Identification of plastic behaviors of structural steels at very large deformation

构件中脆性裂纹的产生降低了钢结构的延性。正如在Hyogoken Nanbu地震中观察到的那样，这种脆性断裂通常是由先前的韧性断裂触发的。因此，钢结构的极限性能和变形能力的估计，关键取决于其构件的延性断裂。然而，由于缺乏对这种材料特性的定量方面的了解，目前研究构件延性断裂的数值方法还很少。我们提出了一种替代的实验方法，以绕过使用基于有限元的逆分析的困难。这种方法提供了一种通过简单的单轴试验来确定材料常数的一致方法。在本文中，我们认为用Gurson模型进行有限元分析可以作为估算钢构件延性极限的一种合适的工具。一旦材料参数被适当地确定，就可以用孔洞体积分数来相当准确地预测塑性断裂的起始。然而，我们也注意到，实验结果表明在某些情况下需要进一步研究。考虑到实际应用的重要性，我们提出了Gurson模型的一个简单扩展，即所谓的运动硬化模型。修改后的模型被编写为有限元程序DIANA的插件用户子程序，用于实际的数值分析。

The initiation of brittle cracks in members degrades the ductility of steel structures. As observed in the Hyogoken Nanbu Earthquake, such brittle fracture is often triggered by preceding ductile fracture. The estimation of ultimate behaviors and deformation capacity of steel structures, therefore, critically depends on the ductile fracture of its members. However, few numerical method to investigate the ductile fracture of members is available because of the lack of the understandings of the quantitative aspects of such material characteristics. we have proposed an alternative experimental method to circumvent the difficulty using an FEM-based inverse analysis. This method provides a consistent way to determine the material constants by a simple uniaxial test. In the present work, we show that the FEM analysis with Gurson's model could be a suitable tool for the estimation of ductility limit of steel members. Once the material parameters are properly determined, the initiation of plastic rupture can be predicted with reasonable accuracy in terms of void volume fraction. However, we also note that the experimental result suggests the needs for the further investigation in some cases. In view of practical importance, we proposed a simple extension of Gurson's model to so called a kinematic hardening model. The modified model is coded as a plug-in user subroutine of an FEM program DIANA for practical numerical analysis.