Theoretical prediction for maximum residual cross-sectional deformation of thin-walled cylindrical steel tubes under pure plastic bending

Cross-sectional ovalization（ovalisation）usually occurs when thin-walled tube is subjected to large plastic bending. This paper is concerned with residual deformation of tube's cross-section in radial direction when external bending moment is removed. A combination of experiments and theoretical modeling is used to study this issue. Firstly the four-point pure bending experiments involving fifteen stainless steel specimens with a range of diameter-to-thickness ratios are carried out to observe the shape of the ovalized cross-section and to obtain the cross-sectional flattening (value of ovalization) along the circumferential direction. Secondly the theoretical modeling procedure is performed to derive a rational model for predicting the maximal residual cross-sectional flattening of the thin-walled tube subjected to pure bending after unloading process, employing the thin-shell kinematics, the principle of virtual work and the classic unloading rule. Finally the model is validated by comparing the theoretical results with the experimental ones with less than 10% error. And the relationships between the residual flattening and the bending radius as well as the wall thickness are also revealed by the numerical results. The application of this investigation will play a positive role in thin-walled steel tube bending operation.