Most appropriate method for measuring breaking strain?


Well-known member
Jun 13, 2016
I wrote a series of articles for Sexyloops regarding the strength testing of various materials and knot. I also did an ageing test on nylon that had been left in sunlight for a couple of months.

Firstly, I'll say that if you want accurate data then you need to be using a load frame such as an Instron mechanical tester, fortunately I have access to one of these. Here the material under test is wound onto 'spools' and these are loaded into the machine, thus the testing can be done in the absence of knots. This gets the true tensile strength of the material.

When it comes to knots, there are very, very few in my experience that retain anything like 90% of the materials strength. The exception is the Bimini Twist which retains all the lines strength due to the fact that knot itself twists the line into a multi-strand 'rope' that is stronger than the starting material. A simple knot like a overhand loop (i.e. a 'wind' knot), in contrast loses 30% of the lines strength immediately. From my fairly extensive testing, I think an angler should assume that they've retained 60% of the materials strength once they've finished making up their leader and attached the fly.

What a machine like an Instron produces is a stress/strain response of the material under test. It should be noted the strain is the physics term for stretch. As such, breaking strain is actually the extension at which a material ruptures quoted as a percentage of its starting dimension. This term has been bastardised by anglers to mean tensile strength though. For example, the breaking strain of nylon is typically about 50% (irrespective of diameter), but if you answer the question of 'what breaking strain are you using' with 50%, although technically correct, you'll get a blank stare :)

The stress/strain response gives some interesting information about the material. The area under the stress/strain curve is a measure of the energy required to rupture the material, i.e. the brittleness. This fracture energy/brittleness is important when it comes to shock resistance. At this point I should say that the stress/strain behaviour of most materials is affected by the strain rate. I think in practice we all know this - e.g. if you want to snap a piece of leader it's 'easier' to do so with a sharp jolt rather than a steady pull. Materials with higher fracture energies are more resistant to failing under shock-loading conditions, thus stretchy materials tend to do better in this respect. There is a difference in fracture energy between nylon and flourocarbon, but it's nothing like some of the estimates made in this thread.

Hope this is useful, James.
I found this -

do you have any other links to the other articles? I couldn't find them. Maybe that would end the constant repetitive threads on tippet materials (ha ha!).


Well-known member
Feb 27, 2009
West Lothian Scotland
So why does everyone else say that the water absorption reduces strength? Did you take the line straight off the spool? If so, then it may still be covered in the lubricant that is used on the die when being manufactured. This would stop the water getting into the nylon, but only for a short time as in use this lubricant would be removed.
I couldn't say if it was right off the spool I'd only be guessing but it's certainly possible it was and had oil's on it?
I'll try it again ASAP it would be good if others tested it wet and dry too?

I think most people myself included believe or repeat what the manufacturer says idk?


dave b

Well-known member
Jan 24, 2010
There's only so much you can do with a formula. I've found Fluoro to be unreliable regardless of brand. As some have stated it's brittle, lacks stretch and is often quite thick in dia, depending on brand. That said there are times when it still has it's uses and I'll use it however I normally step up accordingly.

Pre-stretched lines vary considerably in their characteristics and stated/claimed breaking strains.

Good old fashioned mono is thick in comparison to pre-stretched lines and some fluoro's however when it comes to strength and durability, lines like Maxima that have stood the test of time still have their place.

I use the stated breaking strain as a guide only and I always measure the dia of any new lines I try. If somebody asks what I'm using I'll always state 0.12 low dia or fluoro rather than breaking strain because there is so much variation and too many factors that affect breaking strain.

The only exception to the above is if I'm using Maxima in which case I'll state 3 lb, 4lb or 5lb Maxima.

This is another thread that could be debated for ever so my advice would be simply to use a suitable line that your confident in and forget the claimed breaking strain and instead concentrate on catching fish.