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Comment by SoftTalker | Hacker News Reader

The problem with carbon fiber is that it's fiber. Think of a rope. It's very strong in tension; you can hang from it, climb it, hoist things with it, etc. Carbon fiber is useful in an aircraft fuselage because the inside pressure is a tension load on the fuselage.

Rope is useless under compression. You can't push a rope, or climb a rope that's only attached to the floor. So under compressive loads, you're relying on the glue mainly. The carbon fiber gives the glue something to stick to. A submarine is the opposite of an aircraft: the high pressure is on the outside, so the hull is under compression not tension.

It's a terrible choice for a submarine.

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Some good points from first principles. But there are ways. If you manage to make the composite with a very good process so the fibers are straight and closely packed, then it can have good compression strength as well.

This can be achieved for example in pultruded carbon rods, where the carbon fiber is under tension when it passes through an epoxy bath. They have been used in aircraft wing top spars which receive compression loads (when taking positive gees).

How to make a cylindrical vessel that can take compressive loads? The creation process certainly needs some thinking and attention.

Thinking about this more from first principles, if I got it right, the cylinder hoop stress is twice the longitudinal stress. So you could pultrude a unidirectional layer of hoops, or very shallow angle spiral to handle half of the the hoop stress only on the outside. Have normal cross wrapping layer on the inside probably.

This is misleading. Only raw CF is like a rope, once made into a composite with resin it becomes a stiff member with entirely different properties. It's more analogous to a wood beam - very stiff and resistant to bending but once you get past it's limit it cracks.

I suspect the failure mode in a submersible like this isn't so much the carbon fibers themselves but either the carbon > metal interface at the ends or gradual delamination between layers of fibers due to the cyclical pressure loads.

Under outside pressure, the fibers in the matrix take close to no load, while under tension they take a lot. That alone is the reason why carbon fibre composites are the least suited materials for submarines and submersibles.

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