Measurement and calculation method for circumferential plastic strain ratio of anisotropic aluminum alloy tubes

To improve the finite element analysis (FEA) accuracy of forming hollow tubular components, it is urgent to determine the circumferential mechanical properties of thin-walled tube blanks, especially the plastic strain ratio r_θ, and further investigate their anisotropic deformation and hardening behaviors. In this paper, a new segment-type ring expansion test (SRET) method was established for directly measuring r_θ based on digital image correlation (DIC). It was shown by theoretical analysis that an approximately uniaxial and uniform stress state can be generated when the number of segments is 12 and the initial width-to-diameter ratio of the specimen is about 0.10. It was experimentally proved that the relative error of the measured r_θ of 304 stainless steel welded tube was less than 1 % compared with the r-value of the original 304 sheet. Then, the r_θ of aluminum alloy (6061) tubes was obtained by the SRET method, and the biaxial tensile deformation of the tubes was realized by a controllable biaxial tension test. It is shown that the axial and circumferential plastic strain ratios were 0.460 and 0.638, respectively. The strain path of equal-biaxial stress deviated from the equal-biaxial strain path, and the strain paths of σ_z / σ_θ = 0.75 and 1.333 (reciprocal) were asymmetrically distributed along the equal-biaxial strain line. These results indicate the tubes’ apparent anisotropic deformation behaviors. Finally, the effect of r_θ and yield criterion on predicting the anisotropic hardening behavior was analyzed using the effective stress–strain curve. The results illustrate that r_θ must be considered, and the Balart89 yield criterion with higher order has higher accuracy compared with the Hill48 yield criterion. This research is significant for improving and evaluating the prediction accuracy of plastic constitutive models.