Effect of pre-straining and grain size on the limit strains in sheet metal forming

Forming processes of metal sheets are generally limited by plastic instability phenomena and flow localization. The occurrence of these phenomena is dependent on the material properties such as strain-hardening exponent, strain rate sensitivity, anisotropy parameters and grain size and is also dependent on the strain path. The formability of the sheet metals can be assessed by the forming limit diagram (FLD). In this study, a theoretical model using the 'many slices' approach is introduced to simulate the neck growth. The effects of changing strain path and grain sizes on the limit strains are then investigated both theoretically and experimentally. The low carbon steel ST12 and austenitic stainless steel 321 are used in the experimental approach. The theoretical and experimental FLDs of these sheets are obtained for different grain sizes and after pre-straining in uniaxial and biaxial tension parallel to the prior rolling direction. It is shown that the limit strains are quite sensitive to the grain size and strain path. Thus, by selecting the proper strain path and grain size, better formability properties can be achieved. Also, good agreement is obtained between theoretical and experimental results.