A new forming limit test method for incremental stretch flanging

Incremental stretch flanging (ISF) has a more complex deformation process than conventional incremental forming, resulting in varying forming limits and strain characteristics. This work pointed out that D_t/D (D_t: tool diameter; D: flanging diameter) can be used as a dimensionless parameter to represent the flanging process, and proposed the incremental flanging forming limit (IFFL) test method. The IFFL test's idea is to conduct incremental hole flanging experiments using different D_t and D, in which the opening diameter of the sheet metal is continuously reduced until a fracture occurs. The incremental flanging-fracture forming limit (IF-FFL) was obtained by fitting the ultimate strain points, which successfully predicted the occurrence of fracture in ISF, and its accuracy has been experimentally verified. In addition, the strain path in ISF can be divided into three zones, and the ratio β of the minor strain and the major strain in zone Ⅰ has an exponential relationship with D_t/D. The fracture may also occur at the edge or wall under different flanging parameters, and the critical flanging diameter Dc of the two fracture forms is obtained by using the membrane strain analysis model. Moreover, the theoretical variation of the process forming limit with D_t/D is studied, effectively supporting the industrial production practice.