Prediction of Residual Cross-sectional Ovalization of Thin-walled Circular Steel Tubes under Pure Bending

Cross-sectional ovalization usually occurs when thin-walled steel tube is subjected to large plastic bending. It is hard to predict the residual deformation of the cross-section when the external bending moment is removed. So four-point pure bending experiments involving eight stainless steel specimens with different geometric parameters under different bending radii are carried out to periodically monitor the cross-sectional flattening along the circular direction. From the experimental results it is observed that the appearance of the maximum residual flattening is usually found in the direction normal to the neutral surface. Then using the digital image processing methods the points on the outer edge in the image of the ovalised cross-section are fitted by the elliptic equation which is validated to be best in fitting prices. Finally a rational model for predicting the maximal residual section flattening of the thin-walled circular steel tube after pure bending is presented using the classic unloading rule. The new model is validated by experimental datum. The results show that the model agrees well with the experiments with less than 10% error. And based on the analytical results, the relationships between the residual flattening and the bending radius as well as the wall thickness are also revealed. This new model is expected to find its potential application in thin-walled steel tube bending operation. © 2018 Journal of Mechanical Engineering.