Law Cycle Fatigue of heavy steel welded joints

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The 1995 Hyogoken-Nanbu Earthquake caused an unacceptable damage level of brittle fracture in steel structural buildings which was the first event in the history of Japanese seismic disaster. Since then, considerable efforts have been devoted to identify the causes and establish the method to prevent fracture. Especially, this study is focused on super-heavy steel welded members which are highly susceptible to brittle fracture under earthquake loading.Conventionally, the cracks which are generated in several cycles of plastic strains are called very-low-cycle fatigue. However, a precise observation by means of micro-fractography revealed that such cracks are ductile cracks. On the basis of this fact, the factors related to the initiation of ductile cracks under plastic strain reversals and the conditions on which the ductile cracks are forced to be transient to explosive brittle fracture are investigated in this study.From this study, followings were found : the crack initiation strain … More is governed by the stress triaxiality at the hot spot of crack generation as well as the uniform elongation capacity inherent to the material ; the crack initiation under cyclic plastic strains can be predicted from the crack initiation strain under monotonic tensile strain within the mechanism of ductile cracking ; and the plastic strain capacity from the crack initiation to the final brittle fracture is governed by the member thickness and Charpy impact energy. It is hopefully suggested that these findings can be applied to the fracture of welded joints in consideration of the changes of material properties due to welding heat input as well as the configurational discondinuities due to joint beads.It was recognized, however, that the brittle fracture is accompanied by a high degree of uncertainties, and thus general conclusive remarks were not derived. More test data will be definitely necessary to establish an effective design method to prevent and control brittle fracture of steel buildings under the hazard of earthquakes. Less