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)

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.

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.

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"

A dive to the Titanic needs to deal with a pressure differential of ~340 atmospheres.