Identification of plastic behaviors of structural steels at very large deformation

钢结构构件中脆性裂纹的产生降低了钢结构的延性。正如在兵库县南部地震中所观察到的那样，这种脆性断裂往往是由先前的韧性断裂触发的。因此，钢结构的极限行为和变形能力的评估，关键取决于其成员的延性断裂。然而，很少有数值方法来研究构件的延性断裂，因为缺乏对这种材料特性的定量方面的理解。我们已经提出了一种替代的实验方法，以规避困难，使用有限元为基础的逆分析。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。在目前的工作中，我们表明，有限元分析与Gurson的模型可能是一个合适的工具，用于估计延性极限的钢构件。一旦材料参数被适当地确定，塑性断裂的开始可以预测与合理的准确性方面的空隙体积分数。然而，我们也注意到，实验结果表明，在某些情况下，需要进一步的调查。考虑到实际应用的重要性，我们提出了一个简单的扩展Gurson的模型，所谓的运动硬化模型。修改后的模型被编码为一个插件用户子程序的有限元程序DIANA的实际数值分析。

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.