Bending graphlite rod

The data on the Marske website about the graphlite (TM) rod they use
indicates
a glass transition temperature of 212F. So I tried heating some with a
heat gun
to see if I could put a permanent bend in it. No luck.

I think maybe it uses a phenolic resin which is thermosetting, rather
than thermoplastic. If I understand these things, heating a phenolic
resin raises the glass transition temperature so that you really
cannot soften it.

I've thought about wetting a tow with epoxy, and then wrapping it
around a form. The problems I anticipate include flattening of
the tow, and not keeping approximately equal tension on the
individual fibers as is done with pultruded rod. The second problem
might be addresses by flattenting and spreading the tow,
clamping it between squeegees, and then pulling on it so that
the fibers with the most tension slip in the clamp, shifting the
tension to the others, until they are all approximately equal.

But it would be a heck of a lot easier to buy a coil of rod and
shape it.

So some questions are, "Is there any practical way to work graphlite
(TM)
rod to bend it into a permanent curve? Does anybody make pultruded
carbon fiber ros using a thermoplastic resin? Is it possible I just
didn't
heat my sample enough?


--

FF

Earlier, wrote:

So some questions are, "Is there any practical way to work graphlite
(TM) rod to bend it into a permanent curve? Does anybody make pultruded
carbon fiber ros using a thermoplastic resin? Is it possible I just
didn't heat my sample enough?

Ooh, yay, a question I can sort of partly answer, and posed by a fellow
SpamCop supporter besides!

If you want to bend one little portion in the middle of a long chunk of
the stuff, you can't just heat the area where you want the bend and
leave the rest cool. That's because you're asking the carbon fibers on
the inside of the bend to shrink, and the fibers on the outside of the
bend to stretch, and carbon don't play that.

I suspect that what you would need to do is heat the entire length to
tg and crank in the bend so as to get a permanent shear displacement
between the inner and outer fibers - and in order for that to work, the
displacement has to propagate all the way to the end of the rod. And
I'm guessing that's going to be a very touchy hit-or-miss operation
that's likely to weaken or disrupt the bonds between the fibers.

So, what to do? What I do and have done in exactly that sort of
situation is to use many smaller Graphlite elements, bent and then
bonded together, instead of one big Graphlite rod. As an example, check
out the bend in the bottom spar cap on this Graphlite-based wing spar:

http://www.hpaircraft.com/hp-24/update_21_march_04.htm

That bottom spar cap is about 1-1/2" wide and 3/8" deep, and if it was
solid Graphlite there's no way I could bend it and make it stay. But
since it's made up of many smaller elements, bending the Graphlite
strips into the mold and making them stay is pretty much painless.

As another poster suggested, Jim Marske really is the expert in using
Graphlite. He's designed lots of neat tooling for coaxing it into
position and making it stay until the part is cured.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24
http://www.hpaircraft.com/glidair

Earlier, wrote:

I'm thinking like making a stick rib.

Maybe try making the rib out of straight pieces, and let the rib truss
lock in the curve for the upper and lower cap strips. I bet that'd
work.

Also, Graphlite comes in a variety of round, square, and rectangular
cross-sections. I think the standard round sections go down to 0.020";
it might not show on the Web but Jim Marske can get it for you.

Thanks, Bob K.

Bob Kuykendall wrote:
Earlier, wrote:


I'm thinking like making a stick rib.


Maybe try making the rib out of straight pieces, and let the rib truss
lock in the curve for the upper and lower cap strips. I bet that'd
work.

Also, Graphlite comes in a variety of round, square, and rectangular
cross-sections. I think the standard round sections go down to 0.020";
it might not show on the Web but Jim Marske can get it for you.

Thanks, Bob K.

Go to this page on graphlite
http://www.aist-nara.ac.jp/~rieken/projects/composites.html The
stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.

John

J.Kahn wrote:
The stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.

Yup, that's the way I get it, spools of 1000 to 1500 feet.

J.Kahn wrote:
Bob Kuykendall wrote:
Earlier, wrote:


I'm thinking like making a stick rib.


Maybe try making the rib out of straight pieces, and let the rib truss
lock in the curve for the upper and lower cap strips. I bet that'd
work.

Also, Graphlite comes in a variety of round, square, and rectangular
cross-sections. I think the standard round sections go down to 0.020";
it might not show on the Web but Jim Marske can get it for you.

Thanks, Bob K.

Go to this page on graphlite
http://www.aist-nara.ac.jp/~rieken/projects/composites.html The
stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.


Yes, mine came rolled into a circle. When you cut the tape
it straightens back out. I want it to hold a curve by itself. If
I have to add other materials to hold it into the curve I want
that kinda defeats some of the advatange to using it in the
first place.

If you are using a material to reinforce something, pre-stressing
the reinforcement may or may not be a good idea, depending
on whether it is pre-stressed in the same orientation as the
applied stress, or contrary to it.

For wing spars, it may be helpful to droop the rod down as it
runs along the spar flange from root to tip. The leading edge
of a stick rib would be another matter.

Yes, there are other ways to make ribs.

--

FF

Bob Kuykendall wrote:
J.Kahn wrote:
The stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.

