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

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.