CF composition construction questions

[dtandy]

Hello,

I thought I'd tap the collective experience of those who have built with carbon fiber composites. I'm looking to build a large diameter telescope tube out of carbon fiber and I have a few materials and process questions. Sure, it isn't sailing but I have sailed to Bermuda twice so maybe you can cut me some slack for a slightly off-topic post. :D

The tube is 24" OD by 56" long. The ID isn't critical. Obviously, lighter is better but it needs to be stiff and durable. The tube will also be drilled to hold various parts so the wall needs to be thick enough to mount hardware although nothing will see a heavy load. I was planning to vacuum bag the assembly but I'm concerned about collapsing the mold so I need to do a bit of experimenting to see if the mold can handle the pressure before painting myself in a corner, so to speak.

I am planning to use a carbon fiber weave fabric around 5.5 oz/yd weight. Should I use a heavier or lighter fabric? How many layers should I plan on laying up? Is there any need to use a Kevlar layer? Should I use a honeycomb core ? Which type? How many layers on the inside and outside of the honeycomb? Any suggestions are appreciated as I am a novice at this. I plan to make a few small pratice runs at small parts prior to tackling the big tube.

Thanks,

Dave

[marshmat]

Hi Dave, what you describe is certainly doable. You'd probably want to lay this up over a cylindrical mandrel of the desired inside diameter, rather than using a mould (you'd need to split the mould, build 2 half pipes, and laminate those together). 5.5oz is a pretty light cloth, easier to work than the heavy ones but more total labour due to more layers being needed. If it were my project, I'd be likely to use four or five layers of the cloth on each side of an overexpanded honeycomb core, perhaps 1/4" or so thick. If you decide to core, you really do need to vacuum-bag if you want it to work out well. (If you use a pipe that's about a foot longer than your tube as a mandrel, you can tape the bag to the pipe hanging past the end of the telescope tube, leaving the centre open... then only the part, and not the whole pipe, is under vacuum and you wouldn't be able to collapse it.)
Stock tube this size would not be easy to find, but it might be worth looking.

[dtandy]

Matt,

Ahh, excellent suggestion regarding how the bag the part. Also, you're right, I meant mandrel rather than mold.

Sonotube comes in a 24" diameter size and I could probably use that. It doesn't have a perfectly smooth surface but that wouldn't really matter since the inside of the tube doesn't have to be perfect and it's actually beneficial to have some roughness to help trap light.

Do you have a good suggestion as to which type of honeycomb to use?

Dave

[marshmat]

Hexcel is one of the biggest core manufacturers.... http://www.hexcel.com/Products/Core+Materials/ Overexpanded Nomex (aramid), aka Flexcore, is what I'd consider in your case, it's flexible enough to make a cylindrical shape but has almost the same stiffness as regular honeycombs once the laminate is cured.

[fiberglass jack]

before you do anything wrap the tube with a layer of light cardboard and tape it down tight then wrap a layer of mylar using 3m 77 spray adhesive, this way u will be able to slip the tube out then tear the cardboard and mylar out from the finsh tube, if your going to vac then renforce the tube or it will colapse maybe fill the inside of the tube with expanding foam , for a nice look finsh with 2x2 6 oz carbon twill and paint with clear coat which will sand smoth and polish nice, put two pegs at each end of the tube to let u spin the tube on a couple of stands or saw horses ,also get a 1x2 cut it so it will be the lenght of the floor to the tube and hammer a nail at one end, when u spin the tube use the stick as a brake by placing the nail into the foam trust me its a nightmare to hold the tube and spin so grab a friend

[marshmat]

You wouldn't bag the whole tube, would you? Cut the bag an inch narrower than the mandrel is long, and wrap it around the circumference.... the only pressure differential will thus be in the skin, and not trying to crush the whole tube..... good advice from jack about the cardboard/mylar, that'll make life a lot easier.

[fiberglass jack]

if you going to vac why not infuse ,resin line with sprail tubing on one end vac on the other end ,no messing with wet cloth just tack the carbon with some 3m 77

[dtandy]

A couple of questions....

Could I go with a heavier cloth (maybe 19.7 oz/yd2, 0.03" thick), no honeycomb, and layup four layers followed by a light cloth finish layer and achieve good stiffness and durability? It seems like four layers would give a wall thickness of around 1/8". This is thick for aluminum and much thicker than I've seen on the carbon fiber tubes I've used to build a bike. So, it seems like it would be adequate for the tube.

Using a honeycomb core has structural benefits but it seems like it adds to the complexity of making the part. Also, how easily does the honeycomb cut? I'd like to achieve a nice finish on the ends of the tube and it seems like honeycomb would be hard to finish easily whereas a good cut through carbon fiber is easily sanded smooth with not much work.

Are there any benefits to adding a layer or two of Kevlar if I go without a honeycomb core?

