We're talking about a bunch of layers of fibers, and woven cloth impregnated in epoxy. It's a pretty messy, irregular thing with voids, gaps, broken fibers, etc

We're talking about a bunch of layers of fibers, and woven cloth impregnated in epoxy. It's a pretty messy, irregular thing with voids, gaps, broken fibers, etc. even if you lay it up robotically (as they did). The modeling they did assumes a perfect material, and then adds some arbitrary factor of safety that will hopefully account for imperfections. It probably would have been fine if only used for one dive, but not after a substantial number of fibers had snapped as the hull flexed across numerous dives.