I didn't pay attention to this until the day they were supposedly running out of oxygen. I searched for pictures to find a hollow long cylinder made of carbon fibre with end plates. That and reading their Wikipedia entry made me angry. It was obvious that the reason for losing contact days before was that it had imploded killing them instantly. My background is physics.
The operators decided this was too much work. They gambled and they lost.
Compression loads on submarines are extreme and cyclical. This causes the fibers to delaminate loosing strength. Furthermore, De-lamination is difficult to measure non-destructively (I think only xrays work?)
Manned conposite subs should always have their chief designer or financier aboard
Also note that the hull was 5 inches thick (safety factor > 2.5). Depending on the dispersion properties of the medium ultrasound might not penetrate and get a good result. In Ti (some alloys at least), for example, you cant get much signal past a couple of inches due to the scattering (from the grains?)
Ultrasound for delamination is like medical ultrasound and looks for a discontinuity in the sheet from the delamination.
Both for ceramics and for composites the failures are caused by microscopic defects that are never the same for different samples, so the conditions for failure are very variable from sample to sample. Therefore the results of experiments with such materials have lower predictive ability for the behavior in a real application.
Metal fatigue is understood because we have a mechanistic understanding of what it is, how fatigue accumulates, and how different metal crystal strictures respond to fatigue.
We know, for example, that metals like aluminum will always fail from fatigue given enough loading cycles, no matter how small the applied stress.
We also know that other metals, like iron and titanium, have a "fatigue limit" below which fatigue doesn't accumulate and these metals can endure infinite loading cycles.
We have, to some extent, the ability to repair metal fatigue.
We build airplanes from aluminum knowing their aluminum hulls and wings will fail (whereas if it were built with Ti fatigue failure could be eliminated) because metal fatigue is very predictable and we can withdraw a hull from service after a regulatory determined number of landings.
So yes, until we developed our current understanding of fatigue, people died. But, often, this was from a callous disregard to traditionally accepted safety factors by cowboy "innovators".
(Im a materials eng. PhD in polymers w/ background in Eng. Phys. Im not a metallurgist for what its worth)
And after the first dive, even if it survives, you know that 100% that it does.
Weight is not nearly the problem it is for a sub as it is for spacecraft or aircraft. The problem wasn't safety protocols or XBox controllers or any of that. It was conflation of "big idea people"[1] with real engineers.
[1] see Jobs, Musk, etc.
However, there are a lot of things that are absolutely designed and built to be single-use, especially in fields like space, where rocket engines have been single-use until very recently.
As for weight, I think the reason that was important was because they didn't have a big enough ship that could operate a crane and haul the submersible out of the water. Really, the whole operation was a bad idea: this kind of exploration is expensive as hell, and the only way to cut costs (with current tech) is to do really dangerous stuff.
Rope is useless under compression. You can't push a rope, or climb a rope that's only attached to the floor. So under compressive loads, you're relying on the glue mainly. The carbon fiber gives the glue something to stick to. A submarine is the opposite of an aircraft: the high pressure is on the outside, so the hull is under compression not tension.
It's a terrible choice for a submarine.
This can be achieved for example in pultruded carbon rods, where the carbon fiber is under tension when it passes through an epoxy bath. They have been used in aircraft wing top spars which receive compression loads (when taking positive gees).
How to make a cylindrical vessel that can take compressive loads? The creation process certainly needs some thinking and attention.
I suspect the failure mode in a submersible like this isn't so much the carbon fibers themselves but either the carbon > metal interface at the ends or gradual delamination between layers of fibers due to the cyclical pressure loads.
Steel is a significantly better material for this task. I'm stunned they even considered carbon fiber.
These challenges are on the order of "non-flammable paper" or "non-magnetic iron"
These exist, but are not common.
> "non-magnetic iron"
Austenitic stainless steels are a thing.
https://en.wikipedia.org/wiki/Austenite
So I'm not sure if I like those examples. It's more along the lines of using baked clay under tension, or a rope under compression.
Not sure whether this would actually work in practice though or whether it would be better than the simple metal sphere design.
Seriously, this isn't a tent.
A dive to the Titanic needs to deal with a pressure differential of ~340 atmospheres.
I really dont think ppl appreciate what 4000 m of water means! 1 atmosphere is only a 10 meter water column. 4000 m is 400 atm. Actually more because water can no longer be assumed incompressible.
