TENSILE DUCTILITY AND DEEP DRAWABILITY OF RATE-DEPENDENT SHEET MATERIALS

The aim of this work is to clarify the role of appreciable rate dependency of the flow stress in practical sheet metal stamping operated at room temperature. The uniaxial ductility of rate-dependent sheet materials which have the ability of strain hardening as well as strain-rate sensitivity is examined. An analytical description combined with conventional instability theories predicts the significance of the rate of change of strain rate during uniaxial deformation in estimating the ductility. In accordance with the prediction, an Instron test machine is improved so as to be able to accelerate/decelerate the strain rate. Computational mechanics is introduced also to simulate the test. The precise examination on the influence of the rate dependency on the total elongation clarifies the nature of the rate of change of strain rate but results in a somewhat diverse conclusion. In deep drawing where the ductility of sheet metals is of little concern, the effect of continuous change of punch speed on the limiting drawing ratio (LDR) is investigated experimentally also. Sheet metals of pure aluminum and zinc are used in the test and the LDR is determined for each tool geometry. Measured LDRs show distinct differences, depending upon the punch-speed conditions, but systematic explanation of the trends displayed has not yet been accomplished.