for long-fiber composites, I don’t THINK the matrix is designed to carry any load. That should not be going into any stress calc. Maybe in short-fiber...but I am less familiar with them.
Iirc the matrix only supports the long fibers - keept them in place and oriented. Kinda like a web of an i beam. I could be incorrect.. The main concern is the fiber in the geometry of the layup that are most stressed starting to fail, which in turn transfer the load to the next fibers, which are in a slightly less mechanically advantageous position to bear the increasing stress due to reduction in stress bearing area...it cascades until catastrophic brittle failure.
Carbon fibers have tensile strengths on the order of 400,000-700,000 psi (high strength steel is 180,000 psi) but the epoxy resin strength is on the order of 24,000 psi. If the vessel had no carbon fiber in the longitudinal direction it had virtually no strength or structural rigidity in the long direction. I simply can't conceive of anyone actually designing or building such a vessel because it breaks pretty much every design and fabrication rule in the composite world, but James Cameron seems to confirm that the videos are representative of the manufacturing process and the vessel was pretty much a hoop wound design. Which scared the heck out him - and rightly so.