An anisotropic constitutive model for forming of aluminum tubes under both biaxial tension and pure shear stress states

In forming of complex thin-walled tubular parts, such as tube hydroforming with axial feeding, both biaxial tension and one axial tension & one axial compression (around the pure shear stress state) deformations are involved. The deformations under both biaxial tension and pure shear stress conditions cannot be accurately modeled and described with the symmetrical constitutive models based on the tensile properties. It is due to the fact that the anisotropic yielding and plastic flow behaviors of tubes are not well articulated over the entire range of stress states. To address this issue, an enhanced constitutive model was proposed based on the Yld2000–2d model (Barlat et al., 2003). The enhanced model improves the applicability of the Yld2000–2d by introducing an additional term, which can flexibly describe the change of the curvature of yield locus around the pure shear stress state. The yielding and plastic flow characteristics of the tubes under both biaxial tension and pure shear stress conditions can be well considered by the enhanced model and the model convexity can be strictly guaranteed. Moreover, the predictability of the proposed model for the deformation behaviors under these states was analyzed and compared with the quoted anisotropic constitutive models. The accuracy and efficiency were also verified and corroborated with the forming of AA6061-O tubes. It showed that the enhanced constitutive model can accurately predict the experimental yield loci and the plastic flow directions of the AA6061-O tube under both biaxial tension and pure shear stress deformations. The in-depth comparisons with the three benchmarked models and experiments were also conducted in terms of the predicted profile and wall thickness distribution of tube hydroforming, in such a way to further validate the enhanced model in terms of the efficiently and accurately modeling of the forming of complex tubes.