The main problem with the design was that it used dis -similar materials, carbon, fiber, and titanium, and it had a cylindrical shape rather than spherical. The carbon fiber could not handle the repeated cycles of high pressure, it lasted a few times down to the Titanic and then it just gave out right at the contact point with the titanium ends.
actually, the carbon fiber tube delaminiated internally from repeated pressure changes and thereby lost structural integrity, and stockton rush knew that and kept using it anyway ... that’s all explained in the documentary ““Implosion: The Titanic Sub Disaster”” and the final Coast Guard report ...
Also note that the sub was left OUTSIDE DURING A CANADIAN WINTER.....................