| NC precision bending process of thin-walled tube is showing an inevitable tendency to improve and develop tube bending process into advanced plastic forming technology. However, it is a complex astable process with coupling interactive multi-factor effects and wrinkling phenomenon as a compress instability often occurring , when the process parameters are inappropriate, especially for tubes with large diameter and thin wall-thickness. Thus, the wrinkling and forming limit have become one difficult and key problem urgently to be resolved in the R&D of the process. In this dissertation, a systematical and thorough investigation on the wrinkling and the forming limit of the process has been carried out by using rigid-plastic FEM simulation combining with analytical and experiment methods. The main achievements of the project made are as following:(1) Some of the key techniques related to the 3D rigid-plastic FEM simulation and wrinkling prediction of thin-walled tube NC bending process have been studied and improved methods and algorithms have been proposed: The instability predicting model has been improved which make prediction results more precise. Shell element with absolute-relative degree of freedom and corresponding setting method of velocity boundary conditions make the simulation results more efficiently and more accurately. Frictions on four areas in the forming process and side pushing velocity have been added in to the finite element equations, which makes FEM model more correspond the reality. Based on OOP ( Object-Oriented Programming ) theory, one numerical wrinkling prediction system TBWS-3D for visual NC Tube Bending have been reconstructed successfully with VC++. The validity of system has been verified by experimentation and comparison with data in the literature.(2) By using improved wrinkling system TBWS-3D, a systematical and thorough investigation into influence of forming parameters on wrinkling onset has been carried out and influence laws are obtained, which may be helpful to the parameters design and determination of forming limit.(3) Influence law of friction factor and clearance between tube and die on wrinkling initiation is obtained. The results show: with the friction factors between tube and mandrel and the one between tube and wiper die turn bigger, the wrinkling factor may become lower to some extent. With the other friction factors between tubeand pushing die, as well as between tube and bend die increase, the wrinkling factor rises a little. Namely the friction factor between these dies have little influence on the wrinkling onset, when clearance between tube and wiper die rises, the wrinkling factor increases; the wrinkling factor increases sharply, when clearance is larger than 0.5mm.(4) The main parameters are gained: relative diameter, bending radius, hardening exponent, mandrel extension length, friction factor between tube and wiper die, friction factor between tube and mandrel, clearance between tube and wiper die. Based on a virtual orthogonal experiment, information regarding the relative importance of every processing variables for wrinkling onset has been investigated. The results are listed according to relative importance for wrinkling: relative diameter, relative bending radius, hardening exponent, clearance between tube and wiper die, friction factor between tube and wiper die, friction factor between tube and mandrel, friction factor between tube and bend die.(5) According to analysis of forming characteristics of thin-walled tube bending, concept of forming limit for the process has been put forward. A searching algorithm of forming limit by using developed FEM wrinkling system TBWS-3D as virtual experiment has been brought forward. Employing the algorithm, wrinkling limit under various process conditions can be obtained conveniently and quickly, and one primary forming limit data base has been established. Influence law of main forming parameters on wrinkling limit has been investigated thoroughly. Thus measures to improve the wrinkling limi... |
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