Destruction Testing of Ferrules

[Daryl and Daryl]

Kitebuilder.com has done some testing of spar breaking strengths that might be of interest to some of you.

I use Skyshark 400s and was surprised to see that the spar was stronger with an internal ferrule than with an external ferrule.

Here's a link:

Ferrules

[Roger Caffin]

Fascinating, and thank you!

> surprised to see that internal ferrules did better than external ferrules
Not all that surprising. When the ferrule is outside the forces are mainly tension around the circumference. When the ferrule is internal a lot of the forces on the ferrule are compression. The tension is then longitudonal. Thee can be a lot more lengthwise CF strands along the length of the material. Mind you, if you use purely pultruded CF rod or tube as the internal ferrule, buckling is not hard to achieve as the only strength there is the polyester resin – which is weak.

Sorry for the techie blurb, but I had done some testing myself.

If you want a really slim-line outside ferrule, find a matching Easton arrow shaft of the right gauge. They exist, and work well.

Cheers

[Daryl and Daryl]

Roger,

Thanks for the info and aluminum ferrule suggestion.

One of the kitebuilder members suggested that the Skyshark 400 tubing did particulary well because it is wrapped with fibers around the circumference of the spar and therefore resisted splitting at the end.

Daryl

[Daryl and Daryl]

I edited my original post.

The testing is of the spar breaking strength, not the ferrule. My original wording suggested the opposite.

I really shouldn't be allowed on these public forums.

Daryl (bum scoop) Clark

[Sam Farrington]

Two things I've found to be important here:

1. The fit between the ferrule, internal or external,and the spar should be snug. Otherwise, when the pole is flexed, pressure will concentrate in small areas, or conversely, pressure will not be spread out over a larger area, and breakage will occur sooner. This observation is not new to this forum, but worth repeating, as the amount of wiggle between the ferrule and the spar will markedly change the results obtained in break tests.

2. The relative elasticity of the ferrules and the spars is also a big factor. Not being a scientist, I can't give you numbers; but you can still avoid using ferrules much stiffer than the spars, or vice-versa, as either will result in sooner breakage.
The elasticity of flexible tubes is called the "spine," and is a number expressed in the USA in inches, and represents the deflection of the tube by a weight around 2# over a span around 2'. Typically, the spine will be in the 0.2 to 0.5" range, and is helpful for purposes of comparison.

[Daryl and Daryl]

Samuel,

Thanks for the introduction of the word "spine". That's a new one for me. Knowing this for the various spars would be helpful.

Daryl

[Roger Caffin]

Hi Daryl

> wrapped with fibers around the circumference of the spar and therefore resisted
> splitting at the end.
Yes indeed.
There are three main sorts of CF tubing available:

1) CF fabric wrapped around a mandrel. This is easy to do by hand in a small cottage factory, and the result is quite robust, but it is usually not as stiff as anything else.

2) CF fibres pultruded. All fibres run down the length of the tube, and only the resin holds the tube together. Pultrusion: the fibres are pulled through a die and resin extruded around them as they go. Very stiff, and used widely in kites, but can NOT handle bending: the tube suddenly splits full length at a low amount of bend. Many small pultrusion factories make this in both glass and CF.

3) 2D wrapped CF tubes. Typically the good ones will have 5 layers. Going from inside outwards: lengthwise, wrapped around, lengthwise, wrapped around, lengthwise. This makes a superb tent pole of very high strength and reliability: I use this myself for all my tents. It requires a huge machine to make this properly: I know of one in China at Zongshan Pulwell Composites. If anyone knows of others, please let me know! But companies such as Victory Archery and Gold Tip import and sell the shafts.

You can also find pultruded glass fibre tubes with a cosmetic CF wrap on the surface, just for appearance. Some of the Carbon Express arrow shafts are made this way. Do NOT buy them for tent poles!!!! (Yes, I bought some … sadly)

Cheers

[Daryl and Daryl]

Samuel,

"The fit between the ferrule, internal or external,and the spar should be snug."

One place I read recommended a difference of .014 inches between the od of the internal spar or ferrule and the id of the external spar or ferrule. This would give .007 inches of void to be filled by the glue.

Roger,

Thanks for explaining the pultruded/wrapped thing. I've been a little fuzzy about the whole process.

Daryl

[Sam Farrington]

Daryl,
Ideally, the fit would be just right, as snug as possible without creating binding.
In reality, one takes what one can find in the consumer market. Right now I am burning from another kick in the teeth by a major thread manufacturer, so will say no more until I cool off.

[Daryl and Daryl]

Here's some more info about ferrules that I found helpful"

ferrules

[Sam Farrington]

Daryl,
Great article.
Elmer's polyurethane glue is inexpensive, can be bought in small quantities, and works well with ferrules. It contains no hardeners than can weaken the resin in the carbon composite.
The mini crosscut saw from Harbor Freight is under $40, and with a fresh blade, cuts carbon tube up to 1/2" very cleanly.

[Daryl and Daryl]

Samuel,

Thanks for the glue tip.

I searched but could not find the mini crosscut saw.

Daryl

[Sam Farrington]

My bad – here's the link:

http://www.harborfreight.com/power-tools/cutoff-saws/bench-top-cut-off-saw-42307.html

[Daryl and Daryl]

Samuel,

Found cut off saw.

Looks like it could be handy for a number of things.

Daryl

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