I've seen a picture of graphlite (TM) rod bent into a loop with a
radius
of about an inch or so. Now, imagine that you want a 2" diameter
ring, like a key ring. Would you bend the rod into a circle and glue
the ends together?

I think not!


Yup, that's the way I get it, spools of 1000 to 1500 feet.

As you probably noticed, when you take it off the spool, it
straightens out. I want to shape it so that it is curved without
any applied stress.

--

FF

Fred,

The graphlite derives its strength from the fact that the individual
carbon fibers are all kept very straight. Even when slightly bent, the
fibers in any local vicinity are still relatively straight. If you try
to heat it past its transition temperature while bent, the fibers under
more tension will try to displace toward the fibers under less tension.
I believe that you could do this; transition temperature is transition
temperature; I don't think you got it hot enough... But then you would
lose the basic strength characteristic of the material.

Bob's suggestion of using/joining smaller diameter rods is a logical
one. The pre-stress here may be something that you can easily live
with...

When joining with other same diameter rods or larger diameter rods for
axial loads, you need to have enough length such that the surface area
of the adhesive around the rod times its psi strength equals the psi
strength of a rod. For a round rod in tension L has to be approx. 80 D;
assuming an adhesive strength of 1,000 psi. So you can see that smaller
diameter rods have to have less length joining.

I have been giving some thought as to how to use this material other
than in a strictly axial loading, e.g. wing spars. Still thinking...

Blue skies,

Tom





wrote:
Bob Kuykendall wrote:
J.Kahn wrote:
The stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.

I've seen a picture of graphlite (TM) rod bent into a loop with a
radius
of about an inch or so. Now, imagine that you want a 2" diameter
ring, like a key ring. Would you bend the rod into a circle and glue
the ends together?

I think not!


Yup, that's the way I get it, spools of 1000 to 1500 feet.

As you probably noticed, when you take it off the spool, it
straightens out. I want to shape it so that it is curved without
any applied stress.

--

FF

wrote:
Bob Kuykendall wrote:

J.Kahn wrote:

The stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.


I've seen a picture of graphlite (TM) rod bent into a loop with a
radius
of about an inch or so. Now, imagine that you want a 2" diameter
ring, like a key ring. Would you bend the rod into a circle and glue
the ends together?

I think not!


Yup, that's the way I get it, spools of 1000 to 1500 feet.


As you probably noticed, when you take it off the spool, it
straightens out. I want to shape it so that it is curved without
any applied stress.

I don't think it's necessary to eliminate all preload. In the general
scheme of things, I would think that the amount of stress built up in
hand bending a graphlite rod to an airfoil profile is insignificant. In
wood rib construction it's only necessary to steam the forward part of
the top cap where the bend is extreme due to limits in the flexibility
of the wood, but in most of the rib the wood is under bending preload.
I would extend a graphlite top rib cap as far forward as it can be
comfortably bent by hand and use a wood nose block for the very front
for the nose radius.

The bigger problem is designing an effective way to join the pieces so
the joints are as strong as the graphlite pieces if it's going to be a
truss rib, using fibreglass, or maybe using pieces of carbon fibre sheet
cut into gussets and gluing it together like a wood rib.

John

J.Kahn wrote:
wrote:
Bob Kuykendall wrote:

J.Kahn wrote:

The stuff is rolled into coils for shipment. Scroll down and check out the
flat version bent by hand into a half circle.


I've seen a picture of graphlite (TM) rod bent into a loop with a
radius
of about an inch or so. Now, imagine that you want a 2" diameter
ring, like a key ring. Would you bend the rod into a circle and glue
the ends together?

I think not!


Yup, that's the way I get it, spools of 1000 to 1500 feet.


As you probably noticed, when you take it off the spool, it
straightens out. I want to shape it so that it is curved without
any applied stress.

I don't think it's necessary to eliminate all preload.

Nor do I. But if I want a curved structure made from the
rod to hold its shape without a lot of built-in stress it is
necessay to eliminate MOST of the pre-load.

...
I would extend a graphlite top rib cap as far forward as it can be
comfortably bent by hand and use a wood nose block for the very front
for the nose radius.

I wouldn't even consider doing that. If I'm going to use materials
in addition to graphlite rod, I'll use foam, monofilament tapes,
fiberglass,
or some such.

I love working with wood. I just don't see that as a efficatious
use, if I'm already using composites.

As noted before, there are a myriad of perfectly servicable
alternatives, all of which are outside of the scope of the
original question. There is no problem at all finding alternatives
and how to use them.

Perhaps more to the point, I just suggested a rib as
an example. Being able to bend graphlite rod like
one steam bends wood opens up a lot of design
possibilites.

Imagine, instead of reinforcing other materials with the rod,
making a wing, or wing and fuselage built like a birdcage
and then covered with Dacron. Not the cheapest and
maybe not the lightest or strongest and certainly not
the most practical way to go. But an interesting concept.

How about a dirigible made the same way? (Here one could
probable accept the pre-loading as there would be no really
tight radius curves.)

How about a kayak? How about a key-ring?

If it CAN'T be done, then it can't be done.

--

FF