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

Forming processes of metal sheets are generally limited by plastic instabil ity phenomena and flow localization. The occurrence of these phenomena is d ependent on the material properties such as strain-hardening exponent, stra in rate sensitivity, anisotropy parameters and grain size and is also depen dent on the strain path. The formability of the sheet metals can be assesse d by the forming limit diagram (FLD). In this study, a theoretical model us ing the 'many slices' approach is introduced to simulate the neck growth. T he effects of changing strain path and grain sizes on the limit strains are then investigated both theoretically and experimentally. The low carbon st eel ST12 and austenitic stainless steel 321 are used in the experimental ap proach. The theoretical and experimental FLDs of these sheets are obtained for different grain sizes and after pre-straining in uniaxial and biaxial t ension 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 sel ecting the proper strain path and grain size, better formability properties can be achieved. Also, good agreement is obtained between theoretical and experimental results.