Spar cap material

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BBerson

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What percentage of the individual fibers that make up the roving would actually be tensioned?


BJC
If clamped tight enough they should all be the same. You can adjust the tension like a sewing machine with friction.
I don't know how much tension is desired. I don't have any knowledge about pretensioned roving.
I suspect some factory pultrusion glass rods are pre-tensioned and have stronger properties. But if you make your own caps you get 100% fill unlike round pultruded rods.
 

wsimpso1

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Bill, my interpretation is that the other Bill was asking about using the unidirectional glass fiber that is fed to chopper guns as a potential source of uni for spar caps. No suggestion there of chopping it.

The question that I would have is about the quality of the material.
Oh... My typing got way out front there. Sorry. I should have read the post a little more carefully.

The cited spools of fiber are called out as E-glass. No mention is made of the fiber treatment for laminating, but does say it is suitable for polyester and vinylester resins, so it might be OK with epoxy. The big question how to fabricate with it on spools. Usually a bunch of small spools with a wetting machine to make the amount you are laying down per trip along the mold reasonable, but this might have a thick enough bundle to work directly. Usually E-Glass is just laid, then brushed and squeegeed from center towards the ends to straighten and settle the fibers into a high fiber fraction stack.

I suspect that you could do some spool winding with pins in your fixtures at each end. I sure would not want to do the fixturing like they do on suspension bridges with individually supported wires in the cable. The big reason is that the few hundred wires in the cable can each be wound on their own pulley at each end. Attempting to do that with a few million fibers is bound to result in as many or even more untensioned and wandering fibers than in say a wet tape lamination.

The book method of building the Defiant spar caps is just laying a bundle of S-glass yarns in wet without tension and then brushing and squeegeeing from middle to ends. Has anyone got any test data showing the difference between wound structures and laid ones?

Billski
 

BJC

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If clamped tight enough they should all be the same. You can adjust the tension like a sewing machine with friction.
You could be fight, Bill, but my first hand experience with textile machinery that automatically controlled the tension of multiple fibers (1,200 IIRC - its been 45+ years ago) suggests that it will be almost impossible to achieve uniform tensioning starting with roving.


BJC
 

poormansairforce

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3" wide uni clamped between rubber faced wide jaw clamps lubricated with epoxy. One clamp is fixed and the other is pulled using rachet straps, etc. Pull hard enough to induce slippage to load all strands. Might need to do several layers at a time. After initial cure do the next batch.
 

Marc Zeitlin

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... it will be almost impossible to achieve uniform tensioning starting with roving.
You may be right, but the first question to ask is why anyone thinks that there needs to be pre-tension on the fibers. Any pre-tensioning (strain) will only decrease the total amount of tension (strain) that the fibers could accept in use. The only reason I can see to put any tension on the fibers would be to ensure that they're straight in the cap so that they can maintain full strength/stiffness.

I've used the 3" fiberglass cap tape many times. Generally, you leave a bit sticking out over each end of the layup so that while you're wetting them out and squeegeeing them, you can pull either end and keep the fibers straight - keep doing that for each layer, after removing the cross fibers.

I've also laid up a number of spars using carbon rovings - generally 10K (10,000 fibers/bundle). Same deal in this case - we built a wet-out machine that wets the fibers as you pull them off the roll. Then you lay them into the cap trough and pull each end to keep them straight while squeegeeing - keep doing that for a number of hours, and you've got a cap full of very straight carbon fibers. With the carbon, we then bag the layup - with the moldless construction Rutan EZ aircraft, no bagging of the fiberglass.

My guess is that when they pultrude fibers to make the rods (round or rectangular), they put just enough tension on the fibers to keep the straight - it's not like prestressed concrete with reinforcing rods. But that's just a guess.
 

