How many plys?

[dtandy]

I am making a large diameter (~23" OD, ~21.5 ID ) tube with a 20mm thick H8PP NidaCore core.

The tube will be used for a large telescope and needs to be stiff but it will not by carrying any significant weight as the entire telescope will only weigh about 85 lbs.


How many plys of 11 oz 8HS carbon cloth should I use for the inner and outer layers? I would think that 1 ply (with perhaps a 12" overlap at the seam) would be sufficient and result in a very stiff tube.

I have made one other tube about this size and used two plys of 11 oz cloth for each layer. The tube had a .250" Flexcore core and was super stiff. I tried to flex the tube by giving it a bear hug and I couldn't get it to budge in the slightest. I'd like to save money where possible yet not compromise strength, hence, my question about going with 1 ply for each layer versus 2.

I will be vacuum bagging the tube in probably three layup steps (inner, core, outer) using peel ply to get a good bondable surface between the inner layer and the core.

Thanks,

Dave

 

[rstupplebeen]

I tried to find an equation for a composite tube in bending however I could not. In the absence of a hand calc I would do a quick and dirty FEA with better units than 1 bear hug . Equations of composite beams in bending might give yourself enough insight. I hope this helps.

Rob Stupplebeen

 

[RPstress]

It sounds as if you need to quantify the issue somehow to be numerate about the thicknesses.

Maybe you could come up with a bending stiffness requirement for the telescope under its own weight when horizontal.

You'd need some sort of estimate of an acceptable deflection from an optical point of view. How much angular misalignment along the tube would compromise the image.

You could then work out the bending angular deflection of a tubular beam with one ply and two plies quite simply using Roark or maybe a bit of FE as rstupplebeen suggests, perhaps with a cheap FE system like MYSTRAN at

http://www.mystran.com/

or LISA at

http://www.lisa-fet.com/

. (MYSTRAN works ok, I've used it - it used to be free. LISA still apparently is, but I've not used it.)

On the other hand if two plies were definitely ok, you know that one ply will be twice as flexible. You might be able to find a way to check the adequacy of the two-ply tube. If it's more than twice as stiff as it needs to be you're ok with one ply.