Fly Line Stretch and Viscoelasticity
Posted: Sun Nov 14, 2021 8:06 pm
Motivation
Bernd has written a nice front page about "Fly Line Stretch"
https://www.sexyloops.com/index.php/ps/fly-line-stretch
and we had already a in-depth discussion at the tackle section here:
https://www.sexyloops.co.uk/theboard/vi ... =18&t=3622
Some fly line companies claim indeed that low-stretch fly lines have several advantages, but to my knowledge no objective studies exist that would prove these claims.
See for instance:
Tim Rajeff explains barstool Line low stretch technology
RIO InTouch Lines with ConnectCore
So I thought it makes sense to create a topic also here in the physics section, where we can collect ideas for measuring fly lines, equations and models. When we have enough information gathered, we should be able to check the plausibility of the above mentioned supposed advantages. Also when we have measured these properties we can insert them into existing fly line models to make them more realistic.
We can find most definitions/equations on Wikipedia, so I'll link/cite these pages.
Properties to characterize fly line stretch
A simple model for the fly line would be a linear spring and stiffness is usually employed to describe their behaviour:
From https://en.wikipedia.org/wiki/Stiffness
"The stiffness, \(k\) of a body is a measure of the resistance offered by an elastic body to deformation."
\(k = \frac {F}{\delta}\)
where
\(F\) is force on the body and
\(\delta\) is the displacement.
Measuring stiffness of a fly line at home:
(1) Mark a section of a fly line at a distance of 1m e.g. with a china marker
(2) Take several test weights and measure the elongation of the fly line between the markers
\(l1\) = 1m
\(l2\) = your measured value
\(k = \frac{m_{test} * 9.81 \frac{m}{s²}}{l_2 - l_1}\)
The guys from the German fly fishing forum used always a 1,8 kg test weight. Here are some of their findings (German term "Dehnungswerte" in the reviews):
https://www.fliegenfischer-forum.de/flischnu.html
Example 5% @ 1,8kg
\(l2 = 1.05m @ 1,8kg\)
\(k = 1.8kg * 9.81 \frac{m}{s²} / (1.1m - 1m) = 353.16N/m\)
Relationship to elasticity
Another possible property would be the elastic modulus (of the fly line material).
https://en.wikipedia.org/wiki/Elastic_modulus
"An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it."
https://en.wikipedia.org/wiki/Stiffness
"The elastic modulus of a material is not the same as the stiffness of a component made from that material. Elastic modulus is a property of the constituent material; stiffness is a property of a structure or component of a structure, and hence it is dependent upon various physical dimensions that describe that component. That is, the modulus is an intensive property of the material; stiffness, on the other hand, is an extensive property of the solid body that is dependent on the material and its shape and boundary conditions."
If you know the cross-sectional area, you can compute the elastic modulus from the following equation:
\(E = k * L / A\)
\(A\) is the cross-sectional area
\(L\) is the length of the object
for above example, if we assume a fly line diameter of 1mm:
\(A = \pi * r²\) where r is the fly line radius, thus
\(E = \frac{353,16 N/m * 1m}{\pi * (0.001m / 2)²} = 401274175 N/m² = 0.4 GPa\)
The slight challenge is here, that a fly line consists of multiple materials; the core has a different elastic modulus than the coating. You could assume an average cross-sectional area, but I'd say that choosing the stiffness@1m as property makes more sense.
Viscoelasticity / Damping
Here you will find a good introduction to this topic:
https://www.roush.com/wp-content/upload ... nsight.pdf
"Damping is the conversion of mechanical energy of a structure into thermal energy"
When you're stretching (and un-stretching) a fly line, a part of the energy is stored as potential energy and another converted into thermal energy.
"A purely elastic material is one in which all the energy stored in the sample during loading is returned when the load is removed."
"A complete opposite to an elastic material is a purely viscous material .. All the energy is lost as “pure damping” once the load is removed."
"For all others that do not fall into one of the above extreme classifications, we call viscoelastic materials."
Now my idea is to measure the "loss" with simple tools: a line sample, a test weight, recording the decay of the oscillations with a smartphone and then determining the damping by video analysis. More about this in one of the next postings.
