5 ms·

Carbon fibers have great tensile strength. Why would they be a good choice when a sub faces compressive forces?

by brnt 3y ago

_yid9 3y ago
Exactly my thought. But, then I’m a 50+ year old white guy, so unqualified to speak. In my personal experience w/ carbon fiber overwound tanks for compressed natural gas — my very first thought when I first heard they used a carbon fiber “pressure vessel” was: “Wow, ballsy; carbon fiber composite is great in tension, worthless in compression”.
- pjkundert 3y ago
  Funny, the downvotes from people who (like the CEO, apparently) … don’t have a clue. When you hear a “pop” or a “crunch” from a carbon-fiber wound composite tank: it means some part of the fiber/resin matrix has delaminated, or allowed intrusion of gas/fluid into its matrix. This means that this layer of the vessel now allows relative motion under torsion or compression. If the intruding material is higher pressure than the average within the matrix — it will push the matrix apart, causing more delaminating, at ever deeper levels (now exposed to the intrusion). At the massive pressure differentials in this situation (800 bar), failure of this vessel once compromised would predictably happen in milliseconds. As it turns out… There was a time people came to HN because of the interesting comments by subject-matter experts. Now, it’s a woke smack down. Sad.
  - kllrnohj 3y ago
    The downvotes might be from people that don't realize that "But, then I’m a 50+ year old white guy, so unqualified to speak." is a reference to something else the OceanGate CEO said
  - timeon 3y ago
    I have not downvoted you but not relevant to topic you seems to be pushing some kind of agenda for no obvious reasons: > But, then I’m a 50+ year old white guy, so unqualified to speak. > Now, it’s a woke smack down. Sad. Why?
    - jacquesm 3y ago
      That was in reference to something Stockton Rush said, he said that 50+ year old white guys are 'not inspiring enough'.
      - timeon 3y ago
        That was unfortunate.
- mhb 3y ago
  Composites are used all the time in applications in which they undergo cyclic compression. So your first thought is one you should have dismissed. You know what else is good in tension but not in compression? Bicycle spokes. And yet... There are plenty of other potential design flaws. The assertion that fibers are not good in compression is facile.
  - pjkundert 3y ago
    Any compression pressure gradient that causes the N-layer composite to yield beyond its natural elasticity, and causes any breakdown of level 1 in the N-layer matrix allowing intrusion of fluid beyond that level, has now caused a permanent failure. You now have a vessel of effective thickness N-1 (the outer layer will be sheared away and lost). However, your new N-1 layer vessel now has an average pressure gradient across N-1 instead of N layers, increasing the deflection of the remaining layers, resulting in accelerated cracking, failure and loss of the next layer. This all occurs over < ~1 millisecond at an 800bar pressure differential. This failure mode is radically different than the failure modes of carbon fiber composites under tension. The capacity of such vessels for compression vs. tension is so comparatively low that it would be deemed effectively zero in most cases.
mhb 3y ago
The same reason they're a good choice in airplane construction which also undergoes many cycles of compression. In order to conclude that the choice of composites rather than how they were integrated, interfaced with other components, or a myriad other possible flaws you're going to have to do better than this.
- brnt 3y ago
  Airplanes are not crushed though, they actually are overpressurized compared to their environment. In any case, the level of pressure differentials is huge compared to a sub. In any case, I'm a basic scientists, so it's all I know about carbon fibres. Seems like a weird choice to start a composite with that needs to deal with pressure, love to be educated!
- jacquesm 3y ago
  In aircraft they are mostly used for their tensile properties. "compression" in terms of an aircraft hull is compression inside the hull, not from the outside. The result is that in an aircraft the forces acting on the carbon fiber do so in tension.
- pjkundert 3y ago
  -‘ve pressure vs. +’ve pressure. Aircraft fuselages never undergo “compression”, only expansion. They would fail immediately under even mild compression.
  - dehrmann 3y ago
    The big, open question right now is how would Asiana 214 have gone in a 787.
- snowwrestler 3y ago
  It seems like folks don't understand the reference to compression in airplanes, so let me share some examples. Airplanes are supported by their wings, which deflect air downward and are in turn deflected upward. The wingtips flex upward, which puts the bottom surface of the wing under tension and the top surface under compression. This is true of all wing materials, but the point is that carbon fiber is used for wings and works fine as long as it is specced and constructed to handle the load and cycles. The fuselage is supported during flight by the wings at the wing root, so the nose and tail of the aircraft want to droop down; this puts the bottom of the hull under compression and the top under tension. Again--true of all aircraft hulls but carbon fiber works fine in hulls too--again, as long as it is engineered correctly. Finally composites (of which carbon fiber is only one type) are used extensively in marine applications. Fiberglass, aramid, carbon, and other fiber-based composites are commonly used in boat hulls, where they experience static and dynamic compressive loads. A jet ski smacking into the face of a wave at 50 knots puts a lot more than 1G of compressive force on its composite hull. There's really no physical reason that a composite hull could not work in a submersible. However there are a lot of reasons folks don't usually use composites for that purpose--primarily that the cost-benefits of composites disappear as compressive forces go up. A composite hull that performs as well as a metal hull, at the pressures we're talking about, is possible but would be more expensive and have a shorter lifespan than the metal hull. This is why huge ship hulls are still built out of steel, even though almost all smaller boats have composite hulls these days. It's cheaper. Coming back around to the Titan, the concern with the carbon fiber hull was not that it's physically impossible to do right, but rather suspicion that it only looked like a good idea because they were cutting corners in the engineering (because if they weren't cutting corners, it would not have looked economical).

