Law Cycle Fatigue of heavy steel welded joints

1995年的小古根-南部地震造成了日本钢结构建筑脆性断裂的不可接受的破坏程度，这是日本地震灾害史上的第一次。从那时起，人们投入了大量的精力来确定原因并建立预防骨折的方法。特别针对在地震荷载作用下脆性断裂的超重钢焊接构件进行了研究。通常，在几个塑性应变循环中产生的裂纹被称为极低周疲劳。然而，通过显微断口学的精确观察表明，这种裂纹是延性裂纹。在此基础上，研究了塑性应变逆转作用下延性裂纹萌生的相关因素，以及延性裂纹被迫过渡为爆炸脆性断裂的条件。研究结果表明：裂纹萌生应变受裂纹产生热点处应力三轴性和材料固有的均匀伸长能力的影响；循环塑性应变作用下的裂纹萌生可以由单调拉伸应变作用下的裂纹萌生应变来预测；从裂纹萌生到最终脆性断裂的塑性应变能力受构件厚度和夏比冲击能的支配。希望这些发现可以应用于考虑焊接热输入引起的材料性能变化以及接头珠粒引起的结构不连续性的焊接接头断裂。然而，人们认识到脆性断裂伴随着高度的不确定性，因此没有得出一般性的结论性评论。为了建立有效的设计方法来预防和控制钢结构建筑在地震危害下的脆性断裂，需要更多的试验数据。少

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