BBerson

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I think Prestressed concrete beams use cables. Prestressing allows longer span and lighter beams according to the Precast/Prestressed Concrete Institute:
"Because precast concrete is essentially custom designed and manufactured, it has the flexibility for unique applications, lending it to project optimization. Precast/prestressed concrete also provides for longer open spans with fewer columns and obstructions, as well as smaller cross sections through the use of high-strength concrete and prestressing. This allows floor plans to be more easily adapted as a building’s function changes throughout its service life."

I don't know exactly how prestress reduces weight, but I wouldn't dismiss it offhand.
 

Marc Zeitlin

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I don't know exactly how prestress reduces weight, but I wouldn't dismiss it offhand.
I'm not dismissing the IDEA of prestressing, but it has to be used in appropriate situations.

Concrete is really good in compression and very crappy in tension. Hence the desire to compress the crap out of it when it's going to be in bending and will have part of it in tension (bridge beams, etc.). This allows it to take a lot more tensile stress than it otherwise would (and still deal with the compressive stresses on the other side), which means you need less of it (saving weight). In this case, the pre-stress is of the matrix, not the fibers (cable or rods).

Pre-stressing the FIBERS will have a very different effect - in pre-stressed concrete, you care about the matrix - in our composites, you care about the fibers, which take the load, not the matrix, which is just there to hold the fibers in place.
 

BJC

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The only reason I can see to put any tension on the fibers would be to ensure that they're straight in the cap so that they can maintain full strength/stiffness.
Yes, that was the intent; applying enough tension to the fibers in a roving that is designed to feed material into a chopper gun, to achieve reasonably parallel fibers, in response to BB’s question about using chopper gun feedstock as spar cap material.


BJC
 

BBerson

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Pre-stressing the FIBERS will have a very different effect - in pre-stressed concrete, you care about the matrix - in our composites, you care about the fibers, which take the load, not the matrix, which is just there to hold the fibers in place.
I don't see a difference. Both concrete and epoxy are weak in tension. The fibers or steel cable are strong in tension, so it takes both.
I read that pultruded products (like graphlite rods) are formed under tension. I don't know how much tension.
 
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mcrae0104

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I don't see a difference. Both concrete and epoxy are weak in tension.
The difference is that pretensioned concrete structures are generally loaded in one direction only. In a conventionally reinforced structure, there is much more reinforcement below the neutral axis. A pre- or post-tensioned structure aims to pre-stress the tension flange (or bottom half of a slab). The concrete doesn't need the help up top in compression, and generally uplift is smaller than live load+dead load.

I can't really speak to composites like Marc or Billski but that's the story with concrete anyway.
 
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Steve C

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Concrete is weak in tension, so they load up the steel so that it takes tension well before the concrete does. The first x amount of tension load takes stress off the concrete.

Composites aren't as wildly one sided as concrete, so I think we just want straight fiber

Loose tow gets twisted. Uni tape seems much easier and slightly more efficient since the fiber is very orderly.
 
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MadRocketScientist

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This is a slightly different application of using gun rovings in place of uni cloth, but when I made my CriCri landing gear I used gun roving under tension.
When I was laying up the roving in the landing gear mold I ran it through a resin bath and then wound it around pins at either end of the mold. The tension wasnt a huge amount as I pulled it by hand. The interesting thing is that there are a few reports of the original landing gear bows sagging slightly under the weight of the plane sitting on them. Mine has sat on the gear for 9-10 years and has no sag at all.
 

BBerson

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The difference is that pretensioned concrete structures are generally loaded in one direction only. In a conventionally reinforced structure, there is much more reinforcement below the neutral axis. A pre- or post-tensioned structure aims to pre-stress the tension flange (or bottom half of a slab). The concrete doesn't need the help up top in compression, and generally uplift is smaller than live load+dead load.

I can't really speak to composites like Marc or Billski but that's the story with concrete anyway.
Well they may not need pretensioned cables on top, but I bet they do have some untensioned steel on top.
They pretension the bottom because it is stronger and lighter than no pretension.
No reason a wing spar couldn't be pretensioned on both top and bottom.
I was thinking of twisting the wet roving to squeeze out excess resin and compress the strands like cables.
 
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