Getting Young's Modulus of a Fiber Optic Cable

[ihopeitworks]

I've been reading through a bunch of these forums without creating an account and now I came upon my own dilemma that needs some outside help.

We're modeling a hybrid cable, fiber optic and a few electrical conductors with a strength member. The model is in air from a host vehicle traveling at X m/s with the cable hanging out the back. Through our modeling software, we need to input a modulus of elasticity, but are stumped at how to come up with the value. Since it's multi material in various layers making a single cable, we don't know where to begin.

Would this young's modulus even work since the axial strength is a lot stronger than the bending strength of this cable? If we pull in the axial direction, the cable will only bend so much since there are kevlar strength members, but if we were to have a 1m section of cable extend horizontally from a wall, it would bend easily.

Looking for input on where we can get started. Thanks!

 

[MintJulep]

Get sample.
Measure length.
Pull.
Measure length.

 

[ihopeitworks]

Would that be accurate? I thought the young's modulus also would characterize how it bends if it were sticking out horizontally?

 

[handleman]

You've not made it clear what you want out of your model. Are you trying to figure out how much it will stretch as you drag something with it, or trying to figure out whether it'll whip around like a snake on meth when you get over a certain speed due to the vehicle's turbulent wake?

Are you a student?

 

[rb1957]

i guess hope springs eternal ?

Your cable has a structural member, this is designed to take the structural loads and should be much stiffer than the optical cables around it. It would be reasonable IMHO to model the cable as the structural element. If you want "truth" then test the structural element on it's own and/or with the complete cable (to compare). If you need Youngs modulus, well it's easy to test (stress/strain) though what does "stress" mean (for your cable) ?

A cable is expected to work with axial tension and limited bending (unless yo're got some special unusual installation).

another day in paradise, or is paradise one day closer ?

 

[drawoh]

ihopeitworks,

Do you know the elastic modulus of each component of your fibre optic?

Try calculating backwards. Assume a deflection to your fibre optic. You know the cross sectional area and the elastic moduli. Work out the force of each element. Sum them up. Now you can work out the spring rate of the fibre optic, and an equivalent elastic modulus.

--
JHG

 

[drawoh]

ihopeitworks,

Is your cable being pulled in pure tension, or is it being bent? I have analysed a couple of fibre optic cables. The glass fibre can withstand a bend radius of around 10mm without breaking. A bend radius of less than 50mm reduces the transmitted energy. I wonder how glass fibre copes with sudden, violent yanks, as your cable end catches obstacles on the ground.

My analysis assumed a single glass fibre with a plastic outer coating. Your cable sounds way more complicated. Maybe your modelling software cannot cope with this. Sometimes, you have to do stuff by hand.

--
JHG

 

[ihopeitworks]

We are dragging an object in the air (cable or object won't be touching the ground). We are calculating where the cable would be behind the vehicle as it would make a shape similar to this:

https://i.imgur.com/qCBPj6f.jpg

handleman, we're trying to see how much it'll stretch and bend. We know it won't whip in actual practice.

drawoh, our cable is being pulled in tension with air drag acting on it.

 

[rb1957]

yes, your cable is in tension (not bending).

another day in paradise, or is paradise one day closer ?

 

[JStephen]

If you have the model already set up, try some different values and see if it really makes a difference.
I would expect that neither bending nor axial elongation would be too critical in that kind of analysis.
If you use a single value for modulus of elasticity in tension, you'd need to use some adjusted value of moment of inertia for bending, not the actual moment of inertia.
For wire rope, the manufacturers give you an effective modulus of elasticity, which incorporates the effects of steel actually stretching, plus the additional stretching due to it's being in a cable configuration and not a solid iron bar. If the material you're using is ever used in some load-bearing application, that information may be available from the manufacturer. I would guess not, but worth checking if it's a ready-made tether.
Temperature changes may make a bigger difference to deflection that tension loads.

 

[Compositepro]

All wire and fiber will be wrapped around the Kevlar strength member in a helix and thus will contribute little to the tensile modulus. Thus, when flexed the wires and fibers do not kink or stretch. The mass per unit length has a big effect on how much the cable droops.

 

[3DDave]

There are problems from being a composite material. For example, the properties in tension merely add no matter where they are in the section. The properties in bending change a lot. For example, if this was a combination of metal and rubber, if the rubber is on the outside, the metal will have a smaller contribution to bending than if the metal was formed as a tube with the rubber on the inside.

Typically in sag calculations the resistance to bending is so slight it is ignored. Tension and unit-length weight dominate the behavior. This is especially true for items that are designed for minimum resistance to bending where the outer layers are helically wound and not constrained, such as in wire rope.

You may need a different model, one where the loads are combined based on angle of attack to the air and the effect of gravity on the unit mass and then develop the tension and the shape to make those balance.

This is the sort of problem that is best solved by testing; it is realistic to do testing if the item will be deployed as described it will be tested then.

 

[prex]

What are you looking for Young's modulus for? That problem depends just on the dragging force on the flying object, on the unit weight of the cable, and presumably much less on the air drag on the cable.
And your sketch is wrong, if the dragging vehicle is the small circle: the cable will start with a smaller angle, then go up with more slope, or, in other words, the curvature will be towards the sky, not towards the floor.

prex
[URL unfurl="true"]http://www.xcalcs.com[/url] : Online engineering calculations
[URL unfurl="true"]https://www.megamag.it[/url] : Magnetic brakes and launchers for fun rides
[URL unfurl="true"]https://www.levitans.com[/url] : Air bearing pads

 

[ihopeitworks]

We went ahead and modeled without Young's modulus since we decided that the drag was the larger force acting on the cable.

prex, just wanted to chime in and actual flight test showed the path of what I drew in for our configuration.

Thanks everyone for your input and help!

 

[rb1957]

"We went ahead and modeled without Young's modulus since we decided that the drag was the larger force acting on the cable."

don't understand why you think Young's modulus isn't a factor for drag (axial) loads ??

another day in paradise, or is paradise one day closer ?

 

[handleman]

RB, as I read it he doesn't care about cable stretch. It's minimal. Since it's some sort of fiber optic, I'd wager that if stretch is large enough to model, it's too much on the cable. What he cares about is the position of the thing he's dragging in the air behind the vehicle. The "cable" he's using has some sort of stiffening member that can resist bending loads to some degree. However, he's modeled his non-homogeneous cable as homogeneous in the software and is looking for some sort of isotropic Young's modulus to simulate the effects of the sorta-stiff assemblage. Then he (somehow) decided that whatever stiffness the cable has is insignificant compared to (I guess) aerodynamic effects on the dragged object, so he's modeling the cable as a pure tension member with no stiffness.

 

[rb1957]

that's be a pure tension member of infinite stiffness (as extension is minimal) ?

another day in paradise, or is paradise one day closer ?

 

[handleman]

Basically, yes. The point of this simulation (as I read the OP) is to figure out where this dragged thing is going to sit in space relative to the vehicle as it rolls down the road at like 60mph or so. I'm picturing (of course, I could be wrong) some sort of sensor array/glider/kite/etc. Likely the aerodynamic effects, turbulence, heck an immeasurable crosswind will affect the position of the dragged thing orders of magnitude more than the stretch of the cable unless the cable is way longer than I'm picturing.