Evolution of plastic deformation behavior upon strain-path changes in an A6022-T4 Al alloy sheet

Abstract

The plastic deformation behavior under various strain-path changes in an A6022-T4 Al alloy sheet was studied, focusing on the evolution of the direction of the plastic strain rate θ and the stress ratio φ after abrupt strain-path changes. A cruciform specimen was used to measure the evolution of the plastic deformation behavior after abrupt strain-path change experimentally. Before the strain-path change, the deformation was represented well by the associated flow rule with the Yld2000-2d yield function and isotropic hardening assumption. After the abrupt-path change, the deformation temporarily deviated from the associated flow rule, and it could be represented again by the associated flow rule after the plastic work increased roughly 4.0 MJ.m-3, which corresponded to the strain increment of approximately 0.024 under uniaxial tension in the rolling direction. The transitions of θ and φ as a function of plastic work showed that both θ and φ tend to converge to certain values regardless of the strain path if the final strain-path angles are identical. It was also found that the relationships between φ and θ temporarily deviate from the associated flow rule immediately after the abrupt-path changes because φ cannot follow the rapid change of θ. Crystal plasticity finite-element simulations reproduced the qualitative tendencies observed in the experiments, but the deviations from the associated flow rule were much more pronounced. Parametric studies showed that φ tends to converge to different values depending on the strain rate sensitivity, whereas θ tends to converge to a same value irrespective of the rate sensitivity exponent. The rate sensitivity exponent m = 0.044 gave the best fits with the experimental results in terms of the evolution of both θ and φ under an abrupt-change path, although the rate sensitivity exponent determined from macroscopic strass-strain curves were m = 0.002.