Strain rate dependence of dynamic flow stress considering viscous drag for 6061 aluminium alloy at high strain rates

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In order to evaluate the strain rate dependence of the dynamic flow stress of aluminium alloys, 6061-O and -T6, high strain rate tests are performed at strain rates ranging from about 1000/s to 30000/s, and strain rate reduction tests are also conducted in the strain rate range from about 10000/s to 20000/s. A steep increase in the flow stress is observed for 6061-O at the strain rate of about 5000/s. The above phenomenon, however, is not observed for 6061-T6 in the strain rate range where the strain rate reduction tests is conducted. A simplified model for a dislocation kinetics under a dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate ranges, which reveals that the steep increase in the flow stress of 6061-O is attributed to the rate dependence of the viscous drag on the dislocation motion. It is estimated that the velocity of moving dislocation of 6061-T6 is lower than that of 6061-O at a given strain rate, and that 6061-T6 is higher in a mobile dislocation density than 6061-O. The model predicts also that the increase in the mobile dislocation density shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side.