Struggling to understand stretch and strength in fishing line

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I'm an angler and having extensive discussions on a fishing forum about the properties of monofilament fishing line.

There are two forms: various nylons - mainly we think PA6 and PA66 - and fluorocarbon.

Our experience and 'garage testing' of these lines tell us that fluorocarbon while it can have the same diameter and tensile strength is also what I call 'brittle' - it breaks with shock with a much lower force. Eg

I tested 6lb (0.22mm) Maxima Ultragreen against 6.4lb (0.22mm) Greys Fluorocarbon by hanging 48” lengths of each from my garage ceiling joists and attaching a 1lb weight.

I then dropped the weight fom 47” then inch by inch until the lines broke from the dead drop.

The fluo broke at an average of 3” while the Maxima broke at 14”.

The same test with Sightfree G3 6lb, 0.18mm fluorocarbon broke at 2"

In contrast 6lb 12oz, 0.17mm Reflo Power which advertises itself as resin impregnated mono and is therefore presumably nylon broke at an impressive 9”.
The difference seems to be around stretch, deformity and elasticity - fluorocarbon though capable of having the same amount of stretch seemingly reacts badly to sudden shock. But when I'm researching this I get lost very quickly not being a material's scientist. Can anyone here help explain what is happening?
 

Answers and Replies

19,541
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Did you test the advertised line strengths in your garage by adding weight gradually until they broke?
 
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No, nothing so precise, but we do know that the nylon will be a little overspec. But there won't be enough variation from the advertised specification - certainly not enough to create the difference in what I'm calling shock strength.

it seems to have something to do with the ability to absorb energy, fluorocarbon snaps much faster than nylon and I'm looking for a material's explanation for what's happening. Not being a materials scientist, it's proving difficult.
 
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For what it's worth I've been trying to write my own explanation but I have no idea whether I'm talking nonsense.

Elasticity
Fact 4.
Basic fluorocarbon and nylon materials are almost equally elastic. They both stretch.

Although you'll often hear it said that fluorocarbon is low-stretch, it's not intrinsically more low-stretch than nylon. Both monos are thinned during manufacture by heating and stretching in a process called drawing. So just as you’ll find both low-diameter nylons and low-diameter fluorocarbon so you’ll find lower stretch versions of each too. Drawing aligns the molecules in the monocreating a stronger line

images%2fArticle_Images%2fImageForArticle_16014(3).jpg


Understanding Drawn Fibers


You don’t get anything for nothing though, so

Speculation: a bi-product of stretching and drawing (ie thinning), may be increasing brittleness.

One of the surprises to me in the table is that base fluorocarbon has a pretty good comparative impact strength (resistance to shock). This seems to fly in the face of experience.

There are many complaints on this forum and elsewhere of unexplained breakages using fluorocarbon leaders and 'back yard' tests suggest that it has much lower resistance to shock than nylon. It may be that the shock test applied to a block of bulk nylon or fluorocarbon - essentially surviving a blow from a swinging hammer - is not particularly relevant to shock testing monofilament. For that, a drop test is more appropriate and is an easy test to do for yourself. Results of one test:


I tested 6lb (0.22mm) Maxima Ultragreen against 6.4lb (0.22mm) Greys Fluorocarbon by hanging 48” lengths of each from my garage ceiling joists and attaching a 1lb weight.

I then dropped the weight fom 47” then inch by inch until the lines broke from the dead drop.

The fluo broke at an average of 3” while the Maxima broke at 14”.

The same test with Sightfree G3 6lb, 0.18mm fluorocarbon broke at 2"

In contrast 6lb 12oz, 0.17mm Reflo Power which advertises itself as resin impregnated mono and is therefore presumably nylon broke at an impressive 9”.

We can see that base fluorocarbon has very low elongation. Elongation is the amount that a material can stretch before breaking. Part of the stretch is elastic - that is the material will return to its original length when the load is removed - part of it is plastic deformation. In deformation the line is permanently lengthened and also weakened. Nylon undergoes stretching in it's elastic phase more or less until it snaps, fluorocarbon deforms quite early in the process of being loaded.

Nylon's high elongation allows it to absorb energy, a useful feature when fighting a fish keen to stress test your line but when it’s drawn, it increases in strength per diameter but decreases in elongation.


Speculation: A thin line (in relation to its strength) will probably have lower shock strength than a thicker line because it will have been stretched more. This brittleness may be exacerbated in fluorocarbon because of its inherently low elongation.

So this may be why we hear complaints of fluorocarbon’s breakages in circumstances of sudden shock – catching two fish simultaneously, violent dashes for freedom at short range where there is less elasticity in the whole system of rod, line and leader, excessively hard strikes and so on.
 
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In my judgment, this has to do with the viscoelastic properties of the two polymers, nylon 6, 66, and fluorocarbon (presumably teflon). I expect that the nylon polymers exhibit a greater component of viscous behavior than the flouro polymer. The viscous behavior kicks in at high deformation rates, and helps to partially supporting the tensile stress in the filament. You can get an idea of these features by measuring the small amplitude oscillatory response of the two polymers, and comparing their so-called stress relaxation spectra.
 
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The fluorocarbon is PVDF (Polyvinylidene fluoride).

I put together the properties of the material if that helps. I assume that these are bulk material properties that change a lot during drawing

1574263630682.jpeg


I came across viscoelasticity a couple of days ago. Out of my depth!
 
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Many thanks, that will keep me busy!
 
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That book goes far beyond my understanding almost immediately! But I'll struggle with it for a while.

On elongation, elasticity and deformation. As I currently understand it elongation is the total amount of 'stretch' the line extends to before breaking and it's made up of an elastic component and a deformation component. Is that the case? I'm trying to understand whether two different lines can have the same amount of total stretch but different amounts of elasticity. It's claimed that nylon's elastic phase is much longer than PVDF's.
 

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