How are submarines designed to withstand the pressure of deep sea submersion?

Answer 1

Modern submarines rely on their hull design and material construction to deal with extreme pressures in the ocean while submerged. Most NATO countries use a flexible, compressible steel alloy that allows the hull to compress and expand as the submarine goes deep and shallow. The decks aren't actually connected to the hull - if they were, they'd be severely damaged as the boat goes deep. They're actually suspended by cables, the sides of the decks several inches from either side of the hull to prevent contact.

Not all submarines use this type of construction, and it can be a problem later down the road in the life of the boat. For example, the Russians have used Titanium to construct some of their boats. While strong and allowing them to go deeper, Titanium isn't flexible, and over time under repeated pressures it will become brittle. Others like China use GRP (fiberglass) to construct their boats. The problem with having non-flexible material is the egg-shell effect - egg shells are strong, but under enough repeated pressure they become brittle because they cannot compress and expand.

A spherical based design is also the strongest for extreme pressures, and why virtually all modern submarines are based on a cigar or teardrop shaped hull design.

Answer 2

The Russians use titanium to manufacture the pressure hulls of their submarines. The Chinese use a material called glass-reinforced plastic, and the U.S. and most other countries use a steel alloy that is specially designed to withstand the cyclic stress that it experiences when it is subjected to the wide range of pressure changes during submerged operations. Cyclic stress is the same type of stress that you put on a piece of wire or plastic when you bend it back and forth. It is almost impossible for a human to pull apart a steel clothes hanger, but if you bend it back and forth several times, the metal will weaken at the point of the bend and eventually fail.