Comments by "Tx240" (@Texas240) on "Crazy Hydraulic Press"
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@rodh1404 - there is an inherent problem with using carbon fiber in a submersible.
Anyone here is probably aware that carbon fiber has been used in aerospace and automotive uses for decades. So, why not a submersible?
In an automotive application, carbon fiber is able to be used in applications that leverage its great tensile strength to weight ratio.
In aerospace, carbon fiber is able to leverage its strength against expansion forces. A passenger plane will have its passenger compartment pressurized at about 12 psi, slightly less than normal atmospheric pressure at sea level. At cruising altitude, the outside atmospheric pressure is about 4 psi.
The pressure inside the airplane is pushing out against the lower pressure outside. This is carbon fiber under expansion stress.
In a submersible carbon fiber is exposed to its weakness against compression forces. The pressure inside the sub is 12 to 15 psi. The pressure outside the sub, at the depth of the Titanic is around 5,000 psi (yes, thousand).
So, carbon fiber, used in a submersible is fighting against its own weakness under compression forces and having to deal with both MUCH greater pressure differentials and the greater wear from compression/expansion cycling.
Here's where things get really ugly for carbon fiber in an application where there is no "off ramp". In other words, if a problem develops during use, you can't simply surface the way a car can stop or a plane can make a much quicker descent.
Unlike Steel and Titanium alloys which will have a known stress fatigue limit where stress cycles below the limit shouldn't cause a failure, carbon fiber (and aluminum) have no such limit and will acrue fatigue each and every stress cycle.
As we saw above, the type of pressure differential and aggressive compression and expansion cycling carbon fiber goes through when diving deep is insane compared to what it goes through in aviation and automotive uses. Again, those uses cater to the strong points of the material while a compression (as in a submersible) use is fighting against a weakness of the material from the start.
As you point out, aside from cutting it open, there's no current way to determine the structural integrity of carbon fiber the way we have processes to examine metals without destroying them.
The Titan was using an interesting, experimental way to try and measure strength of the carbon fiber pressure hull.
Based on the landing gear appearing mostly undamaged, it's initially suspected that the Titan dropped the gear to try and add buoyancy and surface quicker. That might indicate that the system worked and did detect a problem with the hull.
However, this merely tells us that a problem is happening "now". We already know that the hull will lose integrity on every dive /surface cycle. We KNOW carbon fiber will fail.
Knowing "it's happening NOW," isn't useful if you can't quickly get above a depth where a critical failure is imminent AND where humans can surface without suffering ill effects from a rapid ascent if they are able to escape the vehicle if failure is imminent. Atmospheric pressure increases much more rapidly diving under the sea than it does rising into the sky. This is why you could jump out of a plane at 30,000 ft and be ok if you have a working parachute.
You can't get out of a submersible at 300 feet and rapidly ascend without risking your life due to how compressed nitrogen interacts with body tissue as the pressure is rapidly reduced.
So, knowing in real time that there's a problem does no good if there's nothing you can do about it. The fact that a sub can experience different amounts of fatigue on different dives makes tracking and having a pro active maintenance program difficult and expensive if it's going to err on the side of safety.
Basically, while it MIGHT someday be possible to use lightweight carbon fiber in a deep sea submersible, doing it safely in a repeated use program (what OceanGate was hoping) would probably be just as expensive as going with Titanium in the first place.
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@GravityAnalyticaLLC - that's the point thought, a thing CAN be done, but that's not what OceanGate wanted.
OceanGate wanted to do the thing cheap enough to be able to turn a profit through a low enough ticket price to encourage enough business volume.
Right out the gate, their business goal, volume, is at odds with using materials that don't have a fatigue limit (materials with a fatigue limit that keep stress below the limit won't degrade much, if at all, during stress cycles).
OceanGate: "We want a cheap enough, light enough vehicle to continually make trips using the same vehicle."
Engineers: "You want titanium or steel alloys."
OceanGate: "No. It needs to be cheaper and lighter!"
Engineer: "Cheaper in the short or long term?"
OceanGate: "Short term! We need to get this business running!"
Engineer: "Right, carbon fiber will do... For a while."
OceanGate: "Great! Wait, for how long, though?"
Engineer: "Duno. It's carbon fiber, mate, not titanium or steel."
OceanGate: "Great! We'll innovate around it!"
June 18, 2023: OceanGate "innovating around it".
Basically, doing it right would be prohibitively expensive to a for profit deep sea tourism business. Either you need steel or titanium or you need to replace the entire vessel if using materials with no fatigue limit at an overly safe schedule that would prevent the known fatigue from compromising the hull (realistically, that's probably a dive or 2, not the 20 something that Titan was at, counting all its aborted runs that were stopped early for other problems, hello "red flag!")
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