Lets talk about rope....

There have been quite a few topics discussing the particular properties of one type of high-tech line vs another and which is the better choice for a given application.

Of course there has also been alot of discussion using some technical terms, both correctly and incorrectly I'm sure, to validate one's opinion.

Being an engineer, it bothers me that there is not a clear understanding on what exactly people are saying when they say things like stretch, creep, elongation, etc... Each has a very different meaning as to what is happening to the material and why.

Figuring that since I'm not a rigger, I should do some research on what exactly all of this means. My observations of the discussion here is that there is no consensus on the description of these terms, and researching the manufacturer's literature about their products, there is woefully inadequate information.

I thought a discussion would help all of us learn a thing or two about high-tech lines and why we might want to use one over another in certain applications. So, would somebody qualified to do so please briefly outline the technical terms and qualities about rope, and how that might affect a sailor's decision to use one or another for a few applications?

For instance, I find it very frustrating, confusing and misleading that(near as I can tell) there are no published stress strain curves for the different materials! What is up with this??

 

Tige

Member
108
0
Do you have any relatives who have pursued your same career path in ..say... the Seattle area?

 

Tige

Member
108
0
Nah man, my family is all out east, save an uncle in Bingen WA who is my ma's bro, so not a miller.
No worries, was just being snarky as to a certain CA ex-poster who claimed to an engineer and seemed to think by mentioning that it somehow made his posting beyond reproach.

As to lines and ropes, all I can say is that it really hinges on it's intended use. Breaking it down by use would be the first step in comparing and contrasting different materials and braids.

ninja edit

 
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ADK

Anarchist
For instance, I find it very frustrating, confusing and misleading that(near as I can tell) there are no published stress strain curves for the different materials! What is up with this??
I think there may be a couple pretty reasonable reasons for that. First, there is always going to be some significant difference between the performance of the raw material and the performance of that same material when woven into a rope. A good bit of that is what we end up seeing as an initial stretch when the line is really loaded for the first time. The second thing is that a lot of the materials that are used for the modern high tech lines are visco-elastic materials (i.e. their strain is not dependent just on stress, but also on time and temperature). So, it's not really possible to publish a normal stress-strain curve. Examples of this are Spectra, dyneema and all related products that are all some type of polyethylene derivative. I'm pretty sure that Vectran & technora are aramid fibers which are not visco-elastic, but not 100%. The other thing, of course, is that the vast majority of people who are looking at rope specs would have no idea what to make of a stress-strain curve. I don't think the manufacturers are trying to hide the data, it's just that their intended market wouldn't know what to do with it or be particularly interested, so why bother. Anyway, just some thoughts.

 
For instance, I find it very frustrating, confusing and misleading that(near as I can tell) there are no published stress strain curves for the different materials! What is up with this??
I think there may be a couple pretty reasonable reasons for that. First, there is always going to be some significant difference between the performance of the raw material and the performance of that same material when woven into a rope. A good bit of that is what we end up seeing as an initial stretch when the line is really loaded for the first time. The second thing is that a lot of the materials that are used for the modern high tech lines are visco-elastic materials (i.e. their strain is not dependent just on stress, but also on time and temperature). So, it's not really possible to publish a normal stress-strain curve. Examples of this are Spectra, dyneema and all related products that are all some type of polyethylene derivative. I'm pretty sure that Vectran & technora are aramid fibers which are not visco-elastic, but not 100%. The other thing, of course, is that the vast majority of people who are looking at rope specs would have no idea what to make of a stress-strain curve. I don't think the manufacturers are trying to hide the data, it's just that their intended market wouldn't know what to do with it or be particularly interested, so why bother. Anyway, just some thoughts.

Right, so what do you call that initial stretch? I think the mechanism is fairly obvious, the line's weave becomes tighter. But there are other things happening as well. What are they and whats the proper name for it all so we don't all say different things but mean the same, or say the same thing and mean different. Its a picky point, I understand, but exactly what you say when discussing highly technical subjects matters if you wish to communicate clearly.

You mention correctly that all SK-XX products are Ultra High Molecular Weight Polyethylene derivatives, dyneema and spectra are just trade names, DUX falls into this category as well. On a molecular level, this means that they have long molecular chains and often cross-link. This matters to us because it means that their elastic strain range is (compared to aramid fibers) much smallwe. They do, however, have a very long plastic strain range. We can apply this knowledge by knowing that this is what helps to give the material its great abrasion resistance and the mechanism for how the line conforms to what its tied around or bent through w/o losing alot of it's strength. We also can figure that, due to the large plastic strains it develops, its probably not a great substitute for standing rigging unless you can often re-tension the line (leave long lashings).

