Fracture in hole-flanging produced by single point incremental forming

Abstract Hole flanging produced by single point incremental forming is a new manufacturing process in which a sheet, with a concentric pre-cut hole and the outer periphery rigidly fixed by a blank holder, is progressively forced with a tool to produce cylindrical or conical smooth flanges. The formability limits of the process are known to be higher than those commonly found in hole-flanging produced by conventional press-working due to suppression of necking with a low growth rate of accumulated strains above the forming limit curve. The aim and objectives of the paper is to evaluate the critical values of fracture toughness and ductile damage at crack initiation according to different damage laws and to correlate these values with independently determined values of the forming limits at fracture. The presentation starts by tracing the strains and stresses of various positions over the surface of the hole-flanged parts at different intermediate stages of deformation. It is revealed that the strain loading ratios are constant for different material elements. From these data, the experimental values of accumulated damage at various positions are determined, particularly at the site of fracture.