Re: ? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders & Diffraction Spikes

["matt" <mariusrf@bellsouth.net>]

not quite true. Carbon fiber laminate , if designed and engineered properly
can be significantly stronger than the majority of steels . More precisely
it can have much higher tensile strength than steel . However , it has a
twofold problem which could prove to be a show stopper:

 #1 it really needs to be designed , engineered and manufactured properly
for these qualities to happen , or else the laminate will achieve only a
small fraction of its potential . Among the things that are needed would be
to use unidirectional carbon tow, properly engineered metal inserts for
attachment points, vacuum bagged and oven cured , needless to say use a good
structural epoxy for laminating  . IF these things aren't done properly, the
laminate will be much weaker than its theoretical strenght and you'll need
to design in this weakness by using a much larger safety factor , in other
words overbuilding it and negating any advantages carbon may otherwise have
if executed properly.

Issue #2 is not so easy to avoid, and that might prove to be fundamental.
Carbon fiber has tensile strength that exceeds steel but is extremely poor
in compression. Failure mode is buckling of the laminate and fracturing the
fibers. Steel and more generally metals do not have this failure mode.
Another failure mode is puncture . Simply put, you have to not hit, bend ,
or abuse your carbon spider , because it will only take axial loads. Bend it
sideways for example to attach it or insert it into the tube and you can
kiss it good bye.  Hit it with a screwdriver tip and it's done . The
puncture issue happens especially with thin flat skins , and that would be
the case with thin spiders . Round thicker walled tubes are more robust in
this respect .
So unless you're making a spider that you know nobody will touch for a long
time (for example a spider that goes into a closed tube in an observatory
setting) plan on it breaking often and make lots of spares that are easy to
replace in the field, or don't use carbon fiber for it . Use titanium, steel
, piano wire, racing bike blade spokes, but not carbon fiber.  Make your
mirror cell , truss tubes or other scope parts of carbon where it's not
exposed to these catastrophic failure modes.

best regards,
matt tudor


-----Original Message-----
From: Ross Sackett <rsackett00@yahoo.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Monday, November 15, 2004 9:08 AM
Subject: Re: ? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders &
Diffraction Spikes


>What advantages do you see for a CF spider?  It
>certainly would have low thermal mass, but I doubt it
>will be as stiff as you hope.  Carbon-epoxy has high
>stiffness for its density, but you'll probably have to
>laminate it pretty thickly to match the stiffness of
>conventional steel vanes. (I haven't looked
>systematically, but having played around with single
>and double layer BID CF it doesn't seem as stiff as
>the same thickness of steel.)  There may be a weight
>advantage, but I suspect matched stiffness for
>thickness steel has the advantage.  (OK I admit I am
>being lazy here--if I had my materials tables handy it
>would be easy to check up on all this.)
>
>If you go the CF route you may want to consider
>unidirectional cloth--the vanes are loaded primarily
>in tension, and the weft of the BID isn't going to
>help you much in that case but will add thickness to
>the weave.
>
>Ross
>
>
>
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