Misc. Heterogenous fiber rope strength

[ldanielrosa]

TL;DR Summary

Calculating/approximating failing strength of mixed fibers (carbon,aramid) in rope

I have a textile braider that does 8 or 16 carriers in round configuration. I plan to have a 33 carrier flat braider as well.

I currently have the wild idea of making a ribbon of carbon fiber and examining it's usability for livestock electric fence, or some other ludicrously overpowered application. I may try to put in some aramid yarns for a contrasting color.

I see that the carbon fiber (available in a wide range of tensile strength) is stronger but less elastic than aramid. Can someone explain to me which characteristic in the technical specifications to use to determine how much strength the aramid is contributing when the entire item is stretched to the failing point of carbon fiber?

 

[Baluncore]

Can someone explain to me which characteristic in the technical specifications to use to determine how much strength the aramid is contributing when the entire item is stretched to the failing point of carbon fiber?

That will depend on the construction of the tape.

What is Young's modulus for aramid fibre, and for carbon fibre.
https://en.wikipedia.org/wiki/Young's_modulus
One will stretch more than the other, transferring load to the least stretchy. Ideally, since each fibre wraps around every other fibre in a plaited tape, the fibres will spread and share the load equally.

Since fibres balance tension, the failure under tension will be determined by the least tensile strength of the fibres employed. The cross-section of the aramid and the carbon fibres will need to be selected to balance the strength of the fibres.

One spark zap from the electric fence, in the oxygen atmosphere, will probably burn some of the carbon fibre out of the tape.

 

[ldanielrosa]

Thank you for getting back to me so quickly. I followed the link. I think the curve will have a different profile for non-metals. I noticed that the modulus portion appears linear, and what I managed to dig up for aramid and for carbon fiber has similar behavior up to the knee.

"One will stretch more than the other, transferring load to the least stretchy. Ideally, since each fibre wraps around every other fibre in a plaited tape, the fibres will spread and share the load equally."
I had assumed this much, but found in the past that writing too much repels some of the help. Thank you for validating this.

"Since fibres balance tension, the failure under tension will be determined by the least tensile strength of the fibres employed. The cross-section of the aramid and the carbon fibres will need to be selected to balance the strength of the fibres."
I'm under the impression that the failure point may be determined by the elastic limit of the least elastic portion, if that also has the highest tensile strength.

I found https://www.omnicalculator.com/physics/stress which helped me through this.

The carbon fiber in this case (what I ordered from surplus) has 4.501GPa strength and 1.8% elongation at failure, but the aramid has 2.69GPa (minimum) tensile strength and 67.25GPa tensile modulus (target) with a range given of 3.0~4.5% elongation at failure.
If I assume the carbon's elongation of 1.8%, then the aramid is loaded to 1.21GPa stress. About 45% of the aramid's ultimate strength.

Thank you for your help.

 

[Baluncore]

I am still concerned that a naked carbon fibre would be too resistive to carry the current, and would oxidise where a spark formed in air. I feel that carbon fibre would make better insulated poles as a composite, to support a conductive metal wire for an electric fence.

Spider webs that cross your path are invisible, so they get broken quickly before you realise they are there. An electric fence must be highly visible, and then provide the zap, so the avoidance behaviour can be learned by the approaching animal.

High tensile strength without stretch, is not needed for an electric fence, indeed it is a liability. The fence needs to stretch while an animal pushes into it, the fence must survive contact, then pull itself back into place. Cows are mean, and so dominant animals will deliberately nudge or push another cow into a visible electric fence.

Like power lines, a thin wire or fibre will be invisible to bats that feed at night. They will be injured when they fly into, or cut if they fly along the taut fibre. Thin fibres, at neck height, pose an injury hazard for people on bikes.

 

[ldanielrosa]

I am still concerned that a naked carbon fibre would be too resistive to carry the current

"1.7 x 10-3 ohm-cm"
Which at 0.0071mm diameter, comes to about 429kOhm/m for 1 fiber. However the yarns appear to be standardized as 3k (3000 fiber), 6k, 12k, 24k and 48k. So one 3k yarn would be about 143 Ohms/meter. The 3k yarn in the 33 carrier flat braider Should give me a ribbon about 19mm wide- possibly wider if I run warp yarns. So with the yarn path being about 1.55*length, I should get about 6.7 Ohms/meter. I looked up some of the products available, and the "premium" brand that uses copper wire instead of stainless steel boasts about 2kOhm per 1000'.

and would oxidise where a spark formed in air

I hadn't thought about that. Does it happen with metal as well?

An electric fence must be highly visible

I may try to put in some aramid yarns for a contrasting color.

Cows are mean,

Possibly moreso with mine- they're midgets with dreadlocks.

 

[Baluncore]

I hadn't thought about that. Does it happen with metal as well?

Not with metal, only in very thin fibres that have little heat capacity. The problem comes from the discharge point, where the arc terminates. Very few fibres are involved, and the arc is a plasma, so expect more fibres to be converted into CO₂ each time there is a spark in the atmosphere. The inductance of the wire will maintain the arc plasma, as more fibres are consumed.

Another problem could be tying a bundle of fibres together electrically, and then groups of bundles together, into the tape. You will need to crimp them, or dip them in a conductive silver paint. There needs to be some way of cross connecting the fibres to get the lower resistance.

Electric fences typically run a few kV, so the resistance of the wire is not so important, it only takes a few mA to punch through the skin, to get a response from the nervous system. The problem comes when very fine fibres do not survive the discharge plasma, and the damage accumulates.

 

[Baluncore]

I may try to put in some aramid yarns for a contrasting color.

Aramid fibres are not UV stable, so they will deteriorate quickly in sunlight.

If you want visibility, you could use a white polyester such as Dacron, which is used for boat sails because it has the best UV stability. If you can employ titanium oxide pigment, you may get a white material, that is bright and self-cleaning.

Most polymers are UV and ozone stable while they are underwater, or while they are embedded in a sacrificial polymer, such as polyester resin.

 

[Averagesupernova]

You are reinventing the wheel. The reliability of this material is certainly not better than solid wire. It does offer visibility which isn't important after the first day. When turned into a pen cattle always investigate the perimeter. They generally don't forget where this is.


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