3.4.1. Aluminum Alloy 6061

A 3 mm thick dog-bone shaped aluminum alloy 6061 sample was utilized for a tensile test, and the test conditions were consistent with those used for DP980. The nonnecking and necking areas were used to analyze the R-value of aluminum alloy 6061. Figures 16 and 17 show the R-value curves of the non-necking area and the necking area using the direct measurement method and the indirect measurement method, respectively. All thickness strain data have been filtered using the RANSAC algorithm before plotting the R-value curves. It can be seen from the figures that the two measurement curves of the non-necking area are very close. The R-values obtained by fitting the two curves with least squares are 0.4895 and 0.4925, respectively. However, the data taken in the necking area shows the curves of the two measurement methods are different in the necking stage before fracture. The thickness strain of the direct measurement method increases more rapidly than that resulting from the indirect measurement method; thus, the R-value becomes smaller. The R-value curve change of this aluminum alloy 6061 based on direct measurement is similar to that of DP980. The constant volume assumption is no longer followed in the necking stage. The possible reason for this is that the crystal structure of the metal material is dislocated during the necking process.

**Figure 16.** R-value curves of aluminum alloy 6061 in the non-necking area.

**Figure 17.** R-value curves of aluminum alloy 6061 in the necking area.
