Experimental studies and numerical modeling of the specimen and grain size effects on the flow stress of sheet metal in microforming

In this research, the interactive effect of grain and specimen sizes on the flow stress of sheet metal in microforming is investigated via the tensile test of pure copper and numerical modeling. Models based on different assumptions are proposed to analyze the size effect phenomenon. It is found that the flow stress decreases linearly with the decrease of the ratio of specimen to grain sizes. The grain boundary thickness decreases and its volume fraction increases with the decrease of grain size. The variation of grain boundary thickness is not proportional to the variation of grain size. Furthermore, the fraction of grain boundary increases with the strain and the ratio of specimen to grain sizes. Based on the FE simulation, it is found that the simulated flow stress, which is modeled based on the identified grain boundary thicknesses using the proposed models, has a good agreement with the experimental result. In addition, the size effect on flow stress is also analyzed based on the surface layer model. The methodology to identify the surface and internal grain properties is proposed based on the experimental result. The identified properties are applicable in modeling of the interactive effect of specimen and grain sizes on flow stress. This research thus provides an in-depth understanding of the plastic deformation behavior in microforming process.