CF composition construction questions

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.

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

 

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.

 

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

 

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.

 

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

 

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.

 

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

 

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

 

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.

 

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.

 

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.

 

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

Yoke.

 

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

 

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.

 

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