Why carbon fibre is a poor pressure-hull material

Why are submarines not built with carbon-fibre pressure hulls?

Because carbon fibre's brittle failure mode offers less warning than the ductile deformation of conventional pressure-hull metals.

Failure behaviour matters as much as strength. Carbon fibre tends to fail by brittle fracture instead of the ductile or plastic deformation expected from conventional pressure-hull metals. Designers receive less warning before a catastrophic loss of the hull. Submersible standards and insurer requirements guard against avoidable engineering mistakes.