Carbon fiber telescope tube 4

[k05t1ka]

Hello, everybody.
I am planning to build myself a 80mm refractor telescope and I want to make the tube from carbon fiber composite.

What type of lay-up should I use for the composite for maximum dimensional stability? I did't find a clear answer searching in the composite books that I own, so I'm hoping that I may find some help here.

I'm leaning toward a quasi-isotropic, balanced and symmetric laminate. I will use slippage-resistant 210g/m2 carbon twill and for now, the stacking sequence is [0,+-45,90]s. I will make two halfs of the tube by using hand lay-up (female mold) and then vacuum bagging. The two halfs will then be bonded together with the internal baffles.

Any help will be greatly appreciated.

 

[rstupplebeen]

A symmetric laminate is probably a good Idea just remember you are not making a planar laminate so the outside plies will have more area.

Will your tube have a taper? If so it will be impossible to keep the weave going orthogonal. You may want to consider using uni tape.

If I were making this I would wrap a single ply around a sacrificial cylindrical mandrel multiple times then place the wrapped mandrel in the 2 halves of the female mold. I hope this helps.

Rob Stupplebeen

 

[RPstress]

What sort of stability? If thermal then woven will give about 5 microstrain/°C (2.8 /°F) in all directions for any layup (with common HS fibre).

Twill probably won't need 0 and 90 & + and -45 or balance in terms of warp-side down vs. weft-side down. Just something like 0/45/45/0 all the same way up should do. In theory you could use just 0/45, but it's probably unwise, as rstupplebeen says.

Bear in mind that making two halves will cause each half to have more curvature than the tool due to spring-in, especially from an elevated temperature cure (even an RT cure will spring due to resin cure shrinkage). True for male and female tooling. This will happen even if the laminate has through-thickness balance.

 

[k05t1ka]

Thanks for the quick replies.

The tube doesn't have any taper, it's a constant thickness cylinder. When I was refering to dimensional stability, I was thinking thermal stability, between -15*C and 30*C. I don't want the tube to bend or twist with the temperature variation.

I cannot use prepreg, so with the male-female mold method, I don't know how to control the fiber volume ratio (usually I use vacuum consolidation).

I also will let the part cure at room temperature to avoid any post curing deformation or the deformation of the tool with temperature.

The exterior finish of the tube is important (I will apply only clear varnish), the interior is not.

After reading your posts, it seems I would be better off with a UD ply with the fibers along the tube length, wrapped around multiple times. For the finish, it would be ok to wrap a twill ply on the outside and one on the inside of the tube to keep it symmetrical?

 

[Compositepro]

Why are you making things so complicated?. By far the easiest way to make a tube is to use a male mandrel and wrap fabric around it. Your CTE will probably depend more on your fiber volume than your ply lay-up. 0-90 with no seams should be your most stable construction. Use shrink wrap or vacuum bag.

 

[k05t1ka]

Thanks for the answer. I will make some tests with the male mandrel method. Then I will see where I go from there.

After wrapping the fabric, the ends of the fabric must be one above each other? I am thinking if they aren't, the tube doesn't have the same thickness all around.

 

[Neubaten]

Off topic and just out of curiosity, how do you get the prepregs/fabrics/resins/expendable materials, etc.. for low volume DIY constructions?

 

[k05t1ka]

I don't use prepreg, though there are some vendors.
The rest I buy it from online specialized shops (I'm not sure I am allowed to post links to their sites) or from local composites distributors.

 

[MikeHalloran]

Posting site links in response to a specific question, as in this case, would be within E-T rules as I understand them.


Mike Halloran
Pembroke Pines, FL, USA

 

[k05t1ka]

Most of the materials I use come from

http://shop.r-g.de/en/home/,

but I have also used some from

http://www.fibreglast.com/.

Of course, there are many others.

 

[RPstress]

Going to UD (maybe with some woven) will require the laminate to be as symmetrical through the thickness as possible to control CTE properly (the risk of getting out of plane thermally induced deformation increases).

