Experimental investigations on forming limit diagram of ultra thin SS 304 steel: effect of circular grid size, sheet orientation, punch size and deformation speed

In the present work, SS 304 sheet of 200 μm thickness was used to experimentally evaluate the forming limit diagram (FLD). In this context, a sub-size limiting dome height (LDH) test setup was developed to deform rectangular specimens of different widths using a 30 mm hemispherical punch. Further, effect of various parameters such as change in circular grid size, stretching direction with respect to rolling direction of the sheet, punch size and deformation speed on limiting strains was investigated. It was observed that there was marginal change in limiting strains due to the change of punch diameter from 30 mm to 50 mm. The FLD0 level changed approximately 7.5% and 6.8% with the change in punch diameter and stretching direction respectively. It was also found that the FLD0 value increased by 15% with change in circular grid diameter from 2.5 mm to 2.0 mm. However, a negligible change in the limiting strains was noted when the grid diameter was further reduced to 1.0 mm. There was negligible change in the FLD0 level with increase in punch speed from 4 mm/min to 100 mm/min, but approximately 13% decrease was found when the punch speed was further increased to 400 mm/min. Moreover, the LDH, strain distribution and failure strains were also analyzed in context of formability.