Hiroshige Kitada

Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 3

In recent years, higher safety and reliability of steel welded structures have been required as it shows growing concern about environmental problems. To prevent fatigue fracture is one of the most important challenges to improve the safety and reliability. A lot of studies how reduce stress concentration at critical areas have been carried out from the viewpoint of structural design as prevention measures while nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges in limited range regardless of material strength. On the other hand, it was found that an appropriate dual phase microstructure could reduce the fatigue crack growth rate remarkably. The newly developed steel plate with high resistance to fatigue crack growth could extend the fatigue life of structures. The developed steels have already been applied to some ships and vessels, and a new bulk carrier applied the developed steels acquired the notation and descriptive note as the valuable ship with resistance to fatigue fracture by Nippon Kaiji Kyokai for the first time in the world. From further studies, it was found the developed steels had also high resistance to fatigue crack initiation as well as the growth even in welded structure. In this study, it was clarified that the fatigue strength of HAZ, where fatigue crack generally initiates, in the developed steel was higher than that in conventional steel and the stress concentration at toe of weld in the developed steel was smaller than in the conventional steel. It was considered the mechanism of suppression of fatigue crack initiation with FEM analysis and fatigue test. The newly developed steel can effectively extend fatigue fracture life of welded structure from the viewpoint of material.Copyright

Development of Structural Steel With Superior Resistance Against Fatigue Crack Growth

The fatigue crack problems that broke out at the end of the twentieth century were fatigue crack properties in hull structures. This motivated research on a lot of technologies against fatigue fracture. It was clarified that the detection of fatigue crack initiation in complex welded structures like hulls is quite difficult, and that the crack length at recognition is mostly long compared to mechanical parts. From these research results, not only stress reduction at critical areas by improvement in design but also newly developed materials with excellent resistance to fatigue crack growth has been desirable for structural integrity. The newly developed structural steel, in which fatigue crack growth resistance is controlled by microstructures, will be introduced in this report. Various fatigue properties of the base steel plates in air and in synthetic sea water are compared. And the fatigue life extension effects by FCA is observed in fatigue tests of welded joints and welded structural models.Copyright