composites question

[gug]

hi all.

im currently building some composite poles for a rather complicated application. anyway, the things are 6 metres long, hexagonal in shape (100mm tall). we currently build them on a roticary, so the pole is turning during layup and as it cures. the former is a peice of foam, lightly reinforced with fibreglass strips. the main composite is uni-directional carbon fibre and epoxy, with some light carbon at 90 degrees.

the question i have is that the weight of the wet carbon causes a noticable sag in the peice, which dissapears as the carbon sets. i didnt think much of it until i went to an (unconnected) lecture on welding and started thinking about grain size.

do you guys think that a flexing set and cure, as is happening with these peices, will adversly affect strength?

[gug]

hi all.

im currently building some composite poles for a rather complicated application. anyway, the things are 6 metres long, hexagonal in shape (100mm tall). we currently build them on a roticary, so the pole is turning during layup and as it cures. the former is a peice of foam, lightly reinforced with fibreglass strips. the main composite is uni-directional carbon fibre and epoxy, with some light carbon at 90 degrees.

the question i have is that the weight of the wet carbon causes a noticable sag in the peice, which dissapears as the carbon sets. i didnt think much of it until i went to an (unconnected) lecture on welding and started thinking about grain size.

do you guys think that a flexing set and cure, as is happening with these peices, will adversly affect strength?

[deersound]

I'm not sure I'm completely following. The beam deflects during the layup, then straightens itself?

When you say the carbon sets, did you mean the resin setting? Gelling? Curing?

[gug]

yep, thats right.

when the beam is being laid up it is supported at either end. the weight of the carbon causes some sag in the middle of the beam, then once the cabon has gelled the deflection is taken out by the lay-up stiffening (remember that its rotating during this whole process).

the beam has to rotate during cure as well because of the way we apply heat (infrared lights).

hope you can help me!

[deersound]

I'm always a little nervous about not using a vacuum or resin infusion on something that's meant to be structural, cause it's generally hard to produce parts of repeatable strength when you're just laying everything up and squeezing out the excess resin. How are the beams getting loaded?

Since it seems like you're saying the beam is sagging during the layup process, you should just keep in mind that you're laying the carbon onto a deflected beam. That means that on the top of the beam, the carbon is being layed over a distance slightly shorter than the beam, and on the bottom it's being layed on a distance slightly longer. So if the beam is sagging a lot, when it straightens the carbon on top will be pulled on from both ends and the carbon on the bottom will be pushed on from both ends, maybe wrinkling it a little or moving the fibers around. I'd carefully examine the finished part around the center, and look for any signs that the carbon moved around on you. Even if it did, it may not make a difference depending on how the beam is getting loaded or how much the beam was overdesigned.

As for the resin, I wouldn't imagine it being adversely affected if the beam is straightened during the earlier stages of the gelling. You probably don't want it bending during the cure... but I'm not a polymer expert. Even so, the vast majority of your strength comes from the carbon, and the resin's just there to keep the fibers in place and protect the thing in shear. Sure, it's not ideal, but unless you've got a huge budget, most things never are.

[gug]

thanks for the reply.

its very difficult to look at the structure in the middle, because there is a layer of (non-structural) kevlar around the outside of the peice preventing me from even seeing the outside layer. i dont want to make a test peice with all the carbon on it (about $3000 of material).

the beams are hanging under a helicopter. 3 X 6m lengths are joined together, and have to support about 40kgs at the extremeties and are towed from the middle. like i said, the aplication is complicated.

anyway, thanks for the info!

[deersound]

You can still observe it by makeing one of the pieces in two stages. Leave the uni exposed and let it cure, then take a good look at it. When you're done, wrap the kevlar around it. It'll take more time, but it won't cost you extra for material.

Probably, though, you've got nothing to worry about... unless they start breaking.

[Jay Fleming]

I don't want you to have to explain the application, but it's nothing that has to do with like search and rescue does it? Where you're gonna be hanging supplies or people under the helicopter? I wouldn't want to chance it if lives are at stake.

[gug]

nothing like that, no.

its for whats called a towed electro-magnetic system. basically a giant rectangle of wire 18MX20M, that a 350amp current is pumped along. the current is forcibly shut off, and the dissapation of the magnetic field sets up eddy currents in the ground below the helicopter. the decay of these eddy currents tells us what is below the ground.

its used for mapping shallow water, and finding minerals.