Compression of UHMWPE eg Dyneema

[kryg]

Thanks, interesting paper. I'm new to using Dyneema - I just had my boat fully rerigged from S/S wire to dyneema. Saved 240 kilos in weight. However, I have no idea how it will perform when stressed. Next step is to use dyneema soft shackles for some reachers on the outer fore stay? I wonder if I will experience that glassing effect? On Dyneema soft shackle or the Dyneema forestay?

Originally Posted by Breaking Waves

One study on the topic that is more understandable and usable practically 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

[Breaking Waves]

Originally Posted by Seaworthy Lass

I am not aware that anyone else has done so.

edit: sorry this is a bit rambling

There are commercial people using this in lifting and pulling applications. They are comfortable with it given they (the ones I have seen) derate it by 20%. I have never seen it formally tested but it must have been given the liability and care to details in these applications.

I think samson calls as a cow hitch what you call a strop hitch - am I right? This from samson rope use manual:



Personally, I think if samson is comfortable with this construction derated by 15% (and I am sure they have data to support that derating, and as I say above the guys I know add an additional 5% derating for conservativeness) - then one would be comfortable with your 'double modified strop' construction since it should be the same or better (less bend for at least one set of bends).

edit: I could get all this tested, or do it myself, but honestly I have not yet seen much reason to get energized about it because I think the uncertainty is pretty low if you derate per the commercial guidelines. It is not a complicated construction. I would feel differently if I had ever seen one of these sorts of joins (or even what you call the conventional "two in series" cow hitches) broken or severely damaged.

Your compression question is an interesting one - something would be interesting for a dedicated test series/study, with lots of potential aspects to it, and seemingly little actual quantitative knowledge even among the industrial riggers. They don't seem to think it is important, but they may well be wrong, as they in fact have been about some Dyneema construction aspects in the past.

Some of the 'highest tech/most sophisticated/mission critical' applications I have recently worked on have in fact not been Dyneema, but rather using Technora - because it has proven to be less vulnerable to various externalities and uncertainties and 'unknown/unknowns' than Dyneema.

Be interested to see what the Highline guys do with it - I loved their high line vids because I learned about things I did not know (like bolting) but have not been so impressed by their to-date sailing/soft shackle stuff because they are behind the learning curve there and learning/demonstrating stuff that is already well known - their 'sailor' unfortunately does not seem to be up on best practices.

[Seaworthy Lass]

Originally Posted by Breaking Waves

There are commercial people using this in lifting and pulling applications. They are comfortable with it given they (the ones I have seen) derate it by 20%. I have never seen it formally tested but it must have been given the liability and care to details in these applications.

I think samson calls as a cow hitch what you call a strop hitch - am I right? This from samson rope use manual:

Attachment 248994

Personally, I think if samson is comfortable with the construction derated by 15% (and I am sure they have data to support that derating) - then one would be comfortable with your 'double modified strop' construction since it should be the same or better (less bend for at least one set of bends).

Yes, Sampson call a Strop Bend a Cow-Hitch connection in their Rope Users Manual (probably what made me follow suit in error) and state it results in a loss of strength of roughly 15% (rope material unspecified although single braid of some kind is shown in the accompanying photo). This is commonly used to connect sections in series drogues.

What I have not seen used before is a Modified Strop Bend (my term as I have no idea what to call it). Are you saying that commercial people are using a bend where on one side of the bend TWO eyes are treated as one? This is not a connection I have seen described anywhere else before. My guess is the same as yours: that this will not derate the strength of the junction by much more than an ordinary Strop Bend. I felt confident enough to use this join in our system, but this decision was based only on educated guesswork.

SWL

[Breaking Waves]

Originally Posted by Seaworthy Lass

Yes, Sampson call a Strop Bend a Cow-Hitch

good. at least I know what people are talking about

Are you saying that commercial people are using a bend where on one side of the bend TWO eyes are treated as one?

Yes, I have now (i have been keeping my eye out since it has come up) seen it used exactly as you show and in fact occasionally with 4 eyes on the one side (for instance for 4 lines to the four corners of a lift), but still have not found any specific test results for this. More common is to use various steel parts, where you don't have to derate, in the connection but this is recognized/used as an option.

These are generally more 'static' loads than a series drogue, and I have not yet seen this in a high load tow which might be closer to the series load profile.



SWL

......

[Seaworthy Lass]

Originally Posted by Breaking Waves

Yes, I have now (i have been keeping my eye out since it has come up) seen it used exactly as you show and in fact occasionally with 4 eyes on the one side (for instance for 4 lines to the four corners of a lift), but still have not found any specific test results for this. More common is to use various steel parts, where you don't have to derate, in the connection but this is recognized/used as an option.

These are generally more 'static' loads than a series drogue, and I have not yet seen this in a high load tow which might be closer to the series load profile.

Brilliant! Many thanks for reporting this. It is extremely useful information.

I am not overly concerned with a 15-20% loss of system strength, as the diameter of the UHMWPE I used was generous and I suspect close to 20% loss of system strength is already occurring due to the tapes of the cones threaded through and sticking out of the braid. The losses are not cumulative.