Forming limit of sheet metals in meso-scale plastic forming by using different failure criteria

- The FLC and FFLC of copper sheet metal were developed.
- The FLC and FFLC shift down with the increase of grain size at mesoscale.
- The applicability of different necking and ductile fracture criteria was investigated.
- The M-K model and GTN-Thomason model were found accurate at mesoscale.

The ductile failure of sheet metals in micro/mesoscale plastic deformation is influenced by grain and geometry size effects. Based on the forming limit experiments of copper sheet metals with different grain sizes, it is found that there is a significant reduction of forming limit with the increase of grain size under different deformation paths. To describe the size effect induced decrease of forming limit, a number of the most widely-used failure criteria and theories were employed to investigate their applicability in meso-scale plastic deformation, including the Swift/Hill criteria, Marciniak-Kuczynski model, ductile fracture criteria such as Freudenthal, Cockcroft & Latham, Ayada and Oyane models, and the Gurson-Tvergaard-Needleman model coupled with the Thomason void coalescence model (GTN-Thomason model). The applicability of these criteria and the mechanism behind them were discussed for better characterization of the failure behavior at micro/mesoscale. In addition, to corroborate the developed method, meso-scale hydroforming experiments of sheet metals was conducted. The M-K model and the GTN-Thomason model are revealed to be able to accurately predict the ultimate pressure and the height at the onset of failure by comparing to the experimental results.