Crystal Plasticity Based Constitutive Modeling of Plastic Deformation Under Complex Strain Paths in AA 5754.

摘要

The strain-based forming limit diagram has been widely used for representing metal formability. However, the strong strain path dependency limits its use since forming operations often involve complex loading conditions. Forming limit stresses have been found to be insensitive to strain paths and thus proposed to be more general failure criteria for metal forming. Such a shift from the strain space to the stress space requires accurate descriptions of material constitutive behaviors.;A bi-linear strain path deformation, consisting of equal-biaxial stretch followed by uniaxial tension, was explored to study the strain path change effect. The evolution of mechanical behaviors with the strain path change was carefully analyzed and fitted to the VPSC model. It was shown that taking anisotropic hardening behaviors into account better predicted the transient behaviors associated with the strain path change. In addition, room temperature recovery in deformed AA5754 samples had significant effect on mechanical behaviors in subsequent uniaxial loading, and should be taken into account when dealing with multi-stage deformation.;Constitutive equations for the multiaxial stress-strain behavior of AA 5754 sheets have been developed, based on crystal plasticity. A Taylor-based polycrystal plasticity model, a tangent formulation of self-consistent viscoplastic model (VPSC) and an N-site viscoplastic model based on the Fast Fourier Transform (VPFFT) were used to fit a single slip system hardening law to the available data for uniaxial tension, plane strain and biaxial stretching. The computations are based on the measured initial texture and material parameters fitted from experimental stress-strain curves. When simulating multiaxial tests using the developed hardening law, models that allow both stress and strain variations in grains give better predictions of the stress-strain curves. Furthermore, generally the simulated texture evolution is too rapid when compared to the experiments. By incorporating a more detailed neighbor interaction effect, the VPFFT model predicts texture evolution in better agreement with experiments. Further efforts were taken to study the dependence of constitutive relations on the Stage IV hardening, rate sensitivity exponent and biaxial strain ratios.