Provided this is done, you can mix UD and woven. If you do not make the laminate quite the same around the tube as along it in terms of fibre direction, the CTE wiil be different around the tube than along it (duh!).

Moving to UD will give you the opportunity to make the CTE a bit less than with woven, if it's critical.

If you make the layup something like twill@0°/UD@0°x4/twill@0° it should come down to about 1.3 microstrain/K along the tube (and maybe 8 microstrain around it). If you go to an expensive high modulus fibre it'll come down even more: Toray M55 as the UD might give 0.15 microstrain along the tube.

Mismatching the CTE of woven with 0° UD would make it a bit more likely to get interlaminar cracking at the laminate edge, even with an RT cure.

For a thing like this tube, it's ok to carefully butt the plies with no overlap. You only really need to overlap things if strength is a major issue.

Thanks for the links.

 

[k05t1ka]

Thanks for the tips.
The most important thing for the tube is that it will not bend with the temperature variation. If it does, the telescope looses collimation (allignment of the optical axis) and it must be collimated again and again. If the tube length or diameter vary with the temperature it's not so critical because you can refocus (however, these variations must be as small as possible).

Given what has been said in the above post, I think I will go with one layer of twill rolled multiple times on an aluminium mandrel. At both ends, the tube will be bolted on some flanges and the interlaminar cracking at the free edges will make things worse.

I began some testing with some aliminized fiberglass twill (the only fiberglass fabric I have with no silane sizing) to see how the fibers allign when rolling the fabric on a mandrel. Right now I have problems with vaccum bagging wich causes some wrinkling on the tube.
I think I will not use vacuum on this project. I will try to make some "poor-man prepreg" by impregnating some fabric with the just amount of necessary liquid resin and then wrap it very tight around the metallic mandrel. Then I will wrap some peel-ply around it to be able to sand the exterior without damaging the fibers and then wrap some bagging nylon very tight. After that I will cure the thing at temperature and I hope the expansion of the aluminium mandrel will strenghten the fibers even more.
I'll let you know how it worked out and post some pictures.

 

[Compositepro]

It is most important to wrap the fabric tightly as it is applied. Any compaction that occurs after the fabric is wrapped will result in wrinkles. Compaction results in a reduction in outside diameter, and hoop fibers cannot shrink. Tape the fabric to a pipe or core with the fill yarns perfectly straight and aligned with the axis of the pipe. Wrap a little more than enough fabric onto the pipe and use this to apply the fabric to the mandrel. Apply resin to the mandrel to tack the fabric down and use a grooved laminating roller to impregnate the fabric as it is applied. Use the pipe to apply even tension on the fabric as it is applied. At first there will be some slippage of the fabric on the mandrel when you tension it. After a full wrap slippage will be due only the the fabric compacting down, which causes excess resin to bleed up to the surface.

You can apply a final layer of peel ply the same way so that you end up with a clean outside surface without resin ridges.

 

[k05t1ka]

Thanks, I will try this too and see what I get.

 

[k05t1ka]

I managed to finish the test tube made from aluminized glass fiber. I used a carboard tube(sonotube) as a mandrel. After I wrapped it in nylon foil, I wrapped the fabric two times around the mandrel, then I wrapped some peel-ply very tight around and then I put another nylon foil around. After that, I wrapped everything with some wide adhesive tape.
The result can be seen in the pictures. The tube is just out of the mandrel and it must be sanded and polished.

I gave up using vacuum consolidation for this project because it kept making wrinkles no matter how tight I was wrapping the fabric.

Now I can make the final tube using carbon fiber and an aluminium mandrel.

Thanks again for the ideas.

 

[gwolf2]

I was talking to Peter Wise last week, a custom telescope maker. see zerochromat.com

He said that he just bought in pre-made carbon tube and didn't much worry about the detail for tubes up to 200mm. He said they are strong, stiff, dimensionally very accurate and "have a thermal expansion coefficient 1/100th that of aluminium"

gwolf.