People make a big deal about the acrylic glass not being rated for the depth, but I think it's almost impossible that it was the window, and think it must have been the hull:
* The properties acrylic are well understood, and the strength of an acrylic disc is trivial to calculate, even if not "rated" you can easily calculate if it's strong enough
* The window should fail visually with a lot of warning before rupture
* If the window failed with the titanium end cap, there would be no reason for implosion of the carbon fiber hull
Nothing was screwed directly into the hull:
> These handle-lights aren’t attached to the carbon fiber. The sub is lined with a big insert, a perforated metal sleeve, that fits snugly against the carbon fiber, like a rolled-up poster in a mailing tube. “The thing with carbon fiber is you can’t cut holes in it. It doesn’t like that,” Rush says. “Nothing touches the carbon fiber.”
— David Pogue in “What I Learned on a Titanic Sub Expedition” https://nymag.com/intelligencer/2023/06/what-i-learned-on-a-...
[0]https://www.researchgate.net/publication/340442621_Predictio...
Why is the hull difficult to accurately model? (assuming the exact materials and precise dimensions were known).
[1] composite materials are not as homogeneous as metals are, so most of the time we actually don't know the exact materials and precise dimensions. Only manufacturers that make enough parts to afford to do destructive testing can characterize the process enough to make reliable guarantees about part strength.
And you can produce reliable composite parts (you don't want homogenous, but them to come out exactly as spec'ed) under strict control, but it doesn't look like they necessarily did that here. They seem to have eg foregone autoclave curing of the tube section probably due to cost.
The modeling they did assumes a perfect material, and then adds some arbitrary factor of safety that will hopefully account for imperfections. It probably would have been fine if only used for one dive, but not after a substantial number of fibers had snapped as the hull flexed across numerous dives.
The window would deform up to 2" during dives, according to passengers.
Somewhat related (possible material fatigue):
A 16-meter (52-foot) high aquarium has burst at a hotel in downtown Berlin (2022): https://www.dw.com/en/berlin-massive-aquarium-home-to-1500-f...
Manufacturer: https://www.reynoldspolymer.com/projects/aquadom/
I feel like if the window failed then the sheer momentum of inrushing water could detach titanium cap on the opposite end of the sub. A bit like you can break the bottom o a glass bottle by hitting the bottle from the top which causes the water in the bottle to separate from the bottom of the container and then come back down with enough momentum to break the bottom.
It is possible that the carbon fibre delaminated catastrophically, possibly as a result of stresses accumulated over multiple descents.
People want to imagine that the simple use of carbon fiber here was the sole mistake, I guess because it's easy to understand and makes us feel smart. But come on, engineering is harder than that. Carbon fiber pressure vessels have a long history. Composite material failure modes are for sure more complicated than metals but they certainly aren't a mystery. We've been using composites for more than half a century, we know how they work.
No, my gut says the root cause here (if we ever find one[1]) is going to turn out to be a more routine kind of engineering fuckup. It'll be just some weak point that the analysis failed to turn up.
[1] And sure, if we don't then the "carbon fiber death cylinder" theory will just become part of the folk truth, I guess.
The analysis is very nonlinear and unfortunately not as widely understood as internally pressurized vessels. It requires a lot more analysis, troubleshooting, and expertise that is short in supply.
Add the additional catastrophic failure modes for composites and there's a reason that industry experts warned against this.
Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model.
(Edit because this was misunderstood: I'm not saying what the failure is. I'm saying that we DON'T KNOW what the failure is, and that given the choice between makes-me-feel-smart pontification about materials and a routine design flaw in a prototype device, I'm going to bet on the routine failure every single time.)
I like how you complain about people making glib drive-by analyses (that are at least based on actual submersible experts saying they shouldn't have used carbon fiber) by making a glib drive-by analysis that's based on pure conjecture.
Which is all to say: basing your opinions on this stuff on easily-consumed, tempting pseudoanalysis clickbait is a terrible way to figure out what went wrong. And in particular putting your ducks in this carbon fiber nonsense instead of "they probably just put a hole in the wrong place" seems poorly grounded to me.
That’s exactly what I have seen reported.
>But Lochridge, according to the suit, raised concerns about the design of the submersible's hull, particularly that it was made of carbon fiber instead of a metallic composition.
https://abcnews.go.com/US/lawsuit-alleged-flaws-titanic-subm...
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)
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.
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.
But isn't that what's external pressure comes down too? The material that keeps the hull from buckling?
Some people might characterize it as likely being the decisive mistake, but I’ve yet to hear anyone describe that as the sole mistake, there being so many obvious failures.
> Carbon fiber pressure vessels have a long history.
But does deliberately playing with the service life of carbon fiber for pressure vessels have a long history?
https://www.washingtonpost.com/world/2023/06/23/titanic-subm...
This submersible involves a field where there is a lot of hard-won experience of how to do things safely, but where much of that experience was overtly viewed by the firm as “waste”.
It deforms, fibers break, and thus the load decreases as the deflection stays relatively constant in that time frame.
With external pressure the load doesn't decrease. Fibers break -> deflection increases -> more fibers break. There isn't load relief as long as the vessel is still holding pressure.
But, that's speculation and I'd need to see more pics with more details.
I also want to mention that the large end caps you are seeing weren't bonded to the hull at all- there was a small Ti flange epoxied to the hull, and then the large domes were bolted to that. The front, with the window, was unbolted and rebolted every time for ingress/egress.
There is no way that CF epoxied to thick Ti is going to make a strong enough bond that it will deform the Ti when it detaches. In this case, the geometry of that "glued" joint was such that it was forced together by the pressure underwater, rather than pulled apart- so it was not necessary for it to have any real strength other than for holding the end caps on at the surface, and filling voids to make it seal.
Those tarps may be covering what's left of the crew