Line Tension v. Strain Testing

[Buoyboy]

I have a new multi-fiber braided mooring line that is to be used for deep sea buoy moorings. I need to generate a function for axial stiffness (EA) in order to model this line in a computer program. We have a line tester and have generated a Tension v. Strain curves, which are expectedly non-linear (5th order polynomials). From these functions we differentiate to get our EA.

There’s the back story, now my problem lies with the behavior of this line under a load. We load the line to 100 lbs and place our sensors on the line, then pull it up to 50% of its breaking strength(~3500 lbs.), then relax it to about 100 lbs again. From there we repeat this loading cycle multiple times. On every test after the initial pull we generate values for strain that are up to 30% less than those of the first pull. The lines length does not change (ie. we have not plastically deformed it and the initial length is roughly the same, within 1%); the line simply stretches less than it does on the first test. If brought back to a zero load and allowed to relax it will pull again identically to the first test. I'm having trouble figuring out what's going on here and was hoping someone might have seen this behavior before, or had an idea on what could be causing it.

Thanks

 

[Compositepro]

What you are observing is probably what is called "contructional stretch". This is the stretch caused by the intitial nesting of yarns into a closer packed configuration and is characteristic for fiber ropes. As long as some tension is kept on the rope the fibers stay nested and then you will only observe elstic deformation and creep. But if you release all tension the fibers will fluff up an lose their packing.

Twisting the yarns can help somewhat at also impregnating the rope with an elastomeric coating like urethane cane help. It will also lessen the flexibility. With some fiber heat setting can be used.

 

[Buoyboy]

Compositepro,

Thanks for you responce. Wouldn't you expect to see some length offset if the line were taking a set? We are mesureing a section of the line to find strain and there it is roughly the same size when we relax back to 100 lbs. as it was before we ramped it up the first time (within 1% of initial lenght).

Thanks again.

 

[DaveVikingPE]

That sounds like a really interesting project! Are you a grad student or something else?

 

[Compositepro]

I don't think there is much more that can be said that wouldn't be pure speculation without knowing a lot more details. The answer to your question in in these details. The best aproach is to carefully observe your testing while it is in progress with some ideas as to what you are looking for. Can termination methods be a factor? What about measurement technique? With high modulus ropes many factors that are insignificant in normal ropes become a much larger percentage of the total stretch. I'm involved in making carbon fiber ropes and while there are similarities to standard ropes you have to pay attention things that usually aren't noticed by users of standard ropes because they are such a small percentage of the total stretch. Carbon fiber is siffer and stronger than steel and will only stretch 1.5% to break.

 

[Buoyboy]

Thanks Compositepro for the help. I was wondering if someone had encountered this behavior before and it was just something I was unfamiliar with. I figured I would check here before I started dissecting our procedures to see what went wrong. I'm guessing that it might be in our displacement measurement, so I guess I'll start looking things over.

DaveVikingPE, I'm not a grad student, I'm something else--I'm working NOAA's Pacific Marine Environmental Laboratory Engineering Division. This line is to be used in a new class of data mooring buoys. They're small and they self deploy. If you'd like moor information you can find it at

http://www.pmel.noaa.gov/pico/.

 

[DaveVikingPE]

"moor information" hahahahahaha!

Interesting stuff, indeed. My major was Ocean Engineering and thus I'm interested, really interested. I didn't see anything regarding anchor tecnology (or I missed it). What kind of anchoring problems were overcome, etc.?

 

[Buoyboy]

DaveVikingPE,

There is a small section giving the basic information on the anchor and mooring line under "The Technology" section. Discussion on the mooring line failures we have experiences in the filed are also under the “Field Experience” section.

The buoy is on a slack line mooring. The line is made up of different segments, as all our mooring lines are, the main difference being that this line all has to reel onto the spool housed in the anchor block. The line is therefore one continuous weave that transitions seamlessly from one line type to the next. The line also incorporates a cable over its entire length that is used to communicate, via an inductive couple, with any sub-surface instrumentation.

The anchor block is like a bobbin that releases the mooring line a constant tension, lowering itself to the ocean floor. One of the major concerns was ensuring that the block would fall in an upright position, so as not to foul the line. It is also envisioned that any sea floor instrumentation required could be placed on this block.