Development of analysis system to assess ultimate seismic behavior of steel piers accompanying fracture due to strain localization

在神户大地震中，薄壁钢桥墩的主要破坏是板单元的断裂和局部屈曲。自神户地震以来，对钢桥墩进行了密集的循环加载试验，以研究板构件局部屈曲引起的延性退化。此外，如果采用适当的循环塑性模型作为钢的本构关系，基于壳单元的几何和材料非线性有限元分析可以准确地分析循环荷载作用下钢桥墩的局部极限屈曲行为。相比之下，在神户地震中观察到的断裂并没有在循环加载试验中类似地发生，因此，这种断裂如何发生以及为什么发生仍然是一个有争议的问题。通过对受损钢墩断裂断面的详细观察得出的唯一事实是，脆性断裂是由小的韧性断裂引发的。这进一步说明，小韧性断裂的发生是脆性断裂发生的必要条件。因此，对钢桥墩韧性断裂的研究将为解释神户地震中观测到的断裂原因提供可能。本文提出了一种预测循环荷载作用下钢墩断裂的数值分析方法，并通过对钢墩的循环荷载试验验证了该方法的适用性。在分析中，考虑到钢的循环行为，对Gurson本构模型进行了顺序修正，并在三维实体单元中实现。Gurson模型是目前应用最广泛的塑性模型，它考虑了与韧性断裂密切相关的微孔洞的形核和生长。为了验证修正的Gurson模型的有效性，对圆形截面薄壁钢桥墩进行了循环加载试验，观察了韧性断裂的起裂过程。数值分析结果与试验结果的对比表明，修正的Gurson模型可以定性地预测钢桥墩韧性断裂的位置。然而，对于韧性裂纹萌生时的变形，分析结果与实验结果存在一定的差异。如何识别延性裂纹的起裂在试验中一直是一个难题。这个问题今后必须进一步研究。少

The major damages observed for thin-walled steel piers in the Great Kobe Earthquake are the fracture as well as the local bucklins of plate elements. Since the Kobe Earthquake, intensive cyclic loading tests on steel piers have been carried out to examine the ductility deterioration due the local buckling of plate elements. Furthermore, it is shown that the ultimate local buckling behavior of steel piers under cyclic loading can be accurately analyzed by the geometrically and materially nonlinear FEM analysis based on shell elements if some adequate cyclic plasticity model is used for the constitutive relation of steel. In contrast, the fracture observed in the Kobe Earthquake did not similarly occur in the cyclic loading tests and, hence, it is still a controversial issue how and why this fracture happened. The only fact obtained by the detailed observation on the fractured section of the damaged steel piers is that the brittle fracture initiated from a small ductile fracture. This im … More plies that the occurrence of the small ductile fracture is a necessary requirement for the occurrence of the brittle fracture. Therefore, the investigation into the ductile fracture of steel piers will make it possible to explain the cause of the fracture observed in the Kobe Earthquake.Herein, we present a numerical analysis method to predict the fracture of steel piers under cyclic loading and examine the applicability of this method by cyclic loading test on steel piers. In the analysis, the Gurson constitutive model is modified in order takes into account the cyclic behavior of steel and is implemented in the 3-dimentional solid element. The Gurson model is the most widely used plasticity model that can consider the nucleation and growth of micro voids closely related to the ductile fracture. In order to investigate the validity of the modified Gurson model, the initiation of ductile fracture is observed by conducting a cyclic loading test on thin-walled steel piers with circular cross section. From the comparison between the numerical analysis and the test results, it is shown that the locations of the ductile fracture in the steel piers can be qualitatively predicted by the modified Gurson model. However, regarding the deformation at the initiation of ductile cracks, some difference is observed between the analysis and the experiment. There still remains a difficult problem how to identify the initiation of the ductile cracks in experiment. This problem must be further examined in the future. Less