We can also use this large plastic strain range to our advantage, as the manufacturers of Dux have. Once you plasticly deform a material, it does not return to its original form once the load is removed. (i.e. Load a bit of SK-XX to a high load, remove the load, and you will have a longer line than what you started with forever more) As I understand, the Dux has been highly preloaded during the manufacture to try and remove all those cross-links in the molecular chains, thus strain-hardening the material. So now, in order for us to experience the large plastic strains of the other SK-XX variants, you have to load the material above what the load during manufacture was, something that we most likely will not do on a sailing boat.

Nearly all materials are visco-elastic, its just to what degree that matters. UHMWP are very sensitive to temperature and load rate, but couldn't we agree on a standard temperature for static loads to easily compare the materials?

Twaron, vectran, technora, and kevlar are all aramid (aromatic polyamide) fibers. The molecular chains are straight and along the longitudinal axis of the fiber. This makes for a very high modulus. The lack of cross-linking means that there is a very small plastic strain range. This is why when aramid fibers fail, they fail spectacularly with very few warning signs, as opposed to UHMWP fibers, which fail like a plastic grocery bag. The straight molecular chains may also be the cause of why the material does not like being bent around things. Thus, aramid fibers are great for applications where you need a high modulus, but shouldn't be asked to make sharp bends under alot of load cycles. If using these for sheets, you've got to be sure that all the hardware involved in the system is up to spec, as the high modulus with no plastic strain does a great job of transferring all of the impulse from the line into the hardware. The plastic strains found in UHMWP fibers helps to provide some cushion in dynamic loading.

So, I guess we just described whats happening microscopically, but what about on a macro level? What, exactly, is creep? Is stretch a plastic strain or an elastic strain? What are the mechanisms of these things?

I know this maybe a bit more to know and talk about for most of us, but if you think this is over your head, then why do you care about which line to choose? Don't you want to be able to call bull-shit on someone if they try to sell you the wrong thing? I don't reckon there is an evil plot to keep consumers uninformed about the materials they're buying, but, if you're buying a product solely based on the material selection, shouldn't we be able to properly learn the materials properties and compare them to other products?

 

LakeBoy

Random Internet Guy
........

So, I guess we just described whats happening microscopically, but what about on a macro level? What, exactly, is creep? Is stretch a plastic strain or an elastic strain? What are the mechanisms of these things?

Creep, is the individual, usually a guy, probably an engineering type, that you really do not want on your crew.

 

dacapo

Super Anarchist
13,606
1,559
NY
madonna mia.............my head hertz

can't we all just go back to hemp :ph34r:

 
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TheTwister

New member
34
0
I have become confused as well. I used to think that for any kind of static application (ie. halyards) Vectran was required as any kind of spectra would creep.

Now I am reading that for most boats <40ft you probably won't be able to load the line enough for creep to show, or long enough if you are just going around the cans. Is this true, and at what point would you start using vectran.

It seems that for most people spectra is the better choice as if you cannot tell the difference in terms of creep it is much more durable, and here I have been wasting money on vecran halyards!

Would be interested in what people think.

 

markvannote

Member
376
25
Newport, RI
I have become confused as well. I used to think that for any kind of static application (ie. halyards) Vectran was required as any kind of spectra would creep.

Now I am reading that for most boats <40ft you probably won't be able to load the line enough for creep to show, or long enough if you are just going around the cans. Is this true, and at what point would you start using vectran.

It seems that for most people spectra is the better choice as if you cannot tell the difference in terms of creep it is much more durable, and here I have been wasting money on vecran halyards!

Would be interested in what people think.
It is true that in most applications on your average sailboat creep does not present itself as a huge problem. That being said, Dyneema stretches more than Vectran which is why Vectran is a good solution for jib and main halyards. Hampidjian (Dux) and Maffioli (Ultra) both have processes which heat the material while under tension which changes the molecular setup of the fiber in a way which I cannot site off the top of my head which affects it's stretch characteristics but mostly increases tensile strength upwards of 15-20 percent as well as compacting it. For the same diameter you typically have more fiber which makes a rope with less stretch. I do not know that anyone has actually compared the stretch characteristics between Dux or Ultra as compared to Vectran.

Back to my original point which is your standard Dyneema jib halyard will stretch more than Vectran. Also, Vectran double braid is typically less money than a Dyneema double braid.

Thanks.

Mark

 

Christian

Super Anarchist
I have become confused as well. I used to think that for any kind of static application (ie. halyards) Vectran was required as any kind of spectra would creep.

Now I am reading that for most boats <40ft you probably won't be able to load the line enough for creep to show, or long enough if you are just going around the cans. Is this true, and at what point would you start using vectran.

