Influence of punch radius in a Nakazima test for mild steel and aluminium

Forming limit curves are usually determined for membrane type deformations. Recently, the influence of simultaneous bending deformation on the forming limits has attracted renewed attention. In this paper, FLCs are determined for a DC06 steel and an AA5051 aluminium alloy by the standard Nakazima test procedure with different punch diameters. The difference in FLC obtained with punch diameters of 100 mm and 75 mm is negligible. For aluminium, the FLC is shifted upwards slightly for a Ø50 mm punch and significantly for a Ø20 mm punch. For DC06, only the experiments with a Ø20 mm punch show a relevant increase in formability. The increased formability cannot be explained by simply transforming the measured (outer) strains to the mid-plane or inner-plane, as proposed by the so-called concave-side-rule. The experimentally observed and numerically validated high pressure on the inner surface for small punch diameters may be responsible for the increased stability of the deformation.