[caffeinated_me]

This isn't the first carbon fiber submarine, although it is the first manned one. The US Navy tried out an unmanned model in the 80s, and got much better results- they were expecting at least 1000 successful dives before stress fatigue was an issue.

Here's a detailed report on it. Pages 32-33 has their take on material analysis, probably the most relevant to this failure

https://apps.dtic.mil/sti/pdfs/ADA270438.pdf

I'm personally more suspicious of oceangates manufacturing process than the material, but I'm far from an expert here.

[Retric]

It’s the unpredictable nature of failure that’s at issue here. For unmanned subs it doesn’t matter if 10% of failures occur well below the expected lifespan but that’s a huge issue for manned subs.

[jerlam]

I'm not even sure it's the first manned carbon fiber submersible.

Deepflight Challenger [...] is the first deep-diving sub to be constructed with a pressure hull (central tube portion) of carbon fibre composite, built by Spencer Composites for HOT. Its carbon fiber design would later influence the tube for the sub Titan,[12] which imploded...

https://en.wikipedia.org/wiki/DeepFlight_Challenger

[esprehn]

Notable from that page is this paragraph though:

""" Based on testing at high pressure, the DeepFlight Challenger was determined to be suitable only for a single dive, not the repeated uses that had been planned as part of Virgin Oceanic service. As such, in 2014, Virgin Oceanic scrapped plans for the five dives project using the DeepFlight Challenger, as originally conceived, putting plans on hold until more suitable technologies are developed. """

[Aunche]

Being manned is a major difference. Humans need a lot of space. Pressure grows with volume, which is cubic, but the hull grows with area. You can also submerge components in oil, which is much better at resisting pressure than air.

[jjk166]

Pressure doesn't grow with volume. The exterior design pressure is constant. The stress on the wall scales linearly with the diameter. Making submarines bigger actually makes it easier because the buoyancy scales cubically with the volume while the weight scales linearly with the perimeter, so the larger the submarine the thicker the walls can be.