Numerical study on plastic instability of circular bending deformation during rotational straightening of thin-walled tubing

Abstract

The rotation straightening process achieves axis alignment by repeatedly bending the pipe material. For thick-walled aluminum alloy pipes with a large diameter-to-thickness ratio, under cyclic bending loads, they are prone to axial wrinkling-type plastic buckling instability on the compressed side, which severely degrades the straightening accuracy and the service performance of the pipe. Taking the thin-walled aluminum alloy pipe of aviation grade 6061-T6 as the research object, a nonlinear buckling finite element model of the pipe material considering the initial axial wrinkling geometric defect was established based on Abaqus. The control variable method was used to explore the influence laws of the diameter-thickness ratio regulation method, the amplitude of the initial wrinkling defect, and the form of defect introduction on the critical curvature, ultimate moment, and evolution of buckling mode of cyclic bending plastic instability. This paper expands multiple sets of absolute pipe diameter and wall thickness parameters, covering the commonly used diameter-thickness ratio range in engineering, and quantifies the response patterns of cyclic bending loads under different working conditions. The results show: 1) Both the constant diameter thickening wall and the constant wall thickness increasing diameter ratio regulation methods will cause the decay rate of the instability critical curvature to slow down with the number of cycles, and the larger the diameter-thickness ratio, the more gradual the attenuation of the cross-sectional bending stiffness; under the same diameter-thickness ratio gradient, changing the wall thickness has a greater disturbance effect on the instability characteristics than changing the outer diameter; 2) The amplitude of the initial wrinkling defect is negatively correlated with the ultimate load capacity of the pipe. When the defect factor exceeds 0.01t, the ultimate moment and the critical instability curvature significantly decrease, and the simulation distortion degree increases; it is recommended that the 0.01t mid-span gradual wrinkling defect model be adopted for engineering simulation; 3) Direct axial modeling of the defect can reproduce the full circumferential wrinkling branching buckling of the pipe material. Compared with the eigenvalue mode superposition defect, it is more in line with the original defect characteristics of the pipe material during rolling and manufacturing; under cyclic loading, the wrinkling defect will continuously amplify the plastic cumulative effect and accelerate the pipe material’s buckling failure. This study reveals the instability mechanism of thin-walled pipe material straightening, providing a numerical basis for the optimization of process parameters in the rotation straightening of aluminum alloy pipes and the control of pipe forming defects.