Stress drop, Luders strain and strain rate during serrated flow

Load and strain were recorded at every 1 ms interval during the serrated flow in annealed Al-4Cu-0.5Mg-0.5Mn alloy. The strain was measured by four minute strain gauges (0.15 mm length) stuck on the specimen at 0.5 mm intervals. The Luders strain was 0.3%-1% and the Luders strain rate (maximum value) was 0.1-0.6 s^-1 for average strain rates between 6×10^-5 and 3×10^-2 s^-1. The Luders band length was about 2-3 mm. In the test with the average strain rate of 3×10^-3 s^-1 two staged Luders deformation occurred, that is, a fast deformation (0.1 s^-1) was followed by a slow one (0.01 s^-1). The current theory, which is based on the concept of the negative strain rate sensitivity of the flow stress, could not explain the experimental results mentioned above.