Any suggestions on how to cut the ends of such a large tube nice and square? My thought is to roll the mandrel and cut around the circumference with a saw.

Regarding infusion, how much risk is there that the epoxy will not wet every part of such a large tube? I'd hate to find out the hard way....

Dave

[Toot]

I might use fiberglass as an inner layer/s to keep the cost down. You can build thicker that way and not be paying $60 a yard or whatever.

[marshmat]

I second Toot's idea about using some FG in the middle. Honeycomb doesn't give nice edges when cut, it needs edge closeouts of some sort. There are no benefits to adding Kevlar in this, unless you plan to be repeatedly dropping it on concrete (which I hope you don't). I wouldn't bother with infusion for a one-off, it's a huge pain to set up and get working.

[Toot]

Heck, if you go to fibreglast.com, the have two things which may interest you.

First, they have black fiberglass. It's not really "black" but it's dyed/treated to look like carbon fiber... for less money. They also have aluminized fiberglass that is silver and shimmery. Looks pretty cool. Some young kids call it "silver carbon fiber", but as long as you don't get wrapped up in the marketing and hype, it's still a pretty cool thing.

Just more options. :-)


Oh... and if you really want something fancy, you could consider adding a little bit of tint to the resin. Red? Blue? Green? Just make it subtle and you can have a little bit of colored glow in the surface. I've never seen it done, but it might be pretty... You could probably also do it with the clearcoat and have a little more control over the process.

[yokebutt]

Kevlar is a very good vibration dampener, it also has the lowest density of readily available fibers. (thickness=stiffness)

Yoke.

[dtandy]

I like the idea of spending less by using FG but weight, stiffness and thermal expansion are considerations. With the size of the telescope I'm building I'm already pushing up against the max weight limits for non-custom mounts so I'm trying to go light where I can. It looks like S-glass epoxy composite is pretty strong stuff (similar specific strength) relative to carbon fiber composites according to one website I saw. It is much less stiff, though. Also, thermal expansion is an important consideration as well since changes in the distance between the mirrors changes the focus of the instrument. I suspect the CTE for fiberglass is still low relative to say aluminum, though.

So, is it safe to say that given equal fabric weights that one can substitute one layer of s-glass for one layer of carbon maintaining comparable strength but with 1/3 the stiffness? For a 24" diameter tube with adequate wall thickness stiffness shouldn't be an issue but weight definitely is.

I really appreciate all of the good info!!

Dave

[dtandy]

Oh, one other question.... what type of surface finish can I expect when vacuum bagging? How much finishing work should I expect? Any tips on the best type of peel ply to use? My goal is smooth as can be!!!!

Any yes, I've already looked at the tints. I'd like a "trick" look to the tube and have been looking at some of the kevlar/carbon color weaves as a finish layer.

[fiberglass jack]

to cut the tube square ,get a sheet of masking paper say 1 foot wide and wrap it around the tube , use this as a guide

[cristofa]

Dave

Producing something for so specific a function, it would be useful to know a bit more about the function. With 24" diameter, I assume this is a reflecting telescope, so I'm not sure why 'Obviously, lighter is better' ... astronomy's not my field; what do folk generally use for this? I'd have thought that (within reason) the greater the mass of the structure, the steadier the image.

Regarding composites, I have become a huge fan of the materials/layups with integral resin infusion mediums - we use two kinds for different purposes. Saercore (http://www.saertex.com/index.cfm?2C7CCD5E2B3E9DFD2BAD66B685E3B48D), or Rovicore (http://www.chomarat.com/pages/accfac_rovicore.htm)(and the others I don't know about) are random mat glass, either side of a knitted plastic stuff. It's remarkably conformable, and it works particularly well in closed moulds. We have used it under a bag, however - in which case you can regulate its thickness to some extent with more or less vacuum. In your case you could apply some carbon either side if you want.

The other material is Lantor Soric (similar to Corecell, I gather), which is a non-porous polyester material with hexagonal flow channels for the resin. You simply lay the Soric between your reinforcement (be it glass or carbon) and, when it's done its job as an infusion medium, the hexangonal ribs of resin act as a core, significantly adding to the strength of the composite.

Both these materials make the infusion process delightfully easy, and cut the cost of 'consumables'.

And regarding the suface finish from a bag - on complex shapes you apply lots a surplus bag to reduce the risk of bridging, and the result can be decidedly wrinkly. With a simple shape like your cylinder, though, you could prevent the wrinkles by stretching the bag quite taut; you will, however, get the texture of the outside reinforcement. If you don't want that, although it would be a bit more fiddly, you could lay your cylinder up on the inside of the tube.

[dtandy]

Christopher,

Thanks for the post. I'll take a look at the materials you listed.

