Many experiments have been conducted to investigate the forming limit in single point incremental forming (SPIF). The forming limit curve (FLC) generated from these experiments follows a linear line with a negative slope in the positive minor strain side of the forming limit diagram (FLD). It is also found that, in general, for the same material subjected to the SPIF process, the failure strain level greatly exceeds the traditional FLC based on theories of the plastic instability [Iseki and Kumon, 1994]. Currently, no theoretical work or no criterion was proposed to predict this FLC in the SPIF. In this paper, the criterion for the ductile fracture [Oyane, 1980] is introduced to try to predict the forming limit of the material in the SPIF. Based on the calculated stress and strain from the finite element simulation and the ductile fracture criterion, the fracture initiation site and the forming limit are predicted. The predicted results are compared with that from the SPIF experiment to verify the feasibility of the proposed method.

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