So is mine, and they made several successful trips which (among other things) makes me think that carbon fiber could be an appropriate material for a submarine. Metallic hulls would also implode catastrophically if not well designed.
The operators decided this was too much work. They gambled and they lost.
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.
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.