For many years, fatigue design has been based on the fundamental that the fatigue strength of welded structures is independent of the steel material and/or strength. Nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges much to the same capacity regardless of material strength. For improvement of fatigue lives, the designers have been advised to improve the geometry of the details, to reduce the nominal stress level or to use some post weld improvement method like toe grinding. In 2001, a new steel material was developed that showed extended fatigue initiation life as well as extended crack growth life, and a new alternative method for fatigue life extension appeared. This steel was denoted FCA (Fatigue Crack Arrester) due to the improved fatigue properties. The improved fatigue strength in welded joints is explained by flat hardness distribution and very fine microstructure at heat affected zone (HAZ). It was clarified that fatigue strength of HAZ in FCA where fatigue crack initiates generally was higher than that in conventional steel. And the improved fatigue crack propagation properties in base steel is explained by a decreased crack growth rate when a fatigue crack passes a grain boundary from a soft phase (feritte) to a hard phase (bainite) that is present in these new dual phase steels. FCA steel has now been used for details of a number of newly built ships, where good fatigue properties are required. In order to establish a general design S-N curve that can be used for the FCA steel, it was agreed in 2007 to start a joint industry project among Kawasaki Shipbuilding Corporation, Det Norske Veritas and Sumitomo Metal Industries. This JIP is now being finished and a design S-N curve has been proposed based on 66 data from small scale testing of specimens made from FCA steel, and 18 data from conventional steel. These test results have been supplemented by some large scale tests of relevant ship details. The discussed results from this JIP including a recommended design S-N curve for FCA steel will be shown in this paper.
- Ocean, Offshore and Arctic Engineering Division
Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 4
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Konda, N, Arimochi, K, Inami, A, Takaoka, Y, Yoshida, T, & Lotsberg, I. "Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 4." Proceedings of the ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering. Volume 3: Materials Technology; Jan Vugts Symposium on Design Methodology of Offshore Structures; Jo Pinkster Symposium on Second Order Wave Drift Forces on Floating Structures; Johan Wichers Symposium on Mooring of Floating Structures in Waves. Rotterdam, The Netherlands. June 19–24, 2011. pp. 297-306. ASME. https://doi.org/10.1115/OMAE2011-49501
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