Mass on a telescope is good (neglecting portability, mine will be housed in an observatory) if it is centered in the pier and mount that hold the telescope in place. A lighter tube is more easily moved and held stable by a given mount compared to a heavier tube. There are two commericially available mounts I am considering and they are rated to 150 lbs payload for photographic use. A 20" scope of the type I am building is pushing those limits so I need to conserve weight where I can. There are no "off the shelf" mounts available with larger capacities and custom mounts are big $$$$$.

The tube will be approx 24" diameter by 56" long. At one end it will house a mirror and a metal mirror cell that weigh a combined 75 lbs or so. At the other end, it will house a smaller mirror with support structure that weighs around 15 lbs. The tube will be cradled by large circumferential tube rings that will be mounted to the telescope mount. Take a look at http://www.rcopticalsystems.com/20inch.html to get an idea of the type of telescope I'm building. I could go with a truss but the cost for materials and machining all of the parts is quite a bit more than the cost of materials for a carbon fiber tube.

I just need to ensure that the tube is rigid enough to support the optics and be held securely by the rings. I'm not sure how many layers of CF at a given weight is adequate. I know that aluminum tubing is available and that it is fairly thin stuff... like 1/10" to 1/16". It seems like 1/16" thick CF may have enough strength but I would think it would be fairly brittle and easily damaged. The tube will be drilled at a number of points to allow bolts to attach the mirror cells to the tube. I think that 1/8" thick walls should be plenty strong and thick enough to support the mounting bolts for the optics. I had planned on using a couple of layers of Kevlar/CF as the outer skins for aesthetic reasons and that should help provide a certain level of toughness for the tube as well.

I am concerned about the compressibility of honeycomb core material as the bolts will need to be fairly tight (pulling tension across the diameter of the tube) to rigidly hold the mechanical assemblies that hold the mirrors. I don't want to be puncturing the surface of the tube as I tighten down bolts.


Dave

[Toot]

I am concerned about the compressibility of honeycomb core material as the bolts will need to be fairly tight (pulling tension across the diameter of the tube) to rigidly hold the mechanical assemblies that hold the mirrors. I don't want to be puncturing the surface of the tube as I tighten down bolts.

That's a concern I agonize over as well with some of my projects. If you aren't concerned about fatigue resistance and such, the easiest way to do it is to drill the hole through the outer surface of the structure, then take a piece of wire (like a paperclip), bent at a 90 degree angle and insert it into a drill. Let the paperclip spin around inside the core of the structure. Then take a vacuum or compressed air and suck/blow the particles out. Then you can pour/inject resin, or a slurry into the area, making a hard point. After it cures, you drill the hole again and mount to the hardpoint.

[marshmat]

Toot- a slightly more sophisticated procedure, but similar in concept, is used to hold gearboxes and engines onto some helicopters. Done well, solid inserts are insanely strong.
In my experience, washers are ALWAYS necessary when bolting to carbon, regardless of the type of load.
1/8" would be what I'd expect if you're making this solid, no core. On a tube that size I'd want to use perhaps a 1/2" overexpanded core. The extra thickness makes it a LOT stiffer! I'd hate to have to recollimate the scope every three days because the tube flexes. Off the top of my head, I'd be tempted to say 1/2" Nomex overexpanded honeycomb sandwiched on each side by four layers of ~300g (9oz) carbon in epoxy. This wall would have a Young's modulus of around 9.3 GPa (depending on core density)- yes that's only 13% of aluminum's modulus, but it's 18 mm thick thanks to the core- compared to 3 mm of aluminum, that's 216 times the geometric stiffnesss at 13% of the modulus, or a total of 28 times stiffer than 3 mm aluminum wall (comparing flat plates, not tubes, but the comparison should hold...). This tube, at 24" dia, would weigh around 8.5 lb per foot of length. Sound like what you're thinking of?
Oh- if you do the cf/kev outer layer, make the carbon axis of this stuff follow the length of the tube and the kevlar follow the circumference, this way the carbon will be of the most benefit....

[Toot]

Washers are important, as the composite structure, regardless of the type of fabric, will not take well to localized stresses.

Washers are ok because they spread the stresses over a larger area. Other ideas for mounting hardware include:

after drilling the hole, coat the rivet/bolt/whatever with epoxy before tightening it down. After drilling through the FRP, you will have strands of fiber that have debonded. This bit of extra glue helps to rebond those areas and prevent stress risers. Not as good as a washer, but it's another option.

Even better, add a small piece of resin-coated fabric over the hole that you intend to mount into. Then push the bolt/whatever through the fabric, but without tearing the fabric... simply force the tows to realign to one side or the other of the bolt. Rewet it, let it cure. Once it's cured, the fibers will transfer the load around the bolt hole. This is far superior to a washer where the surface is not flat. A washer on a rounded surface, for example, isn't really going to work well. This approach, however, can be quite messy.