The typical test case for quick calculations is the middle of a hull bottom plate section between stiffeners, close to or at the midsection of the ship, somewhere midways between the keel and the side of the ship. Ship classification societies such as Det Norske Veritas, American Bureau of Shipping, and Lloyd's Register of shipping have established standard calculation forms for hull loads, strength requirements, the thickness of hull plating and reinforcing stiffeners, girders, and other structures. These methods often give a quick way to estimate strength requirements for any given ship. Almost always those methods will give conservative, or stronger than precisely required, strength values. However, they provide a detailed starting point for analyzing a given ship's structure and whether it meets industry common standards or not. Modern ships are, almost without exception, built of steel. Generally this is fairly standard steel with yield strength of around , and tensile strength or ultimate tensile strength (UTS) over . 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 , ultimate tensile strength of , must elongate at least 19% in an 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.