly variable but under decent load, there was clearly systematic damage. There are also academic papers on compression - one I remember where they drop weights on Dyneema and could document the filaments 'shattering' - but that was really high point compression.

In most 'real world' applications other than the cow hitch it does not seem to be a huge issue although I do occasionally see it (one more reason to test new/novel constructions).

It is hard to say without testing it, but I would just guess a 20%ish loss in that sample you show the picture of. But that would be a super casual guess.

[ggray]

I was going to respond after reading the title and first paragraph of your post, but then realised I wouldn't be answering your eventual question.

But what the hey....

I cut open an old eyesplice I had made in a core dependent line, and was astounded by what I found. The surface of the tail of the amsteel that became the core of the splice was rock hard with peaks and crevices from compression by the amsteel weave that had surrounded it. It did indeed look like the amsteel had been melted and pressed in a form.

Anyway, I was impressed by the mechanical "keying" that seemed to have taken place, and had to help the strength of the splice.

[Breaking Waves]

Originally Posted by Seaworthy Lass

a Dyneema strop with a loop cow hitched on was one of these items.

Just out of curiosity, what specific application was this piece you are showing doing?

As I mentioned there are studies and papers, but I asked around a bit, and among the practical working best heavy lifting riggers and test labs I know it is a poorly understood phenomenon and people just try to design to avoid it. The test lab said they had once tried to study it and got highly variable difficult to summarize results - they 'thought but were not sure' that the glassy look was not compression damage but rather 'plastic flow', and that you needed further compression beyond that to get large strength loss.

[Breaking Waves]

One study on the topic that is more understandable and usable in a practical way on the topic is about tow lines used in the real world. these are high load commercial tows and there is compression of the lines against the winch drums - they conclude "Comparison of these test results indicates that there is strength reduction of
approximately 12 per cent before the line is end-for ended, caused by a compressive
compaction of rope and deformation on the winch drum. "

It is a quite readable paper - https://www.dsm.com/content/dam/dsm/...es_MAY2002.pdf

[gamayun]

Great thread! I've been wondering the same where I have Dynema shackles under high loads. I think the preventive maintenance approach on these should be to replace every 5 years or so also because of UV exposure but that's just my gut check and nothing scientific. They are used in some places that would be really painful if a block were to blow out.

[Seaworthy Lass]

Originally Posted by Breaking Waves

Just out of curiosity, what specific application was this piece you are showing doing?

I thought someone may ask .

The explanation is a little complex. Our yankee sheets pass between the shrouds and we need outboard sheets for wide wind angles, otherwise the sheet rubs on the shrouds. I cannot reach the clew to attach these when the sail is out, so I added an 8mm Dyneema strop (a bit less than 2 m long) and attached the “inboard” yankee sheets to this with a soft shackle. I found that if I did the same for the outboard sheet the soft shackle frequently became trapped on the transfer and could not be undone, so I added an 8 mm loop and attached the “outboard” sheets to that.

The loop was cow hitched to the long strop and that is where the glassy appearance occurred. I am replacing the system after 3 years of use. The headsail has often been out in 30+ knot winds on a broad reach so the loads have been high.

The soft shackles attaching the inboard sheets still look pristine over the same period. I am planning to ditch the loop and replace it with a soft shackle to avoid this issue in future. It has reinforced my dislike of cow hitches .

Originally Posted by Breaking Waves

As I mentioned there are studies and papers, but I asked around a bit, and among the practical working best heavy lifting riggers and test labs I know it is a poorly understood phenomenon and people just try to design to avoid it. The test lab said they had once tried to study it and got highly variable difficult to summarize results - they 'thought but were not sure' that the glassy look was not compression damage but rather 'plastic flow', and that you needed further compression beyond that to get large strength loss.

Thanks for the info. I am not familiar with plastic flow. I will need to do some reading. It is an interesting phenomenon.

SWL

[Seaworthy Lass]

Originally Posted by ggray

I was going to respond after reading the title and first paragraph of your post, but then realised I wouldn't be answering your eventual question.

But what the hey....

I cut open an old eyesplice I had made in a core dependent line, and was astounded by what I found. The surface of the tail of the amsteel that became the core of the splice was rock hard with peaks and crevices from compression by the amsteel weave that had surrounded it. It did indeed look like the amsteel had been melted and pressed in a form…

I have undone several eye splice now in single braid Dyneema (usually as I have either wanted to shorten them or to repurpose the line) and I have seen the same “pressed” appearance of the buried portion. It looks cool . It is not, however, glassy in appearance and it does readily transform to its original configuration with a bit of wriggling.

