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,

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 etc., 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?)

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 etc., 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.)

In addition to the application I'm considering which is aerospace
related,
a potentially very important application to this would be in hydrogen
storage.
Hydrogen fueled automobiles have been given high priority by the US
government.

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 etc., 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.)

In addition to the application I'm considering which is aerospace
related,
a potentially very important application to this would be in hydrogen
storage.
Hydrogen fueled automobiles have been given high priority by the US
government. A key problem that needs to be solved is the storing of the
hydrogen in low weight systems within the vehicles. The US government
has set
a benchmark of 6.5% weight hydrogen to the storage system weight for
hydrogen
fueled vehicles to be competitive with gasoline vehicles.
Liquid hydrogen requires expensive and heavy cryogenics. And gas
hydrogen
storage requires the gas to be kept at high pressures.
In the case of gas storage clearly you want the density to be as high
as
possible while at the same time saving weight in the storage system.
This is
why high tensile strength materials would be useful.
Carbon nanotubes have been a key area of research in this regard.
There were
some early reports that high storage density was achieved but these
results
were not repeatable:

Carbon nanostructures: An efficient hydrogen
storage medium for fuel cells?
http://www.fuelcelltoday.com/FuelCellToday/FCTFiles/FCTArticleFiles/Article_433_Carbon%20Nanostructures.pdf

In addition to the high strength non-metallic fibers mentioned above
there is also
a high strength steel wire marketed known as Scifer wire. This steel
wire can
be up to 5.5 Gpa strong in tension. As is the case with the other
fibers this strength
holds in longitudinal tension for thin wires: the Scifer wire may be
only 8 microns
wide. However, interestingly there is micromaching being done actually
at the
*submicron* scale. Here's a review article by Clark-MXR, Inc. (no
relation) :

Clark-MXR, Inc. Femtosecond Lasers Micromachining Handbook.
http://www.cmxr.com/Industrial/Handbook/Introduction.htm

The key facet that makes this possible is that ultrafast laser pulses
are
used. This prevents the thermal energy from spreading beyond the area
that
needs to be machined.
This method clearly would suffice for creating a small hole cross-wise
in
8 micron wide wires or wider fibers, which would allow tests to be
made of their
cross-wise tensile strength.
In regard to producing tubes from the wire, the method may also work
if the
wire is kept very straight. Also, an interesting advantage of using
ultrafast
pulses is that the heated material moves away quickly from the
illuminated
area because it turns into a highly ionized plasma. We might also help
this
process by using a high pressure suction to remove this material or
high
magnetic fields.
Still another facet of these ultrafast lasers is that they can
actually be
tuned to focus *inside* the material. That is, the exterior of the
material
is left unheated and the laser energy is concentrated inside.

We can get an estimate of the storage weight density of hydrogen we
can achieve using
Scifer steel tubes assuming they are able to maintain their 5.5 Gpa =
55,000
bar tensile strength radially. At very high pressure hydrogen like all
gases
no longer obeys the ideal gas law. I found this page that computes
hydrogen
properties given temperature and pressure:

Hydrogen Properties Package.
http://www.inspi.ufl.edu/data/h_prop_package.html

Lets say we store the hydrogen at 2200 bar. Then at 200K the density is
62.4
kg/m^3. Use the Barlow formula to see how thick the Scifer steel tube
would
have to be to hold a gas at 2200 bar pressure: P = 2*S*t/d, t =
P*d/(2*S) =
2200*d/110000 = d/50 . Since the outside diameter = 8 microns = d +
2*t, this
results in t = .154 microns and d = 7.7 microns. For say a 1 meter long
tube
the volume of the steel would be Pi*(.000008^2-.0000077^2)/4 =
3.7*10^(-12)
cubic meters. At 7800 kg per cubic meter density for steel this would
weigh
2.89*10^(-8) kg.
Now the hydrogen inside the tube would have volume Pi*.0000077^2/4 =
46.57*10^(-12) cubic meters and at 62.4 kg/m^3 density would weigh
2.91*10^(-9) kg. So the ratio of the hydrogen to the steel would be
2.91*10^(-9)/28.9*10^(-9) = .1, which exceeds the benchmark required
for
hydrogen car viability.
Actually looking at the density numbers returned by the "Hydrogen
Properties
Package" we could do better than this by choosing a lower pressure for
the
hydrogen. And indeed the pressure probably doesn't have to be
exorbitant. But
the key factor is of the thinness of the Scifer steel or other high
strength fiber
that would be required to hold it.



Bob Clark

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.

We can get an estimate of the storage weight density of hydrogen we
can achieve using
Scifer steel tubes