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 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

 

[SnTMan]

Kind of a shake down maybe?

 

[CESSNA1]

BOUYBOY: Ropes and flexible cables stretch and the individual fibers move. They are not like a solid steel rod. On the first pull you probably "set" the individual fibers. I do not know how you measure strain and load, but that could be a concern also. When doing ropes and cables you must consider the effect of untwisting.

Hope this gets you started
Regards
Dave

 

[rb1957]

i disagree with cessna1 about permanent set ... the cables relax and repeat their behaviour the next time 'round. this could be something to do with the structure of the cables (since it is repeatable); could the fibers of the cable be tightening up around the core, and the friction induced makes the cable stiffer on the next load cycle ... some sort of hystersis

since this behaviour is so repeatable, you just accept it and move on ... (let some scientist think about it, whilst you're working with the data !)

 

[MintJulep]

Are you measuring the bulk strain of the entire bundle, or strain of individual fibers?

I imagine that while the 100 pound load is applied the strainometer is set to read zero.

What does it read after the first load to 3500 and return to 100 with 100 pounds of load?

I agree with cessna1 and rb (who seem to agree, if if rb doesn't think that they do) that what you are seeing is likely related to the individual fibers moving wrt one another.

 

[GregLocock]

You'll need a creep/relaxation model for the stiffness.

On the first pull you pull all the kinks out of the fibres, and the kinks temporarily lock into a new configuration. If you leave them unstressed then over time they wiggle back to their old locations and you start all over again.

This is well covered in the literature for polymers, talk to your supplier.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Buoyboy]

Thanks all,

This braided line is designed to have zero twist, and does not show signs of twisting during testing.

We have two to linear displacement measurement instruments fixed to either side of a section of known length of the line. Taking the difference between these two readings give us our change in length, which we then divide by the loan starting length to get strain. Load is measure with an integrated sensor in the hydraulic ram.

I would have assumed that the fibers were taking a set, but would have assumed that the length of the line would increase as this occurred. I'm not seeing any major change from the initial length value though when we come back to 100 lbs.

I am like going to base my calculations on the first curve as it seems unlikely that we will be maintaining that load on the line continuously.

Thanks again for all you help.