Don't (always) blame the knot.

Don't (always) blame the knot.

Tracy&James | Thursday, 21 July 2016

On Monday Tracy and I went carp fishing to our local lake, we have a summer season ticket so can turn up as and when we please, so it’s great for a couple of hours after work (especially in the current weather). We fish dog-biscuit imitation flies so it’s certainly not fly-fishing in its purest form, but it’s fun. Carp like to play with their food before taking it, swirling at it, pushing it, hovering millimetres from it etc. As such, timing when to strike varies with one fish to the next and the hook-up to take ratio can be pretty poor. [Part of the fun is just how many takes, or signs of interest, you can get in just a few hours of fishing, so even with the poor hook-up ratio it is possible to catch lots of carp]. I also vary the type of strike I use depending on the distance I’m fishing at and how I’ve perceived the fish to have taken the fly. On Monday evening unfortunately I smashed my leader on a fish that I strip-struck. Obviously every fly-fisher hates doing this, so I thought I’d write about some contributory factors from a materials science perspective.

In my laboratory I have an Instron mechanical tester.  For those not familiar with such machines it pushes and pulls materials to destruction whilst measuring the load (force) applied and the resultant displacement (strain).  As an aside, strain is a unit-less ratio of the induced displacement divided by the material’s original length (dl/L).  So when someone asks you what breaking strain of nylon you’re using they don’t actually mean it.  What they actually mean is ‘what’s the ultimate tensile strength of the nylon’, this can be answered in kg.  However ‘breaking strain’ is in such widespread use throughout the angling world that pedantic engineers and scientists have to put up with it and quote the UTS.  (True breaking strain for nylon would be in the order of 0.25, and this is independent of diameter).

So back to my Instron, obviously I should be using this to further the defence of the nation, but I prefer to test fishing knots, leader materials, carbon composites etc. Smile  One thing I’ve learnt from this testing is that certain ‘constants’ – UTS, fracture toughness, breaking strain etc. are not in fact that constant.  So when, for example, a spool of nylon states it will fail at 5kg, what it means is that under a steady, slow, loading regime it will snap at 5kg.  This figure will be very much lower for a shock scenario (i.e. strip striking a bolting carp).  For the purposes of this FP I ran some tests on some standard nylon material that I had knocking about.  Instrons are not great for going fast so I ran two sets of tests, one set slow (albeit as fast as the equipment goes) and one set very, very slow.  Even with this limitation the results showed a 20% difference in failure load, with the faster of the two data sets failing earlier.  If it was possible for me to start ‘shocking’ the material then this discrepancy between the stated failure load and the measured value would only increase.  [The reasoning behind this is probably too much detail for here, but is tied in with the visco-elastic properties of many polymers, i.e. a time dependence of the response].

So whilst I sat there on Monday evening, cursing my clumsy strike that resulted in the fly parting company with the leader, I consoled myself with the thought that I hadn’t tied a poor knot, despite  it feeling  to me that I had barely ‘hit it’, and it wasn’t a bad batch of leader material or anything like that.  It was simply because I’d exceeded the failure stress of the leader at that given loading rate – a rate that was much higher than expected due to the carp bolting at the same time as I struck.  It was still my fault though.

Nb. The chart shows two typical results from the Instron tests on identical nylon at different loading rates.  A few things of note; the faster (relative) test breaks at a lower load, it also fails at a lower extension (strain) i.e. apparently it is less stretchy.  At the faster strain rate the nylon has a higher modulus i.e. it is stiffer, as indicated by the blue trace rising faster initially.  The area beneath the stress/strain response is proportional to the energy required to break the material – clearly less energy is required in the faster test (remember this is the exactly the same material).

Slow fast nylon