Without knowing the exact laminate schedule or the type of resin used I wouldn’t necessarily believe that the 5” thickness was solid carbon fiber. Many core materials are used to lessen cost and lower the weight, but those foam or honeycomb cores also add levels of inconsistency in construction. There are likely multiple failures In the construction. 1. Resin used was not designed to be exposed in freezing temperatures in the stress load is was put under. Very likely no resin is made that could handle the use this sub was put under, these subs should have been treated as disposable. 2. All the mechanical engineers in the world could have agreed on the sub design, yet if during the fabrication somebody did not apply something correctly or the resin didn’t bond to one of the cores or layers or carbon correctly and that easily could have started the timer on how many deep sea exposures the sub could handle.
It sounded like this sub had been sent down dozens of times. For whatever reason the structure failed and my best guess is the same thing, there was fatigue in the composites from the amount of stress dozens of dives put on it.