What influences the choice of material for a marine engineering component and explain in simple terms?

Answer 1

There are a lot of factors, but some include operational environment (surface or submerged), temperatures involved, if it's a hull valve (exposed to the sea and inner hull) and the mission requirements of the vessel. Salt and water corrosion resistance is also a huge factor, as ships go through a lot during their lifetime (why a lot of stainless is used onboard). Finally, safety and strength are paramount. Bad materials mean accidents, and accidents cost lives as well as a lot of money, or possible loss of ship. Think Titanic - we know today her hull became brittle in the cold water she was traveling in because of how steel was made in those days.

For example, U.S. submarines use a flexible alloy steel for submarine construction. This is because as submarines go deeper, sea pressure exerts much force on the outer hull. Having a hull that can contract and expand with pressure lasts much longer. Conversely, the Russians often use Titanium in some of their submarine hulls. While stronger, it becomes more brittle over time as the boat goes deep and shallow.

As an example of how inferior materials can cause huge losses, when I was still working as a DoD QA Inspector, one of our guys uncovered a company in our area that was selling bogus, hardware-grade fasteners (nuts, bolts, washers, etc.) as military-grade hardware. The investigation and implications were staggering - millions of bad fasteners were being used in major weapons systems, ships, submarines, etc., that weren't designed to withstand the pressures and loads of military use. It's like going to your local Ace Hardware and buying a few bolts to replace those on a critical system under huge loads and pressures. During the late '80's and the early 90's, many military accidents were attributed to bad fasteners.

Most materials the Navy uses have been used for many decades since a lot is already known about how they perform, and though ship/submarine classes have evolved, the environment they operate in hasn't. A lot of testing and analysis goes into materials testing (there are Material Engineers who do nothing else but analyze potential materials for military programs) before they're actually accepted and bought by the Pentagon.