BQ78 said:
Steel flexes under extreme pressure, carbon fiber breeches.
Mechanical Engineer here with too much time playing with both steel and carbon fiber. BQ, you probably know all of this but I'll just quote you because you've summed it up quite well.
There is a material property called toughness. It measures how much energy a material can sustain in damage before it ruptures when loaded. So lets say you have a ceramic mug. You hit it with a hammer. It shatters. Ceramics have very low fracture toughness. You hit steel with a hammer, it just puts a localized dent in it and you can mostly keep on trucking with a minor stress concentration. Ceramics can be designed to resist a large amount of force, but they are very unforgiving of any damage to them. On a more realistic scale, lets say you have a very small crack in a ceramic and you pull the material apart. Something brittle like a ceramic mug will just immediately rupture. Steel wont. The infinitesimally small tip of the crack will yield and blunt a bit, allowing the material to still carry some load - just don't load it up any more and get it repaired.
Another material property is called the glass-transition temperature. And its ironic that this might come into play because it was discovered to be the main cause of failure on liberty ships as the crossed cold water. Most materials, have a temperature threshold that once you get them so cold, they loose a LOT of their material properties. Specifically, its toughness that can be effected. The liberty ships of WWII suffered from poor metallurgy and would break in half in rough, frigid seas because the material was just too brittle in very cold water. If you ever hear someone say Charpy V notch testing. That is a material toughness test that revealed difference in toughness at different temperatures. All the steels we buy have Charpy testing done to them, and Lord help everyone from project managers to supply chain if you don't pass Charpys on a material delivery.
Now onto carbon fiber composite hulls. Oh man. So carbon fiber composites are an extremely strong fiber material in a curing resin matrix. Think about pouring a concrete foundation. The rebar is the fibers, the concrete itself is matrix. Together, the foundation has a mix of material properties of both the steel and the concrete. It is very directional though. The direction of the steel rebar is the direction that it helps the concrete resist loads. Same with a carbon fiber sheet. Its very directional. When you do a carbon fiber layup you alternate the direction of the fiber layup to hopefully align with your stresses. Carbon fiber composites also don't have much toughness. You hit a carbon fiber composite with a hammer, its going to create a dent, but it'll shatter the matrix and fiber bonds and immediately destroy material strength. This is called damage mechanics. When you stress steel and load it up until it ruptures (like pulling it apart) it eventually comes apart in two pieces. For composites, when you pull it apart, what you get is a cascade of microcracks that sound like a mix of popcorn popping and someone playing with guitar strings. Each one of those little microcracks is irreversible damage to the composite.
Now on to fatigue. In materials, you can load them up once to a very high limit and it'll resist loading. But lets say you stay under the rupture load. Like 60% of failure load. But you cycle up to it and back down. Some materials are very forgiving of this. Steel can be one of them. Aluminum by the way, is not. It never forgets and never forgives the things you've done it to. No matter how many times you bring it flowers or apologize. Thats why airplanes are always inspected for cracks.
Carbon fiber composites do not like compression. They work really well in tension. So huge amounts of excessive force on the outside of a hull applying pressure don't do nice things to the composite. And with every cycle, there is a chance they had a bad manufacturing defect somewhere in their carbon fiber layup. Its not 'probable' its a guarantee to be there because of the complexity of laying up composites. There is some spot deep within the resin walls where the carbon fiber didn't get fully wet. With every load up of pressure, microcracks start accumulating. Heck even the difference in thermal expansion rates between the carbon fiber and the resin can pull the matrix away from the fiber and cause damage.
Eventually you damage the composite lattice so much it just ruptures. Between the thermal expansion, manufacturing methods, material selection, and just in general lack of looking up and seeing how everyone else is building subs/submersibles this was not going to end well.