Torsten.
Bernd has written a nice front page about "Fly Line Stretch"
https://www.sexyloops.com/index.php/ps/fly-line-stretch
and we had already a in-depth discussion at the tackle section here:
https://www.sexyloops.co.uk/theboard/vi ... =18&t=3622
Some fly line companies claim indeed that low-stretch fly lines have several advantages, but to my knowledge no objective studies exist that would prove these claims.
See for instance:
Tim Rajeff explains barstool Line low stretch technology
RIO InTouch Lines with ConnectCore
So I thought it makes sense to create a topic also here in the physics section, where we can collect ideas for measuring fly lines, equations and models. When we have enough information gathered, we should be able to check the plausibility of the above mentioned supposed advantages. Also when we have measured these properties we can insert them into existing fly line models to make them more realistic.
We can find most definitions/equations on Wikipedia, so I'll link/cite these pages.
Properties to characterize fly line stretch
A simple model for the fly line would be a linear spring and stiffness is usually employed to describe their behaviour:
From https://en.wikipedia.org/wiki/Stiffness
"The stiffness, \(k\) of a body is a measure of the resistance offered by an elastic body to deformation."
\(k = \frac {F}{\delta}\)
where
\(F\) is force on the body and
\(\delta\) is the displacement.
Measuring stiffness of a fly line at home:
(1) Mark a section of a fly line at a distance of 1m e.g. with a china marker
(2) Take several test weights and measure the elongation of the fly line between the markers
\(l1\) = 1m
\(l2\) = your measured value
\(k = \frac{m_{test} * 9.81 \frac{m}{s²}}{l_2 - l_1}\)
The guys from the German fly fishing forum used always a 1,8 kg test weight. Here are some of their findings (German term "Dehnungswerte" in the reviews):
https://www.fliegenfischer-forum.de/flischnu.html
Example 5% @ 1,8kg
\(l2 = 1.05m @ 1,8kg\)
\(k = 1.8kg * 9.81 \frac{m}{s²} / (1.1m - 1m) = 353.16N/m\)
Relationship to elasticity
Another possible property would be the elastic modulus (of the fly line material).
https://en.wikipedia.org/wiki/Elastic_modulus
"An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it."
https://en.wikipedia.org/wiki/Stiffness
"The elastic modulus of a material is not the same as the stiffness of a component made from that material. Elastic modulus is a property of the constituent material; stiffness is a property of a structure or component of a structure, and hence it is dependent upon various physical dimensions that describe that component. That is, the modulus is an intensive property of the material; stiffness, on the other hand, is an extensive property of the solid body that is dependent on the material and its shape and boundary conditions."
If you know the cross-sectional area, you can compute the elastic modulus from the following equation:
\(E = k * L / A\)
\(A\) is the cross-sectional area
\(L\) is the length of the object
for above example, if we assume a fly line diameter of 1mm:
\(A = \pi * r²\) where r is the fly line radius, thus
\(E = \frac{353,16 N/m * 1m}{\pi * (0.001m / 2)²} = 401274175 N/m² = 0.4 GPa\)
The slight challenge is here, that a fly line consists of multiple materials; the core has a different elastic modulus than the coating. You could assume an average cross-sectional area, but I'd say that choosing the stiffness@1m as property makes more sense.
Viscoelasticity / Damping
Here you will find a good introduction to this topic:
https://www.roush.com/wp-content/upload ... nsight.pdf
"Damping is the conversion of mechanical energy of a structure into thermal energy"
When you're stretching (and un-stretching) a fly line, a part of the energy is stored as potential energy and another converted into thermal energy.
"A purely elastic material is one in which all the energy stored in the sample during loading is returned when the load is removed."
"A complete opposite to an elastic material is a purely viscous material .. All the energy is lost as “pure damping” once the load is removed."
"For all others that do not fall into one of the above extreme classifications, we call viscoelastic materials."
Now my idea is to measure the "loss" with simple tools: a line sample, a test weight, recording the decay of the oscillations with a smartphone and then determining the damping by video analysis. More about this in one of the next postings.
Torsten.