A combined experimental and computational investigation was carried out on the measurements and modeling of spatial characteristics of unstable plastic flow patterns in two commercial aluminum alloys: a solid solution strengthened alloy AA5182-O and a precipitate strengthened alloy AA6111-T4. Using an accurate and robust incremental plastic strain mapping technique based on the interconnected digital image correlation, the initiation and growth of unstable plastic flow patterns on the surfaces of the two aluminum sheet metals were measured at high spatial resolution until necking in a quasistatic uniaxial tensile test at ambient temperature. It is found that a finite level of plastic strain is required to initiate the unstable plastic flow in both aluminum alloys. Once commenced, the unstable plastic flow manifests itself as propagative plastic deformation bands traveling through the gauge section of the tensile specimens. Two distinctive stages are identified for the propagative plastic deformation bands in terms of their spatial characteristics, namely, stage I of uncorrelated, inclined, multiple deformation bands and stage II of correlated, symmetrical, double bands. Based on the plastic strain mapping measurements on both sides of the sheet metal sample, the propagative plastic deformation bands are confirmed to be macroscopic shear bands in nature.;A stable and accurate time integration scheme was implemented via a user material routine UMAT in the nonlinear finite element code ABAQUS for a dynamic strain-aging viscoplastic model that is formulated based on the interaction between solid solute atoms and dislocations in a polycrystalline material. Both 2D and 3D numerical simulations of uniaxial tensile tests of flat sheet specimens were carried out to predict the unstable plastic flow patterns in the two aluminum alloys observed in the experiments. By using the displacement boundary conditions measured in the experiments, the structural or mechanistic effects on the unstable plastic flow patterns were carefully separated from those of the constitutive material behavior. Computational results showed that the distinguishing temporal and spatial characteristics of the unstable plastic flow patterns in the two aluminum alloys can be modeled reasonably well, especially the onset of the unstable plastic flow, the transition from the uncorrelated, to correlated, propagative plastic deformation bands and the propagation of the correlated deformation bands leading to the final necking of the samples. The U-shaped strain dependence of the overall steady-state strain rate sensitivity was found to play the critical role in setting the specific behaviors of unstable plastic flows in the two aluminum alloys. However, the deficiency of the phenomenological dynamic strain-aging viscoplastic model was also clearly identified in this investigation as the spatial details of the uncorrelated, unstable plastic flow patterns in the stage I could not be predicted to any degree of satisfaction for both materials, especially the AA5182-O alloy.
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