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But - sincere question, as I am largely ignorant on such matters: Those pressure vessels made from composites usually are used to contain a pressure on the insi

by lb1lf 3y ago

But - sincere question, as I am largely ignorant on such matters: Those pressure vessels made from composites usually are used to contain a pressure on the inside, taking advantage of the excellent tensile strength of the carbon fibers, no? Having to withstand a large, external pressure is a whole different cup of tea, isn't it? (Please be gentle; I am an electronics engineer)

brnt 3y ago
I have the same question! How can the tensile strength of carbon fibers be an asset if the forces are compressive?
- ajross 3y ago
  Carbon fiber is stronger per weight in compression too, just not by the same amount. Again, composites aren't exotic. We've been building with them for decades and decades. It is surely possible that the root cause here was an incorrect accounting for some or another failure mode in the composite hull. It is likewise possible that given correct analysis, carbon fiber just isn't a good material for this application. But the absolutism of declaring, absent evidence, that carbon fiber hulled submarines are impossible, or that this was the sole reason for the implosion, is just ridiculous. Engineering is more complicated than that.
  - lb1lf 3y ago
    But (and this is my electronics engineer mind still trying to wrap my head around this) - when you pressurize a vessel made from composites, the tensile strength helps keep it all together, which is trivially intuitive. When the same vessel sees an exterior pressure, wouldn't basically the full force have to be borne by the epoxy, making, say, knitting yarn as effective a material for holding the thing together as carbon fiber? (I assume there's some flaw in the above reasoning...) As for the observed absolutism, I mostly agree - my one nitpick would be that from what I've seen, it is a much more challenging material to monitor, the vessels tend to fail catastrophically rather than gracefully, and, well, let's just say that there were a couple of seemingly bold design choices being made by the OceanGate team, choices which weren't validated through sufficient testing afterwards. That would be my main criticism, based solely on news reports: That OceanGate appeared to think 'move fast and break things' was an appropriate design philosophy when the stakes were people's lives. A simple (as such things go...) action like doing numerous unmanned test cycles on a Titan sub to rated depth would have shown what challenges were still unsolved. Instead they basically did the test cycles on manned dives. That was, uh, bolder still.
    - ajross 3y ago
      No. The matrix bears the transverse forces in a composite. The epoxy is there to prevent the fibers from bending or buckling. The carbon holds the compression.
      - brnt 3y ago
        > The epoxy is there to prevent the fibers from bending or buckling. But isn't that what's external pressure comes down too? The material that keeps the hull from buckling?

by lb1lf 3y ago

brnt 3y ago

I have the same question! How can the tensile strength of carbon fibers be an asset if the forces are compressive?

ajross 3y ago
Carbon fiber is stronger per weight in compression too, just not by the same amount. Again, composites aren't exotic. We've been building with them for decades and decades. It is surely possible that the root cause here was an incorrect accounting for some or another failure mode in the composite hull. It is likewise possible that given correct analysis, carbon fiber just isn't a good material for this application. But the absolutism of declaring, absent evidence, that carbon fiber hulled submarines are impossible, or that this was the sole reason for the implosion, is just ridiculous. Engineering is more complicated than that.
- lb1lf 3y ago
  But (and this is my electronics engineer mind still trying to wrap my head around this) - when you pressurize a vessel made from composites, the tensile strength helps keep it all together, which is trivially intuitive. When the same vessel sees an exterior pressure, wouldn't basically the full force have to be borne by the epoxy, making, say, knitting yarn as effective a material for holding the thing together as carbon fiber? (I assume there's some flaw in the above reasoning...) As for the observed absolutism, I mostly agree - my one nitpick would be that from what I've seen, it is a much more challenging material to monitor, the vessels tend to fail catastrophically rather than gracefully, and, well, let's just say that there were a couple of seemingly bold design choices being made by the OceanGate team, choices which weren't validated through sufficient testing afterwards. That would be my main criticism, based solely on news reports: That OceanGate appeared to think 'move fast and break things' was an appropriate design philosophy when the stakes were people's lives. A simple (as such things go...) action like doing numerous unmanned test cycles on a Titan sub to rated depth would have shown what challenges were still unsolved. Instead they basically did the test cycles on manned dives. That was, uh, bolder still.
  - ajross 3y ago
    No. The matrix bears the transverse forces in a composite. The epoxy is there to prevent the fibers from bending or buckling. The carbon holds the compression.
    - brnt 3y ago
      > The epoxy is there to prevent the fibers from bending or buckling. But isn't that what's external pressure comes down too? The material that keeps the hull from buckling?