It seems that for most people spectra is the better choice as if you cannot tell the difference in terms of creep it is much more durable, and here I have been wasting money on vecran halyards!

Would be interested in what people think.
It is true that in most applications on your average sailboat creep does not present itself as a huge problem. That being said, Dyneema stretches more than Vectran which is why Vectran is a good solution for jib and main halyards. Hampidjian (Dux) and Maffioli (Ultra) both have processes which heat the material while under tension which changes the molecular setup of the fiber in a way which I cannot site off the top of my head which affects it's stretch characteristics but mostly increases tensile strength upwards of 15-20 percent as well as compacting it. For the same diameter you typically have more fiber which makes a rope with less stretch. I do not know that anyone has actually compared the stretch characteristics between Dux or Ultra as compared to Vectran.

Back to my original point which is your standard Dyneema jib halyard will stretch more than Vectran. Also, Vectran double braid is typically less money than a Dyneema double braid.

Thanks.

Mark
I have never seen such a cmparison either. My guess is that they will be very similar in terms of stretch and also in terms of weight (as the previously lighter Dyneema has been compacted quite a bit by the heated stretching). It also seems like the heated stretch of Dux makes it like sharp bends as little as Vectran but retains its better abrasion and UV properties. Vectran is significantly cheaper than Dux.

 
...flashing back to last years material science course...actually after that class I really wanted to get some stress/strain curves, I think layline printed the reults of some cyclic testing they did with a load cell a few years back, not sure if they still do that, but it kind of made me want to go back to get an engineering degree, hope it doesn't turn me into the creep on the boat.

 

markvannote

Member
376
25
Newport, RI
...flashing back to last years material science course...actually after that class I really wanted to get some stress/strain curves, I think layline printed the reults of some cyclic testing they did with a load cell a few years back, not sure if they still do that, but it kind of made me want to go back to get an engineering degree, hope it doesn't turn me into the creep on the boat.
Nothing against Walt and the guys who did the testing at Layline, their rig was very basic. Ours is similar which is why I do not go very far in publishing results we find. Funny enough, I asked one of our vendors to do some testing on SK-90, Dux, Vectran and the like yesterday before reading this topic. I will try to share what they come up with when they get around to doing it. I will also ask their permission to post the results they have actually found regarding SK-75 versus SK-90.

Thanks

Mark

 

Steam Flyer

Sophisticated Yet Humble
44,100
9,535
Eastern NC
Nothing against Walt and the guys who did the testing at Layline, their rig was very basic. Ours is similar which is why I do not go very far in publishing results we find. Funny enough, I asked one of our vendors to do some testing on SK-90, Dux, Vectran and the like yesterday before reading this topic. I will try to share what they come up with when they get around to doing it. I will also ask their permission to post the results they have actually found regarding SK-75 versus SK-90.

Thanks

Mark
That would be great, I have been trying to work up a set of useful data for shopping for various lines and not gotten very far. However I am also rather dissatisfied with the options of various rope specifically for halyards.. it's tempting to go back to wire!

FB- Doug

 

t_huebs

Member
236
23
Russ,

don't nerd out too hard, a lot of the data we might obtain from this is specific to material/brand/braid, etc.

- initial "stretch" isn't stretch. It's the application of load to align a suboptimal structure. How much a length stretches depends on it's construction, to include the weave and splices. Maybe better called set-lengthening.

- stretch is what happens to that optimal structure that can be undone - elastic

- creep is what happens to that optimal structure that cannot be undone -plastic

Dyneema does all of these things, you can spend money to have some of them done for you (see Dux 75) to eliminate variables

Vectran and other aramids tend to not do the latter two, but will still set a fair bit.

You could certainly go about determining a standard value for all lines based on an arbitrary length, which some people might find useful, I'd be down for a Miller quotient.

All of you, whatever you do, taper your splice, or I will be able to tell you exactly where your line will fail.

 

savoir

Super Anarchist
4,914
201
For instance, I find it very frustrating, confusing and misleading that(near as I can tell) there are no published stress strain curves for the different materials! What is up with this??
I think there may be a couple pretty reasonable reasons for that. First, there is always going to be some significant difference between the performance of the raw material and the performance of that same material when woven into a rope. A good bit of that is what we end up seeing as an initial stretch when the line is really loaded for the first time. The second thing is that a lot of the materials that are used for the modern high tech lines are visco-elastic materials (i.e. their strain is not dependent just on stress, but also on time and temperature). So, it's not really possible to publish a normal stress-strain curve. Examples of this are Spectra, dyneema and all related products that are all some type of polyethylene derivative. I'm pretty sure that Vectran & technora are aramid fibers which are not visco-elastic, but not 100%. The other thing, of course, is that the vast majority of people who are looking at rope specs would have no idea what to make of a stress-strain curve. I don't think the manufacturers are trying to hide the data, it's just that their intended market wouldn't know what to do with it or be particularly interested, so why bother. Anyway, just some thoughts.