Originally Posted by gamayun

Great thread! I've been wondering the same where I have Dynema shackles under high loads. I think the preventive maintenance approach on these should be to replace every 5 years or so also because of UV exposure but that's just my gut check and nothing scientific. They are used in some places that would be really painful if a block were to blow out.

I understand that for 6mm line with high exposure, 10 year studies have shown strength drops around 50% after two years, then does little for the following eight.

I started off with all new rope work 3.5 years ago and (apart from one instance where chafe was a problem) I am replacing some of the soft shackles and stops for the first time. Most of them have been way over specified in terms of strength, and I have been sailing mainly in high latitudes so UV has not been a big issue, so my current criteria for replacement are physical signs of deterioration eg chafe or “fluffiness”.

Some of the applications are critical like yours (eg the boom brake and attachment of headsails to halyards), but these soft shackles still look pristine so I will leave these a bit longer before routinely replacing them.

SWL

[Breaking Waves]

Originally Posted by Seaworthy Lass

yankee sheets often been out in 30+ knot winds on a broad reach so the loads have been high.

Interesting. thanks. So I guess perhaps 2000kgs ballpark loads - would that be right - I guess possible transient shock loads possibly 50% higher - so 3000kgs? But 2000kgs for decently long sustained periods of time. And 8mm dyneema, so breaking strength around 6000kgs?


Thanks for the info. I am not familiar with plastic flow. I will need to do some reading. It is an interesting phenomenon.

My lab guys say that 'creep', which you will be familiar with in Dyneema, is a symptom of plastic flow along one-axis load path. What you see here (perhaps, if my test labs guys are right, which they are not 100% sure of) would be the same basic effect but 2 or 3-dimensional flow. I am not any sort of polymer chemist so any sort of explanation beyond that is way way beyond my pay grade.

.......

[thinwater]

Originally Posted by Seaworthy Lass

Over the last few years I have posted images and commented on the glassy, rock hard appearance that can occur when UHMWPE line (eg Dyneema, Spectra, Stealth-12, Acera….) is subject to high loads. I have frequently seen this at the base of stopper knots on soft shackles and where lines pass around low friction rings.

I have referred to this as “melting”, but a while back a member (I think it was Thinwater, I can’t find the post) suggested it was due to compression, not melting. Closer inspection has shown me that this is correct. The fibres have not melted at all.

When the area is bent back and forth and scrunched up (particularly after being washed) the appearance reverts to normal, at least to the naked eye.

I am currently replacing some of the UHMWPE ropework on board and a Dyneema strop with a loop cow hitched on was one of these items. The cow hitched portion of the attached loop (and the underlying Dyneema had taken taken on a glassy appearance. The cow hitch could not be undone and had to be cut off, but the underlying eye could be examine easily once this was done.

I have attached a few photos:

1 The cow hitch.

2 The super hard, glassy appearance of the underlying eye. The faint red mark was used during construction to note the peak of the eye. It serves as a reference point now.

3 The same region following being bent back and forth and scrunched up.

4 A zoomed in photo showing the individual fibres can still be seen. No melting has occurred.

Note the general “fluffy” appearance of the strop. One huge advantage of Acera and Stealth-12 is that this does not occur as easily, even after several years of use.

Does anyone know if permanent damage to the line has occurred following compression such as this?

SWL

Nice work and nice documentation, as always.


Two things, from Samson:
COMPRESSION
Often seen on winch drums, compression
is caused by fiber molding itself to the
contact surface while under a radial load.
Compression can be identified by a visible
sheen and stiffness that can be reduced
by flexing the rope. Compression is not a
permanent characteristic and should not
be confused with melted or glazed fibers.

Also see the last page of this document: https://www.samsonrope.com/docs/defa...ing-insert.pdf


Samson seems to consider this normal use. Yes, there is some fuzzing so, guesses of 10-20% loss in strength seem reasonable. The rope is not new. I've seen the same thing on rope used in rope pull test rigs that I know for certain never exceed 20% breaking strength. I'm also guessing your load was not less than that, because that seems to be about what it takes to make it look like that. My feeling is that a few more years would be fine, still withing the safe working load, but not more than that.

I've only seen that much "shine" on test rig eyes. Amazing, how it fluffs back out.

As for when something gets replaced, that depends on the safety factor and how critical a failure would be.

[Seaworthy Lass]

Originally Posted by Breaking Waves

Interesting. thanks. So I guess perhaps 2000kgs ballpark loads - would that be right - I guess possible transient shock loads possibly 50% higher - so 3000kgs? But 2000kgs for decently long sustained periods of time. And 8mm dyneema, so breaking strength around 6000kgs?

