Titan Sub made from Carbon Fiber-some analysis-how it's made
Ain't no Navy sub guy but I do know a lot about carbon fiber (which the sub was made out of ) and they actually were using that in race cars as early as mid 70s-but just wings and aero appendages as no one would stump up the munee to build an entire chassis out of it plus they din't know how it would react in a crash since it was too expensive to crash test-they thought it would splinter and impale any driver inside of it…they later found out it was wayyy better than any steel/titanium/aluminum concoction. How that relates to the sub is I can't imagine how many cycles it takes to permanently damage that weave of carbon fibers and the glue(s)/resins with the repeated pressure loads and cold.
Since they were doing it all on the cheap (logitech controller and bluetoof…C'mom man!) they probably never checked it as you would a race chassis. You can't see cracks on the surface of them either (at least race tubs-chassis) you have to fully scan them-and another reason is that they are made in an autoclave-they have Yuge ones now for entire monocoques. I do remember it wasn't until the early 80s that McLaren got one of it's Arab investors to pay Hercules-who had been doing it for AC purposes for years- to finally build a complete tub out of it. Then once it was proven to be stronger than what they all had it became a race to get supply out of that Mfg (Hercules) or source it somewhere else-costs skyrocketed because of it. They ended up banning the underbody appendages because drivers were blacking out due to the G forces generated by having all the extra aero load (called them 'sliding skirts')created an instant several seconds a lap on the formula cars which then trickled to the sportscars.
The Making of Carbon Monocoque caps 2-4
http://tut-f.com/making.html
*this is all predicated on the story being what they say it is (it probably isn't) but the tech stuff here is relatable