On the effect of plastic anisotropy, strength and work hardening on the tensile ductility of aluminium alloys

Abstract

The influence of plastic anisotropy, yield strength and work hardening on ductile failure is studied by nonlinear finite element simulations and strain localization analyses of tensile tests in different material orientations. Three aluminium alloys with different grain structures and crystallographic textures, heat-treated to three conditions giving rise to different yield strength and work-hardening behaviours, are considered. The anisotropic yield surfaces of the alloys, obtained by the crystal plasticity finite element method, are used in the numerical simulations of ductile failure in the tensile tests. In addition, a yield surface for an isotropic material is included for comparison. These yield surfaces are combined with three stress-strain curves representative of the different heat-treatments, resulting in a range of relevant model materials with different plastic anisotropy, yield strength and work hardening used in the numerical investigations. Finite element simulations of tensile tests in seven in-plane directions are carried out, i.e., 0°, 15°, 30°, 45°, 60°, 75° and 90° to the reference direction, and the non-proportional loading histories are used in the subsequent strain localization analyses. Plastic anisotropy is found to have a marked influence on the tensile ductility and to induce failure anisotropy, i.e., a variation in the failure strain with loading direction. The shape and extension of the regions of concentrated plastic flow in the finite element simulations vary with tensile direction for the anisotropic materials. In agreement with previous experimental evidence, the strain localization analyses predict a variation of the failure strain with tensile direction that appears to correlate with the variation of the Lankford coefficient, indicating that the failure anisotropy is closely linked to the plastic anisotropy. The strain localization analyses predict a higher ductility for materials with lower yield strength and higher work hardening, as these features lead to a more distributed plastic deformation and a stress state with a lower stress triaxiality in the neck. This redistribution of the plastic deformation makes the tensile specimen less prone to strain localization and subsequent ductile failure. The influence of yield strength and work hardening is further found to depend on the plastic anisotropy.