Carbon fibers have great tensile strength. Why would they be a good choice when a sub faces compressive forces?

by brnt 3y ago

Carbon fibers have great tensile strength. Why would they be a good choice when a sub faces compressive forces?

Exactly my thought. But, then I’m a 50+ year old white guy, so unqualified to speak. In my personal experience w/ carbon fiber overwound tanks for compressed natural gas — my very first thought when I first heard they used a carbon fiber “pressure vessel” was: “Wow, ballsy; carbon fiber composite is great in tension, worthless in compression”.

pjkundert 3y ago
Funny, the downvotes from people who (like the CEO, apparently) … don’t have a clue. When you hear a “pop” or a “crunch” from a carbon-fiber wound composite tank: it means some part of the fiber/resin matrix has delaminated, or allowed intrusion of gas/fluid into its matrix. This means that this layer of the vessel now allows relative motion under torsion or compression. If the intruding material is higher pressure than the average within the matrix — it will push the matrix apart, causing more delaminating, at ever deeper levels (now exposed to the intrusion). At the massive pressure differentials in this situation (800 bar), failure of this vessel once compromised would predictably happen in milliseconds. As it turns out… There was a time people came to HN because of the interesting comments by subject-matter experts. Now, it’s a woke smack down. Sad.
- kllrnohj 3y ago
  The downvotes might be from people that don't realize that "But, then I’m a 50+ year old white guy, so unqualified to speak." is a reference to something else the OceanGate CEO said
- timeon 3y ago
  I have not downvoted you but not relevant to topic you seems to be pushing some kind of agenda for no obvious reasons: > But, then I’m a 50+ year old white guy, so unqualified to speak. > Now, it’s a woke smack down. Sad. Why?
  - jacquesm 3y ago
    That was in reference to something Stockton Rush said, he said that 50+ year old white guys are 'not inspiring enough'.
    - timeon 3y ago
      That was unfortunate.
mhb 3y ago
Composites are used all the time in applications in which they undergo cyclic compression. So your first thought is one you should have dismissed. You know what else is good in tension but not in compression? Bicycle spokes. And yet... There are plenty of other potential design flaws. The assertion that fibers are not good in compression is facile.
- pjkundert 3y ago
  Any compression pressure gradient that causes the N-layer composite to yield beyond its natural elasticity, and causes any breakdown of level 1 in the N-layer matrix allowing intrusion of fluid beyond that level, has now caused a permanent failure. You now have a vessel of effective thickness N-1 (the outer layer will be sheared away and lost). However, your new N-1 layer vessel now has an average pressure gradient across N-1 instead of N layers, increasing the deflection of the remaining layers, resulting in accelerated cracking, failure and loss of the next layer. This all occurs over < ~1 millisecond at an 800bar pressure differential. This failure mode is radically different than the failure modes of carbon fiber composites under tension. The capacity of such vessels for compression vs. tension is so comparatively low that it would be deemed effectively zero in most cases.

mhb 3y ago

The same reason they're a good choice in airplane construction which also undergoes many cycles of compression. In order to conclude that the choice of composites rather than how they were integrated, interfaced with other components, or a myriad other possible flaws you're going to have to do better than this.

brnt 3y ago
Airplanes are not crushed though, they actually are overpressurized compared to their environment. In any case, the level of pressure differentials is huge compared to a sub. In any case, I'm a basic scientists, so it's all I know about carbon fibres. Seems like a weird choice to start a composite with that needs to deal with pressure, love to be educated!
jacquesm 3y ago
In aircraft they are mostly used for their tensile properties. "compression" in terms of an aircraft hull is compression inside the hull, not from the outside. The result is that in an aircraft the forces acting on the carbon fiber do so in tension.
pjkundert 3y ago
-‘ve pressure vs. +’ve pressure. Aircraft fuselages never undergo “compression”, only expansion. They would fail immediately under even mild compression.
- dehrmann 3y ago
  The big, open question right now is how would Asiana 214 have gone in a 787.
snowwrestler 3y ago
It seems like folks don't understand the reference to compression in airplanes, so let me share some examples. Airplanes are supported by their wings, which deflect air downward and are in turn deflected upward. The wingtips flex upward, which puts the bottom surface of the wing under tension and the top surface under compression. This is true of all wing materials, but the point is that carbon fiber is used for wings and works fine as long as it is specced and constructed to handle the load and cycles. The fuselage is supported during flight by the wings at the wing root, so the nose and tail of the aircraft want to droop down; this puts the bottom of the hull under compression and the top under tension. Again--true of all aircraft hulls but carbon fiber works fine in hulls too--again, as long as it is engineered correctly. Finally composites (of which carbon fiber is only one type) are used extensively in marine applications. Fiberglass, aramid, carbon, and other fiber-based composites are commonly used in boat hulls, where they experience static and dynamic compressive loads. A jet ski smacking into the face of a wave at 50 knots puts a lot more than 1G of compressive force on its composite hull. There's really no physical reason that a composite hull could not work in a submersible. However there are a lot of reasons folks don't usually use composites for that purpose--primarily that the cost-benefits of composites disappear as compressive forces go up. A composite hull that performs as well as a metal hull, at the pressures we're talking about, is possible but would be more expensive and have a shorter lifespan than the metal hull. This is why huge ship hulls are still built out of steel, even though almost all smaller boats have composite hulls these days. It's cheaper. Coming back around to the Titan, the concern with the carbon fiber hull was not that it's physically impossible to do right, but rather suspicion that it only looked like a good idea because they were cutting corners in the engineering (because if they weren't cutting corners, it would not have looked economical).