EDIT: to add another point - submarines are subject to implosion, which is a buckling failure. That sets up a vicious cycle where a small deformation gets worse, whereas a vessel at positive pressure will "straighten itself out"
Just because it works under tension in an aircraft or even a cylinder of compressed gas, doesn’t mean it will work when the pressure is applied from the outside of the vessel.
It’s the opposite of plain concrete for instance, which has got compressive strength but doesn’t do as well for tension.
I understand just saying the words “carbon fiber” it feels like we live in the future but that’s the difference between marketing and engineering.
It's a shame that it got tested out with deadly results by a company that was completely unserious about safety. Now the HN crowd has all watched the same bunch of Youtube videos over the last few days, so now everyone is a qualified submersible designer and has decided that composites are obviously a bad idea for a deep submersible.
What is still a bit of a mystery to me is why the company that built the hull did so knowing what the application was. I'd love to see the correspondence between those two companies and whether or not there was an opportunity for the company that built the hull to flat out refuse to do so because they thought it was a bad idea or if they put an upper limit on the number of cycles.
https://www.compositesworld.com/articles/composite-submersib...
Is a really good article on this particular design, and there are some interesting details there that I know for a fact were different on the sub that imploded. The article clearly states: "Nothing will be mechanically attached to, or penetrate, the composite hull other than the titanium caps. "
A lot hinges on the use of that word penetrate, whether they meant all the way through or just from one side. There are multiple items screwed into the pressure hull on various photos including more than one monitor stand. That alone may have been enough to cause the failure.
What I also miss from the specification as quoted in the article is that there is no number of cycles specified. Finally the safety factor quoted in the article is 2.25, which is in excess of the 1.5 that you'd normally expect so theoretically the pressure hull should have survived this assuming no prior damage. On the first dive...
Finally, the whole 'we're monitoring things and will ascend if there are signs of impending failure' is something that sits wrong with me. Carbon fiber failures do announce themselves by the pulses of individual fibers breaking. But in a pressure vessel you get into a feed-forward loop that would cause such a failure to expand so fast that I don't think there would realistically be a chance to surface timely.
Incidentally there is another way they could have solved this problem; aluminum. With aluminum you can build a DSV that can dive deep enough to visit the Titanic, is light enough to not require additional buoyancy, and large enough to hold up to 7 people. This has been done: https://en.wikipedia.org/wiki/Aluminaut
I think they probably had a fatigue life issue with an aluminum hull (it was only in service for six years..) since aluminum infamously has no fatigue limit. But it should be easier to characterize, so you can retire the hull before it fails.
Interesting that they still have it on 'standby' just in case it is needed despite being out of service for so long. Unique capabilities.
Not a single person I'm aware of except for mr. Rush thought building the pressure hull of a passenger carrying submersible out of carbon fiber was a good idea.
The fact that they managed multiple dives is probably predicated on your understanding that it was the same named vessel, but the carbon fiber cylinder had already been replaced at least once due to damage. Carbon fiber is a fascinating material, but for this application it isn't the best choice and for an application like this you don't take chances.
I've also looked at all of the photos of the interior of the sub and I've seen some things that anybody that understands this material would never ever have done. Such as to drill holes in it to mount stuff, rather than to use clamps or to integrate the mount into the fiber as you're laying it. The fact that it worked at all at those depths is a severe case of survivor bias, I'm fairly sure if you had tested the cylinder to destruction after a single dive that it would have failed at a much lower level of stress than prior to the first dive. And if you use a novel material and you're seeing unexpected behavior then you stress test until you drop rather than that you go sightseeing with paying passengers.
> Now the HN crowd has all watched the same bunch of Youtube videos over the last few days, so now everyone is a qualified submersible designer and has decided that composites are obviously a bad idea for a deep submersible.
Lots of people on HN are knowledgeable about various materials, you can discard them as well as all of the people who said prior to this accident that this was a bad idea from the start. But I sincerely hope that people who are in a position to actually act on all this knowledge (and the results of the investigation) will adhere to their previous mantra: when in doubt: test, test, and test again. Because no amount of knowledge or perceived understanding is going to replace that, it can help guide a design but it simply isn't the final word. The proof for any kind of pressure vessel is repeatedly cycling a design to the point where it fails. And if it doesn't fail when it has reached its design life, to check if the degradation is as predicted.
Note that such testing already starts when you receive a batch of fiber.
Like who is it critical to not write off carbon fiber for extreme depth submersibles?