Japanese-based ship classification society ClassNK has found that the minimum required brittle crack arrest toughness for extremely thick brittle crack arrest steel plates of 100mm used in ultra-large container ships is 8,000 newtons (N) / mm3/2.
In 2013, International Association of Classification Societies (IACS) had released its unified requirements on brittle crack arrest design for the use of extremely thick steel plates in a bid to improve the structural reliability of large container vessels.
In its requirements, IACS has mentioned necessary measures to be taken to prevent brittle crack propagation in the event of occurrence of a crack.
The IACS requirements also noted that brittle crack arrest steel plates with a thickness of 80mm or less must have a brittle crack arrest toughness value (Kca) of 6,000N / mm3/2 or more at -10°C, which is the minimum design temperature for commercial vessels.
However, no specific rules were given for cases where the thickness of brittle crack arrest steel is more than 80mm.
The required Kca value for brittle crack arrest steel plate with more than 80mm thickness is to be specifically agreed with each classification society.
ClassNK noted that verification tests were conducted by ultra-large-scale test specimens simulating the actual construction of the hatch side coaming and upper deck of ultra-large container ships under a joint R&D project.
The results of the tests have showed that brittle crack arrest steel plates with a thickness of 100mm must have a Kca of 8,000N / mm3/2 or more at -10°C.
ClassNK plans to reflect the new finding onto the IACS unified requirements in order to ensure easy use of extremely thick steel plates in the shipbuilding industry.
Last month, the society modified its rules and guidance for the survey and construction of steel ships in accordance with the new industry developments, such as amendments made to the international code for the construction and equipment of ships carrying liquefied gases in bulk (IGC Code).