The forming limit diagram (FLD) is a very useful diagnostic tool for troubleshooting in sheet metal forming industries. These curves can be assessed using theoretical and experimental approaches. In this paper, the FLD is calculated using Hill's non-quadratic yield criterion. It is shown that the theoretical results are dependent upon the Hill index. To achieve better agreement between theoretical and experimental results, the different Hill indices are used for different materials as well as the different regions of the FLD. The optimum Hill indices for left- and right-hand sides of the FLD for different materials are obtained regarding the experimental results.
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