Abstract
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 r-value is greater than 1, and this become more severe as the r-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.
Highlights
The forming limit represents the ability of a metal to transform into the desired shape without necking or breaking
The forming limit diagram (FLD) is a diagram that exhibits the limit strains for the strains from uniaxial tension to equibiaxial tension based on an assumed linear strain path in terms of the minor strain ε2 (x-axis) and the major strain ε1 (y-axis) [1,2]
4.94, which indicates that ZIRLOTM is a material with very high anisotropy compared to other metallic materials
Summary
The forming limit represents the ability of a metal to transform into the desired shape without necking or breaking. Based on the εeq and η values calculated from the aforementioned two yield criteria, it is shown that the two-dimensional FLD data converted to TFD, which is a three-dimensional fracture limit, are significantly different owing to curve up to failure, which has been verified by a bulge test [13]. When the path-independent strain/stress-based forming limit criterion is calculated to avoid the strain path dependence of the FLD, inherent calculation errors can arise owing to the Materials.
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