Abstract: The single point incremental forming has a high forming limit, but the prediction of forming limit is few. Based on the stress-strain data from numerical simulation and combined with the four groups of fracture tests, the Oyane ductile fracture criteria are used to effectively predict the forming limit of LY12 (M) hard alluminum sheet of 1.5 mm thick. In the Oyane criteria, when the integral value I amounts to four, the predicted rupture starting point of the workpieces and the forming limit diagram are more consistent with the experimental results. Incremental forming limit is much higher than conventional forming methods, due to its “partial, alternate, small-increment” deformation characteristics, so that small deformation accumulates to large deformation, “mandatory” achieving a uniform distribution of deformation to gain a high forming limit. During the forming process, the stress path ups and downs, the stress triaxiality degree ( ) is small and unfavorable to the material ductile fracture; local high pressure and small feed rate, frictional heat and good lubrication conditions guarantee the high plasticity of incremental forming workpieces.

Key words: Degree of stress triaxiality, Ductile fracture criterion, Forming limit, Incremental forming