Kim Minsoo, Hong Seokmoo
Department of Mechanical and Automotive Engineering, Kongju National University, Cheonan 31080, Korea.
Materials (Basel). 2020 Dec 16;13(24):5743. doi: 10.3390/ma13245743.
In this study, the experimental two-dimensional forming limit diagram (FLD) data for a ZIRLO™ sheet, which is used in nuclear fuel rod support grids, were converted and presented as a triaxiality failure diagram (TFD). Most previous studies assumed ZIRLO™ to be isotropic when calculating the effective stress and strain. However, for highly anisotropic materials, the anisotropy should be considered for calculations of effective stress and strain; hence, in this study, they were calculated by introducing the normal anisotropy coefficient. To obtain this parameter of the ZIRLO™ specimens, tensile tests were performed on specimens with 0°, 45°, and 90° angles with respect to the rolling direction. It was observed that the average normal anisotropy coefficient measured during the tests was 4.94, which is very high. The von Mises isotropic and Hill 48 anisotropic yield criterion were applied to the FLD data that were experimentally determined using a limit dome height test and were converted into effective stress and effective strain. When the FLD is converted to TFD, the curve will increase in the top-right direction if the -value is greater than 1, and this become more severe as the -value increases. The TFD, which was converted considering the anisotropy, is almost the same to the TFD obtained using the digital image correlation method in the tensile tests of four specimens with different stress states. If anisotropy is not considered, then the formability is normally underestimated. However, a highly accurate TFD can be obtained with the method proposed in this study.
在本研究中，用于核燃料棒支撑格架的ZIRLO™板材的实验二维成形极限图（FLD）数据被转换并呈现为三轴性失效图（TFD）。大多数先前的研究在计算有效应力和应变时假定ZIRLO™为各向同性材料。然而，对于高度各向异性材料，在计算有效应力和应变时应考虑其各向异性；因此，在本研究中，通过引入正各向异性系数来计算有效应力和应变。为了获得ZIRLO™试样的该参数，对相对于轧制方向呈0°、45°和90°角的试样进行了拉伸试验。观察到试验期间测得的平均正各向异性系数为4.94，这非常高。将冯·米塞斯各向同性屈服准则和希尔48各向异性屈服准则应用于通过极限圆顶高度试验实验确定并转换为有效应力和有效应变的FLD数据。当FLD转换为TFD时，如果r值大于1，曲线将在右上方向增加，并且随着r值增加这种情况会变得更严重。考虑各向异性转换得到的TFD与在四个具有不同应力状态的试样的拉伸试验中使用数字图像相关方法获得的TFD几乎相同。如果不考虑各向异性，通常会低估成形性。然而，采用本研究提出的方法可以获得高度精确的TFD。
