Numerical Simulation Of Wrinkling Onset During The NC Bending Process Of Thin-walled Tube

Precision forming process of thin-walled tube NC bending is showing an inexorable tendency to improve and develop tube-bending process into advanced plastic forming technology. However, it is a complex process with coupling; interactive multi-factor effects and the possibility of wrinkling initiation. Particularly, the development of aviation and aerospace requires bent parts much thinner in wall-thickness, larger in tube radius, smaller in bending radius and more precise in the forming process. The prediction and control of wrinkling have thus become a difficult and key problem urgent to be resolved in the research and development of the process. Consequently, a numerical method for predicting the wrinkling onset during the tube bending process, which combines the rigid-plastic FEM with energy criteria, is proposed in the paper. The method can predict the wrinkling phenomena quantitatively, thus achieving optimal processing parameters with less needed experiments. The main achievements of the project are as following:According to the practical NC tube bending process, a reasonable FEM model has been set up and the key techniques of 3D rigid-plastic FEM have been studied in the paper. A shell element with relative and absolute degree of freedom is put forward so that the velocity boundary conditions can be considered by setting "1" or large number in the finite element equations; cubic factor method is applied to calculate the deceleration factor in the FEM iteration which can increase the computation efficiency while ensuring the convergence.Mathematic description of the winkling wave initiated on the compressed wall of tube during the bending process is established. Prediction criteria for the wrinkling onset is derived based on the theory of thin-walled shell bending and minimumenergy principal; then how to combine the criteria with the FEM simulation system to make the numerical prediction of wrinkling into reality is also resolved in the paper.A numerical wrinkling prediction system TBWS-3D is developed for the NC thin-walled bending process. It includes modules for the description of die cavity, auto-meshing of tube being, treatments for dynamic boundary condition and friction condition, rigid-plastic FEM analysis, visualization of deformed tube and field variables and prediction of the wrinkling initial. The system can be used to both predict the wrinkling phenomena and analyze the bending process.The analysis of the bending process of aluminum and stainless tube has been carried out by the numerical system and the simulation results include: (1) the deformation, distribution of equivalent strain and plastic zone and variation of the ration of plastic forming energy and wrinkling energy of aluminum tube are same as those of stainless tube; the variation of stress distribution along the bending direction of these two tubes with bending angle is identical even though the stress of stainless tube is larger than that of aluminum tube. (2) the internal shift of the stress and strain neutrolayer is not noticeable for the bending process is a rotate bending process with booster. (3) the stress along the bending direction increases at the initial forming stage; the forming process comes into a stable process after the maximum compression stress along the bending direction exceeds some value and the stress just varies in a small scale. (4) the plastic forming area extends at the initial forming stage, while the area remains the same at the stable forming stage and the bended part of the tube being unloads continuously. (5) the ratio of plastic forming energy and wrinkling energy of the compressed part of the tube fluctuates and has a maximum value during the forming process.The influence of processing parameters on the forming limit determined by wrinkling is also researched with the above numerical prediction system. The results are as the following: (1) the bending radius has no effect on the wrinkling if the extension length of mandrel is enough to cover the plastic forming zone, but the te...