Distinguishing micro-scale from macro-scale tensile flow stress of sheet metals in microforming

This study describes the effect of grain size on the flow stress of sheet metal under simple tension in microforming. A simple model of the tensile flow stress of sheet metal is firstly developed based on the Hall–Petch equation. Experimental results verify the accuracy of the developed model, which is a function of T/D (sheet thickness/grain size). A critical condition (T/D)c that distinguishes micro-scale from macro-scale tensile flow stress is subsequently proposed based on Li's theory of dislocation with density type. The trend of the predicted (T/D)c with varying grain size is similar to the experimental finding that the (T/D)c decreases as the grain size increases. Therefore, the developed model can elucidate to understand the tensile flow stress of sheet metal in microforming.