High strength fibers for high pressure tubes.

RGClark

I have interest in an application involving a very high pressure tube. The pressure a pipe can resist is given by the Barlow formula P = 2*S*t/d, with P the pressure, S the tensile strength of the pipe material, t the pipe wall
thickness, and d the inner diameter.
I want to get maximum pressure resistance for the weight of the tube for my
application. However, if the tube is made of wound fibers such as carbon fibers, Kevlar fibers, S-glass fibers, you won't have the same tensile strength of the pipe wall material as that of the fibers in longitudinal tension, which can be in the range of 1,000,000 psi. This is because the fibers have to be bound together with epoxy which will reduce the tensile strength of the pipe wall against burst pressures (BTW, how much is this reduction in comparison to the longitudinal tensile strength of the fiber?)
So I was thinking, has anyone tried drilling through these fibers
longitudinally to create tubes? Since the fibers are quite thin this would
create quite thin tubes, but that's alright for my application as I can just
bind them together to get more fluid flow.
The question is would the tensile strength circumferentially be the same as
the tensile strength for the fibers tensed longitudinally?
A couple of ideas occur to me. While forming the fiber you could have
it form around a thin rod covered with some type of lubricating material so
that after the fiber forms, you could slide out the rod to get a hollow fiber.
Or you could have this rod have a much lower melting or sublimation
temperature than your fiber and raise the temperature so the rod will melt or
sublimate then flush the melted or gaseous rod material from within the fiber.
Secondly, to test the fibers circumferential tensile strength without having
to make the fibers be hollow, you could drill a small hole cross-wise through
the fiber. Then send a high pressure fluid through the small hole. You could
deduce the cross-wise tensile strength from the Barlow formula by seeing how
high the pressure can be before the fiber fails cross-wise.


Top view:


-----------------------------------------------

___ ^
/ \ |
| | Cross-wise tensile strength to be tested.
\__/ |
v

-----------------------------------------------

Tensile strength<------------->known high lengthwise.




Side view:

Hole drilled downwards through fiber this way:
|
| And high pressure fluid sent downwards through hole.
v


-----------------------------------------------

^
|
Cross-wise tensile strength to be tested.
|
v

-----------------------------------------------

(Hole drilled cross-wise; so not visible from side.)



Bob Clark

MR. P

rg I have a similar problem. I want to create a large vacuum volume at sea level. This Volume will occupy a sphere 1000 feet in dia. The question is does this have to be accomplished with one layer?