[CarVac]

Carbon fiber is used because it is rigid.

The failure mode for a member in compression is buckling, which is resisted by stiffness, not strength.

It's also a lot easier to make a huge thick tube out of composites than out of titanium. That's why only the spherical ends are titanium, and the middle tube part is CF.

[floxy]

>The failure mode for a member in compression is buckling, which is resisted by stiffness

Hmm, this is probably the right answer. When things start to buckle, you'd be putting part of the surface in tension, which would be resisted by the fiber. I would definitely be very interested to see the plots of strain gauges embedded throughout the thickness of the wall as it goes to depth (in all three axes, hoop, radial, and axial). My intuition completely fails here. Good thing I'm not making submersible vehicles.

[CarVac]

Once buckling occurs, it hardly matters what's in compression and what's in tension: the applied force has a tremendous mechanical advantage over the material strength resisting it once the stiffness fails to prevent it from crossing a threshold.

Secondly, CF does substantially improve compressive strength over neat resin, which would fail in shear.

The fibers individually may not withstand compression, but embedding them in the epoxy resin prevents them from buckling and the composite material exhibits substantially improved performance over either base material.

The exception is tensile stress that causes delamination, for which there is no benefit over the neat resin.

[jacquesm]

It would be a good thing if the people who did make this contraption also didn't make submersible vessels based on the bits and pieces that have made it into the news so far. This sounded like an accident waiting to happen. At those pressures if something goes wrong it will crumple like a tin can and having the hatch sealed from the outside means that even if they didn't die at depth they may not have a way out if their comms have failed. The whole thing strikes me as beyond irresponsible. I wonder if the people that built the sub would take it to depth.

[patrec]

In what scenario would not being sealed from the outside make a positive difference? If they managed to surface, how would being able to "get out" in the middle of the ocean be helpful? If they didn't manage to surface, under what circumstances would have opening the hatch helped?

[jacquesm]

Air.

[patrec]

True, although fortunately the air supply is meant to last for 4 days. As a passenger I would probably be more alarmed about the apparent absence of some kind of distress beacon than the inability to open the hatch without help.

[jacquesm]

Well, something went wrong and one of the things that could go wrong is the air supply. I wouldn't be a passenger for any amount of money, these are experimental tools, not joy rides.

[marksmithhfx]

There was a distress beacon… it’s called a transponder. It is in a separate pressure chamber controlled by a separate battery. It is a completely separate and isolated system. It failed at the same time as regular comms leading some to conclude the vessel was gone. Kaput. Is no more.

[mustacheemperor]

I think part of the issue there is designing a hatch that can be opened on the surface to exchange air without immediately swamping the sub in the sea chop. Alvin sank very fast when it was accidentally dropped into the water with the hatch open.

[actionfromafar]

They could get air.

[Yeroc]

Well it seems like the President and CEO of the company was onboard the craft so I suppose the answer is "yes".

[jacquesm]

I highly doubt he built the sub.

[wnkrshm]

They copied the glass fiber + titanium rings idea from the Deepflight Challenger that Virgin wanted to dive with. [0]

Virgin deemed the design not safe enough for more than a single dive and quietly scrapped the diving events: "Based on testing at high pressure, the DeepFlight Challenger was determined to be suitable only for a single dive, not the repeated uses that had been planned as part of Virgin Oceanic service." [1]

[0] https://www.compositesworld.com/articles/composite-submersib... [1] https://en.wikipedia.org/wiki/DeepFlight_Challenger

[jacquesm]

That makes very good sense. Pity they didn't copy the conclusion too.

[marksmithhfx]

Poor design choice. Too many materials expanding and contracting at different rates leads to friction stress and ultimate material failure. Maybe not at first, but over repeated compression and expansion cycles.