Identification of plastic behaviors of structural steels at very large deformation

构件脆性裂纹的产生降低了钢结构的延性。正如在小谷县南部地震中观察到的那样，这种脆性断裂往往是由之前的韧性断裂引发的。因此，钢结构的极限性能和变形能力的估计在很大程度上取决于其构件的韧性断裂。然而，由于缺乏对这种材料特性的定量方面的理解，很少有数值方法来研究构件的韧性断裂。我们提出了一种替代的实验方法来规避使用基于有限元的逆分析的困难。该方法提供了一种通过简单的单轴试验确定材料常数的一致方法。本文的研究表明，采用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。一旦确定了材料参数，就可以合理准确地用孔隙体积分数来预测塑性破裂的发生。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。考虑到实际的重要性，我们提出了一个简单的扩展Gurson的模型，即所谓的运动硬化模型。将修正后的模型编码为有限元程序DIANA的插件用户子程序，用于实际数值分析。

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.