Re:Debris field found in search for missing submersible
by can16358p on Thursday June 22, 2023 @01:18PM
(#36435517)
Just note that the body was experimental: while the hull indeed have a thick titanium body, it was mixed with (don't know exactly how and where though) carbon fibers, which are known to catastrophically break under pressure if there's even a microscopic impurity or damage. At least that's what the experts say. Carbon fibers is great for lightweight things that need to bend, terrible for things to be relied under pressure.
Re:Debris field found in search for missing submersible
by kamranjon on Thursday June 22, 2023 @01:36PM
(#36435796)
It was just the end caps that were titanium - the entire tube was just a 5inch thick carbon fiber wrap. In one of the videos with the CEO I saw him saying that he was a rule breaker because common consensus was that you shouldn't build a submarine out of carbon fiber and titanium.
Re:Debris field found in search for missing submersible
by jonah on Thursday June 22, 2023 @02:58PM
(#36437043)
It was actually the second try at a CF tube. The first had flaws and issues and was either "repaired" or replaced by other manufacturing companies.
Re:Debris field found in search for missing submersible
by jonah on Thursday June 22, 2023 @05:20PM
(#36439183)
Reference to the repair/replacement of the cf tube: https://techcrunch.com/2023/06/20/a-whistleblower-raised-saf...
"The Spencer-built composite cylindrical hull either was repaired or replaced by Electroimpact and Janicki Industries in 2020 or 2021, prior to the first trips to Titanic."
Re:Debris field found in search for missing submersible
by mechhacker on Thursday June 22, 2023 @03:22PM
(#36437337)
I'd really like more detail on that tube's layup schedule.
Solid 5inch thick carbon composite OR a sandwich design with thick outer facesheets of carbon fiber? I suppose under that pressure not much would take the hydrostatic loads other than carbon, but that seems thick compared to everything I've seen made out of composites.
Re:Debris field found in search for missing submersible
by saberdancer on Thursday June 22, 2023 @04:11PM
(#36438155)
There are videos of them building it. From what I remember, they rolled carbon fiber around a cylinder, making the flat part of the cylinder. Then they mated two titanium half spheres to the end of the carbon fibre cylinder. This was done using some kind of "glue". Meaning that the middle part had no titanium.
I think it was in Sub brief YouTube video. https://www.youtube.com/watch?v=4dka29FSZac&pp=ygUJc3ViIGJya...
Re:Debris field found in search for missing submersible
by mechhacker on Thursday June 22, 2023 @04:43PM
(#36438679)
Thanks, I see now. Wow, that's a thick layup.
Re:Debris field found in search for missing submersible
by SirSourdough on Thursday June 22, 2023 @04:12PM
(#36438175)
I saw a bit of video where they showed the construction- it’s a few inch wide band of carbon fibre wrapped around an inner tube like a spool of thread until it reaches 5” thickness.
Re:Debris field found in search for missing submersible
by floxy on Thursday June 22, 2023 @05:17PM
(#36439143)
Here's a write-up on the composite part of one of Oceangate's vessels:
https://www.compositesworld.com/articles/composite-submersib...
...since that article is from 2017, it is not clear that this was the unit that actually failed.
Re:Debris field found in search for missing submersible
by mechhacker on Friday June 23, 2023 @08:20AM
(#36445829)
Very interesting. Ties sub tech to Steve Fossett, who was working with Scaled Composites before his death.
Not saying they are related at all but I am reminded of this:
Re:Debris field found in search for missing submersible
by WalterBright on Thursday June 22, 2023 @01:45PM
(#36435937)
A cylinder is much less resistant to pressure than a sphere.
Re:Debris field found in search for missing submersible
by samtho on Thursday June 22, 2023 @02:52PM
(#36436949)
It’s been a while since physics for me, but I was under the impression that this only really applies when pressure is greater inside the solid shape. In this case, it seems roughly equivalent to pulling a vacuum inside a soda can at sea level, which fairs quite poorly for the soda can, and I cannot imagine an unfortified sphere-ish shape performing better.
Re:Debris field found in search for missing submersible
by bumby on Thursday June 22, 2023 @03:22PM
(#36437338)
Fun physics: this is why hot dogs in the microwave tend to fail (split) along their length rather than blow out the ends.* The stresses are modeled as a cylinder with two half-sphere end caps. The end-caps tend to withstand pressure to a greater extent than the side walls.
* the way the ends are joined can complicate this, depending on the hot dog
Re:Debris field found in search for missing submersible
by ridgeguy on Thursday June 22, 2023 @02:18PM
(#36436466)
And composite materials are basically threads embedded in glue. Threads can be extremely strong in tension. In compression, you have only the strength of the glue and the fiber/glue interface strength, which isn't a whole lot. Composite materials are in general poor in compression.
Crewed submersible hulls are always in compressive stress from the ocean outside. I can't fathom why somebody would choose a composite for a deep submersible hull. It's just asking for a buckling failure. Bad design choice, IMO.
Re:Debris field found in search for missing submersible
by bumby on Thursday June 22, 2023 @03:19PM
(#36437293)
>Carbon fibers is great for lightweight things that need to bend, terrible for things to be relied under pressure.
Composite Overwrapped Pressure Vessels (COPVs) are fairly common in cases where a pressurized system needs to be relatively lightweight (e.g., spacecraft). To your point though, the failure mechanisms can be hard to model.
Re:Debris field found in search for missing submersible
by dmpk2k on Thursday June 22, 2023 @03:50PM
(#36437786)
In that case it's taking advantage of carbon fiber's strength under tension.
For a sub you have the opposite problem, which carbon fiber is very weak at.
Re:Debris field found in search for missing submersible
by bumby on Friday June 23, 2023 @11:33AM
(#36448297)
Take this with a grain of salt because I worked on the data acquisition side of COPV testing and not the engineering side, but my understanding is that while carbon fiber is strong in tension but weak in compression, epoxy is strong in compression but weak in tension. So the combination is thought to make a vessel strong in both.
Re:Debris field found in search for missing submersible
by quercusa on Thursday June 22, 2023 @02:07PM
(#36436293)
But would make the inside sort of uncomfortable, what with PV=nRT
Re:Debris field found in search for missing submersible
by tedunangst on Thursday June 22, 2023 @02:42PM
(#36436820)
It's hard enough to make a small hole that doesn't turn into a big hole under such pressure when you're trying to. Even harder for a small hole you're not expecting.
Re:Debris field found in search for missing submersible
by onesphere on Thursday June 22, 2023 @03:08PM
(#36437150)
https://en.wikipedia.org/wiki/Brittle_strength
If it imploded, maybe its wreckage is banging against itself and that's the sound we're hearing.
Re:Debris field found in search for missing submersible
by onesphere on Sunday June 25, 2023 @09:27PM
(#36474622)
https://en.wikipedia.org/wiki/Tenacity_(mineralogy)
(the direct relationship wasn't transparent)
I would think another failure mode could be water rushing in without the overall structure catastrophically failing, which would actually relieve pressure on the structure and occur over a much longer time period.