Right, so what do you call that initial stretch? I think the mechanism is fairly obvious, the line's weave becomes tighter. But there are other things happening as well. What are they and whats the proper name for it all so we don't all say different things but mean the same, or say the same thing and mean different. Its a picky point, I understand, but exactly what you say when discussing highly technical subjects matters if you wish to communicate clearly.

You mention correctly that all SK-XX products are Ultra High Molecular Weight Polyethylene derivatives, dyneema and spectra are just trade names, DUX falls into this category as well. On a molecular level, this means that they have long molecular chains and often cross-link. This matters to us because it means that their elastic strain range is (compared to aramid fibers) much smallwe. They do, however, have a very long plastic strain range. We can apply this knowledge by knowing that this is what helps to give the material its great abrasion resistance and the mechanism for how the line conforms to what its tied around or bent through w/o losing alot of it's strength. We also can figure that, due to the large plastic strains it develops, its probably not a great substitute for standing rigging unless you can often re-tension the line (leave long lashings).

We can also use this large plastic strain range to our advantage, as the manufacturers of Dux have. Once you plasticly deform a material, it does not return to its original form once the load is removed. (i.e. Load a bit of SK-XX to a high load, remove the load, and you will have a longer line than what you started with forever more) As I understand, the Dux has been highly preloaded during the manufacture to try and remove all those cross-links in the molecular chains, thus strain-hardening the material. So now, in order for us to experience the large plastic strains of the other SK-XX variants, you have to load the material above what the load during manufacture was, something that we most likely will not do on a sailing boat.

Nearly all materials are visco-elastic, its just to what degree that matters. UHMWP are very sensitive to temperature and load rate, but couldn't we agree on a standard temperature for static loads to easily compare the materials?

Twaron, vectran, technora, and kevlar are all aramid (aromatic polyamide) fibers. The molecular chains are straight and along the longitudinal axis of the fiber. This makes for a very high modulus. The lack of cross-linking means that there is a very small plastic strain range. This is why when aramid fibers fail, they fail spectacularly with very few warning signs, as opposed to UHMWP fibers, which fail like a plastic grocery bag. The straight molecular chains may also be the cause of why the material does not like being bent around things. Thus, aramid fibers are great for applications where you need a high modulus, but shouldn't be asked to make sharp bends under alot of load cycles. If using these for sheets, you've got to be sure that all the hardware involved in the system is up to spec, as the high modulus with no plastic strain does a great job of transferring all of the impulse from the line into the hardware. The plastic strains found in UHMWP fibers helps to provide some cushion in dynamic loading.

So, I guess we just described whats happening microscopically, but what about on a macro level? What, exactly, is creep? Is stretch a plastic strain or an elastic strain? What are the mechanisms of these things?

I know this maybe a bit more to know and talk about for most of us, but if you think this is over your head, then why do you care about which line to choose? Don't you want to be able to call bull-shit on someone if they try to sell you the wrong thing? I don't reckon there is an evil plot to keep consumers uninformed about the materials they're buying, but, if you're buying a product solely based on the material selection, shouldn't we be able to properly learn the materials properties and compare them to other products?
Vectran is not an aramid. The general chemical name is liquid crystalline polymer . . . . . no, I don't understand the difference either but there it is. Twaron, technora and kevlar are all aramids but are not identical. Each manufacturer will tell you that theirs is best but I haven't the faintest idea which really is best for any application. Spectra/Dyneema are a polyehthylene and they are the same.

Spectra/Dyneema comes in 3 grades - 900 = SK60, 1000 = SK75, 2000 = SK78

Kevlar used in boating comes in 2 grades usually called 29 and 49

There isn't too much worthwhile data available on the stretch or creep properties of high tech rope under typical sailing loads but there is on stretch at break. Here are the stretch percentages

SK60 - 3.6%

SK75 - 3.2%

SK78 - 2.9%

Kevlar 49 - 2.8% ( kevlar 29 would stretch more )

Technora - 4.3%

Vectran - 3.3%

PBO - 2.5%

Polyester - stretches way more than all of these at break, roughly 15%

New England ropes say that their Vectran rope stretches 1.4% at 20% break load, PBO stretches 1% while SK75 stretches 1.25%. That is a fairly typical safe working load for a halyard.

If you aren't confused yet, read it again.

 

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