For a negative pressure vessel, it was for all practical purposes made from epoxy resin (with carbon fiber inclusions). Carbon fiber has very high tensile strength. But its compressive strength is similar to string or wet noodles, effectively zero. A negative pressure vessel experiences only compressive forces. So, the hull strength was determined by the integrity of the epoxy resin which bonded the carbon fiber strands together. It would probably have been stronger if they had just used epoxy (as the carbon fiber would just create sharp angles within the resin from which stress cracks can originate), although they would need to control the curing process very closely to ensure that the middle cured properly and that curing was uniform throughout, without cracking or air pockets. The stresses on the cylindrical section of the submarine would be twofold. Firstly, the water pressure acting perpendicular to the surface of the cylinder, trying to crush it inwards. Secondly, a longitudinal stress from the net forces acting on the two end caps. Based on the hemispherical design of the end caps, and the titanium mounting rings, this would create large forces pushing directly down the length of the cylinder. Any small deviation, flaw or fault in the material could lead to buckling (which would almost immediately lead to catastrophic failure). Each dive of this submarine to any appreciable depth would also result in micro cracks forming in the carbon fiber tube, particularly around the interface with the titanium ring where it would experience differential expansion/contraction due to dissimilar materials. This was guaranteed to fail.