Accurate measurement method for anisotropic parameters of metal tubes in arbitrary directions

The anisotropic parameters of metal tubes are of great significance for calibrating the constitutive model and achieving the high-accuracy simulation of forming thin-walled tubular parts. In this paper, a new method of inclined ring tensile test (IRTT) based on digital image correlation (DIC) technology was established to determine the normal anisotropy coefficients (r values) and stress–strain curves of thin-walled metal tubes in arbitrary directions. The method utilizes blocks with asymmetric geometry to support the parallel segment, thereby avoiding straightening deformation. Meanwhile, a precise calculation formula for the tensile stress in the center section was derived, taking friction into account. Furthermore, finite element analysis (FEA) was employed to investigate the uniformity of tensile stress and the effects of frame moment, straightening deformation (block gap position), and interface friction. It is indicated that the center section is subjected to a uniaxial, uniform stress state. Subsequently, the r values and stress–strain curves of aluminum alloy (AA6061) tubes were obtained in their 15°, 30°, 45°, 60°, 75°, and 90° directions using the IRTT. The r values increase first and then decrease from 0° to 90°, and are all less than 1, reflecting the normal and in-plane anisotropy of flow. The hardening curve in the 0° direction is the lowest, while that in the 90° direction is the highest, and the curves in the other directions lie between them. Ultimately, the effectiveness of these measured parameters was demonstrated for enhancing the prediction accuracy of the constitutive model and FEA. The constitutive model calibrated by r₀, r₉₀, and r₄₅ has higher prediction accuracy for the flow behavior under the general plane stress than the model calibrated by r₀ and r₉₀ (reducing error from 14.7% to 5.3%). The prediction error of the load–displacement curves is reduced from 4.4% to 1.7% when the measured stress–strain curves in the 45°, 75°, and 90° directions are used as input to FEA.