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?

 

[ravenprp]

Thank you for sharing your interest in high strength fibers for high pressure tubes. It is clear that you have put a lot of thought into finding the best solution for your application. Let me address some of your questions and provide some additional information on high strength fibers for high pressure tubes.

Firstly, you are correct that using wound fibers for high pressure tubes will result in a lower tensile strength compared to the longitudinal tensile strength of the fibers. This is due to the fact that the fibers are bound together with epoxy, which acts as a weak point in the overall structure. The amount of reduction in tensile strength will depend on the specific type of fiber and epoxy used, as well as the manufacturing process.

In terms of creating tubes by drilling through the fibers longitudinally, this is a feasible option. However, as you mentioned, this will result in thinner tubes which may not be suitable for all applications. Additionally, the tensile strength circumferentially may not be the same as the tensile strength longitudinally, as the fibers may not be evenly distributed around the circumference of the tube.

Your ideas for creating hollow fibers using a lubricating material or a lower melting/sublimation temperature rod are interesting and worth exploring further. However, it is important to consider the impact on the overall strength and integrity of the fiber when removing the rod.

Regarding your suggestion for testing the circumferential tensile strength by drilling a small hole cross-wise through the fiber, this could be a viable option. However, it is important to note that the results may not be an accurate representation of the overall tensile strength of the fiber, as the presence of the hole may weaken the structure.

In conclusion, high strength fibers such as carbon fibers, Kevlar fibers, and S-glass fibers can provide excellent strength for high pressure tubes. However, it is important to consider the trade-offs and limitations of using these fibers in your specific application. I would recommend consulting with a materials engineer or conducting further research to determine the best solution for your needs. Best of luck with your project!