Modeling and simulation for cross-sectional ovalization of thin-walled tubes in continuous rotary straightening process

Abstract

Cross-sectional ovalization of thin-walled circular steel tube caused by large plastic bending usually occurs at initial large bending stage in tube's continuous rotary straightening process. When the bending loads have been removed, there must be residual ovalization kept on the tube's cross-section during the whole straightening process, to induce the poor roundness and the reduction of the tube flexural stiffness. So the maximal residual cross-sectional ovalization should be predict accurately in order to be controlled under the allowable limit during the straightening process. In this paper, this issue is studied by the approaches of simulations and theoretical modeling. Firstly the finite element (FE) simulations for thin-walled tube straightening process are carried out on stainless steel tubes with different geometric parameters under different working rolls' bending radii, to measure the residual cross-sectional ovalization along the circumferential direction. The results imply that the profile of deformed cross-section is not standard ellipse and the appearance of the maximum residual ovalization is usually found in the direction perpendicular to the tube's centroid axis. Secondly the theoretical modeling procedure is carried out as follows. First of all, the normal strain and stress components are derived using the thin-shell large deformation kinematics. Then the cross-sectional ovalization in loading process is analyzed by the principle of virtual work. And then using the general geometric description without the elliptical assumption, a rational model is derived by the classic unloading rule to predict the maximal residual cross-sectional ovalization of thin-walled tube in straightening process. It is solved iteratively using Matlab ode23s function. The theoretical results are compared with the simulation ones. The relative errors are lower than 10%, which validates the effectiveness of the new model. Meanwhile the influences of tube's material, thickness and bending radius on maximum residual ovalization are found.