Our headsail area is around 52 m2 so 2000 kg is probably a reasonable estimate.
The load on the loop and the eye portion of the strop would be halved though.

The breaking strength for 8 mm line used was around 6000-7000 kg (it was actually Dyneema, but I did not take note of the brand when purchased through a Netherlands chandlery). I have seen the same changes with Acera and Stealth-12 as well.

Originally Posted by Breaking Waves

My lab guys say that 'creep', which you will be familiar with in Dyneema, is a symptom of plastic flow along one-axis load path. What you see here (perhaps, if my test labs guys are right, which they are not 100% sure of) would be the same basic effect but 2 or 3-dimensional flow. I am not any sort of polymer chemist so any sort of explanation beyond that is way way beyond my pay grade.

The changes seen are unique and quite bizarre. I was convinced it was melting, as individual fibres could no longer be seen, even under high magnification. The surface becomes uniformly glassy in appearance and rock hard to touch (cannot be indented with a fingernail). I find it amazing that the line coverts to a “normal” appearance with prolonged wriggling and squishing.

I first noted this in 2016 on the underside portion of line passing over the top of low friction rings.

This is an image of the base of a stopper knot that I presented last year:

[Breaking Waves]

Originally Posted by Seaworthy Lass

2000 kg is probably a reasonable estimate The load on the loop and the eye portion of the strop would be halved though. The breaking strength for 8 mm line used was around 6000-7000 kg

yea, you would think that is all more than satisfactory.

The changes seen are unique and quite bizarre.

I'm sure you know you can get 'solid sheet/block' 'dyneema' (eg uhmw in solid chunk forms), and it is like the filaments are flowing into that form. But idk, this is speculation (from some decently informed people, but who are not chemists).

.......

[BigBeakie]

SWL,

Wondering if this factors into rigging the JSD?

Would the cow hitched bridle eyes to the JSD leader eye junction suffer in any way from repeated high loads in a storm?

By the way, I've communicated with the guys at the Youtube channel How NOT to Highline & they have agreed to test this JSD junction ( bridle lines to JSD leader) on their massive test rig.They are expanding out from their climbing/highlining focus to test high load applications in other sports and sailing is one of them. They've done some sailing related dyneema testing already if you haven't seen it.

If you're interested in them doing more sailing related testing, let them know in the comments section of their vids.

[Seaworthy Lass]

Originally Posted by BigBeakie

SWL,

Wondering if this factors into rigging the JSD?

Would the cow hitched bridle eyes to the JSD leader eye junction suffer in any way from repeated high loads in a storm?

By the way, I've communicated with the guys at the Youtube channel How NOT to Highline & they have agreed to test this JSD junction ( bridle lines to JSD leader) on their massive test rig.They are expanding out from their climbing/highlining focus to test high load applications in other sports and sailing is one of them. They've done some sailing related dyneema testing already if you haven't seen it.

If you're interested in them doing more sailing related testing, let them know in the comments section of their vids.

I first need to correct a term I have been using before someone else does . The Cow Hitch comes in various forms. When loaded on both sides (as in the photos in post #1) it is called a Girth Hitch.

When the same style of knot is used to connect two loops or eyes of roughly the same diameter it is called a Strop Bend and it takes on a very different configuration and one that results in less tight turns. It is fairly easy to undo even when heavily loaded in UHMWPE and for this reason I do not think it significantly effects the strength of joining the eyes of two single line strops, as very commonly used between sections of series drogues.

However, a Girth Hitch is the standard connection that is still being suggested for the junction between the bridle and the first section (the eyes of the bridles being individually girth hitched to the eye of the first section) and I have never been comfortable with this for several reasons. I instead constructed our series drogue using a Modified Strop Bend where the eyes from the bridle were treated together as one unit and strop hitched to the eye of the first section.

The best method for connecting the bridle to the leader was discussed at length in this thread:
https://www.cruisersforum.com/forums...es-216499.html

Many methods for this junction were proposed (thanks to everyone who participated for a great discussion) and the one that appealed to me most was made by WSmurdoch. I am not aware of the correct name for this type of join, so I have been referring to it as a Modified Strop Bend as it is near identical in configuration.

The following images show the difference between the various forms of connections (all tied in a similar manner, but with very different results).

SWL

[Seaworthy Lass]

PS This is a side view of a Modified Strop Bend and a top view before load is applied.

I think Fxykty has now also used this at the bridle junction of his series drogue, but I am not aware that anyone else has done so.

It avoids the use of two Girth Hitches at this junction.

It remains untested, so use at your own risk: