A novel experimental approach in determining forming limit diagrams by considering the effect of normal pressure

Forming limit diagram (FLD) is a useful tool for predicting failure in sheet metal forming processes. Many experimental tests, such as Nakazima test, have been developed to determine the FLDs experimentally. In all these methods the FLDs are determined without considering the effect of normal stress. This paper presents a novel experimental approach based on the Nakazima method to determine the forming limit diagrams under various normal pressures. Both necking and fracture forming limit curves of St12 low carbon steel were obtained experimentally with no, and under 50 MPa compressive normal pressures. The fracture forming limit curves under different normal pressures between 0 and 250 MPa were predicted numerically using ABAQUS/Explicit commercial finite element code and applying VUSDFLD subroutine in order to determine fracture. For this purpose, six phenomenological ductile failure criteria, including Rice-Tracey, Oyane-Sato, Ko-Huh, DF2012, DF2014, and DF2015, were implemented. These criteria were calibrated using the six tests of in-plane shear, uniaxial tensile, circle hole, notched tension, plane stress, and Nakazima stretching. The results from the proposed methods and those from numerical simulations have been compared and the efficiency of each ductile fracture criterion was demonstrated. All the experimental and numerical results indicated that a greater normal stress causes the forming limit curve surface to rise.