Bending graphlite rod

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.

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

wrote

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 using flat graphite stock and laminate the shape you want to end
up wit. That would eliminate purdy much all of the preload, no? g
--
Jim in NC

Morgans wrote:
wrote

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 using flat graphite stock and laminate the shape you want to end
up wit. That would eliminate purdy much all of the preload, no? g

The cat's meow would be pultruding it directly into the desired shape.
That's a bit beyond the capabilites of the homebuilder--I think.

--

FF

wrote:

Morgans wrote:

wrote


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 using flat graphite stock and laminate the shape you want to end
up wit. That would eliminate purdy much all of the preload, no? g


The cat's meow would be pultruding it directly into the desired shape.
That's a bit beyond the capabilites of the homebuilder--I think.

Maybe not.

In normal composite work it's fairly normal to make primative molds or plugs
to make parts.

What's the diff?
The minor "preload" is between layers of the laminate.
Even wood laminates work that way.


On the bigger side, I've always thought a large "rigid" airship frame could be
built using geodesic methods.
Like a dome, but stretched out.
It's done by varying the strut length progressively.

wrote:

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.


I think you're missing the forest for the trees. A wood steamer is your
answer. Make a form out of sheet metal, then bend the graphlite around
it. Tg is around 180F for most epoxies, so an hour at 212F should turn
it to it's plastic state throughout. The internal bending stresses will
pull themselves out. Take it out of the oven, and it should hold
whatever shape it had been forced into.

Now, what I understand of graphlite is that it gets it's strength from
having all the fibers aligned and equally tensioned. Doing the above,
you will probably get something only marginally better than a hand
layup. But you won't know until you try.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Ernest Christley wrote:
wrote:

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.


I think you're missing the forest for the trees. A wood steamer is your
answer. Make a form out of sheet metal, then bend the graphlite around
it. Tg is around 180F for most epoxies, so an hour at 212F should turn
it to it's plastic state throughout. The internal bending stresses will
pull themselves out. Take it out of the oven, and it should hold
whatever shape it had been forced into.

I first tried heating the rod in boiling water and bending it.
No dice. I then tried heating it with a heat gun, still
wouldn't bend. But that time I heated the middle of the
rod, not the end. I'll go back and try heating the end.

Then I may take up your suggestion, but bending the
rod to a tight curve while it is at room temperature is
NOT trivial.

Phenolic resins are thermosetting. While they do have
a glass transition temperature, that GTT increases when
the material is heated, making it a moving target. My
impression is that most phenolic resins will char befor
they soften.

There are thermoplastic resins that have a reproducible
GTT. Dunno if anybody makes pultruded rod using them
though. I am also less than confident in the vendor's
published descriptions of the products. Even if they
were accurate when written the vendor may change
their source to a similar product without updating the
description.


Now, what I understand of graphlite is that it gets it's strength from
having all the fibers aligned and equally tensioned. Doing the above,
you will probably get something only marginally better than a hand
layup. But you won't know until you try.

You and a preceding author both raised this point and it is well
taken. Maybe if the rod is bent 'monotonically', I.e. bent
onto the form without wiggling it the fibers will slip in only
one direction and retain their approximately equal tension.

--

FF