Bending graphlite rod

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:

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

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.

"Ernest Christley" wrote in message
...

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.

You are making some assumptions that are incorrect.
First, movement of the fibers inside a resin system heated to Tg
degrades the mechanical properties of the mass after it cools. This is
because Tg is a temperature point that is usually used to define destruction
of the member due to it's inability to maintain designed mechanical
properties.
In your description, epoxy is a thermoplastic that can be heated/formed and
cooled
repeatedly without loss of properties. In reality it is a thermoset and is
not remoldable.
The loss of mech properties happens as a microshear failure inside the
matrix as
the resin and fiber move relative to each other "crumbling" the crystaline
structure of the resin.
Be very careful when suggesting that heating a laminate to Tg, bending it,
allowing it to cool
is an answer to "reshaping" the part without serious degradation of the
properties of the resin.
OBTW, Graphlite is a BIS F Epoxy with a Tg of 100C.