Strength of Ships - Material Response

Material Response

Modern ships are, almost without exception, built of steel. Generally this is fairly standard steel with yield strength of around 32,000 to 36,000 psi (220 to 250 MPa), and tensile strength or ultimate tensile strength (UTS) over 50,000 psi (340 MPa).

Shipbuilders today use steels which have good corrosion resistance when exposed to seawater, and which do not get brittle at low temperatures (below freezing) since many ships are at sea during cold storms in wintertime, and some older ship steels which were not tough enough at low temperature caused ships to crack in half and sink during World War II in the Atlantic.

The benchmark steel grade is ABS A, specified by the American Bureau of Shipping. This steel has a yield strength of at least 34,000 psi (230 MPa), ultimate tensile strength of 58,000 to 71,000 psi (400 to 490 MPa), must elongate at least 19% in an 8-inch (200 mm) long specimen before fracturing and 22% in a 2-inch (50 mm) long specimen.

A safety factor above the yield strength has to be applied, since steel regularly pushed to its yield strength will suffer from metal fatigue. Steels typically have a fatigue limit, below which any quantity of stress load cycles will not cause metal fatigue and cracks/failures. Ship design criteria generally assume that all normal loads on the ship, times a moderate safety factor, should be below the fatigue limit for the steel used in their construction. It is wise to assume that the ship will regularly operate fully loaded, in heavy weather and strong waves, and that it will encounter its maximum normal design operating conditions many times over its lifetime.

Designing underneath the fatigue limit coincidentally and beneficially gives large (factor of up to 6 or more) total safety factors from normal maximum operating loads to ultimate tensile failure of the structure. But those large ultimate safety margins are not the intent: the intent is that the basic operational stress and strain on the ship, throughout its intended service life, should not cause serious fatigue cracks in the structure. Very few ships ever see ultimate load conditions anywhere near their gross failure limits. It is likely that, without fatigue concerns, ship strength requirements would be somewhat lower.

See Strength of materials.