Evaluation of limiting strains and strain distribution for interstitial free steel sheets while forming under different strain conditions

In this work, forming limit diagrams (FLDs) were experimentally evaluated for the interstitial free steel sheets of different thickness namely 0.6, 0.9, 1.2 and 1.6 mm. The limiting strains for these sheets under different strain conditions were studied. The forming limit diagrams for the above sheet metals were compared. The microstructural aspects, tensile properties and formability parameters were experimentally determined and studied. The tensile properties and formability parameters were correlated with FLDs. The longitudinal major strain and minor strain distribution profiles were plotted from the forming experiment. The thickness strain distribution profiles in longitudinal, transverse and diagonal directions were also plotted and detailed analysis of strain distribution profiles was attempted. The fractography of the formed samples was taken using scanning electron microscope (SEM). Using SEM fractography, the fracture behaviour and formability were analyzed in three different strain conditions namely tension–tension, plane strain and tension–compression strain conditions. From the fractography and experimental data, the average size of voids present in the fractured surface, strain ratios and the shear strain developed in the sheet metal at fracture were determined. An attempt was made to relate the void size to the shear strain developed in the sheet metal at fracture. When comparing all the sheets, the limiting strains are higher for the sheet having 1.6 mm thickness.