Research On Mechanical Behaviours And Key Process Parameters For Continuous Straightening Thin-Walled Tubes

In recent years, the high-precision metallic thin-walled tubes have wide application in metallurgical and manufacturing industry, aerospace structures, military and energy industry etc, in order to be high straightness accuracy and ovality accuracy, the thin-walled tubes must be straightened before leaving factory and using. However, little theoretical research has been done on the thin-walled tubes continuous straightening, and there is no special computing models of the key process parameters for use, meanwhile the traditional computing models for the general tubes are not suitable for thin-walled tubes continuous straightening because of the poor precision, the cross-section distortion, even bending the tube without straightening when they are used in production, so in actual production, thin-walled tubes straightening is always viewed as a craft, the work being largely carried out based on the classic experiential data and chart by skilled labourers, whose art is based on long experience and experiments, result in low precision of products, poor stability and efficacy during the straightening operation. Therefore in this research the mechanical behaviour is investigated for continuous straightening thin-walled tubes, a series of theoretical models have been proposed to describe the key process parameters, which are verified to be correct by the FEA and the experiments in the lab also in the factory. The main research contents and results are listed as follows:(1) The mechanical behaviour of straightening thin-walled tubes with single initial bending curvature-radius is investigated based on the elastic-plastic theory and relevant hypothesis of the thin-walled member, and then a new stress-strain model of the deformation zone is proposed to resolve the offset of the strain neutral surfaces, and the straightening moment, subsequently, the model of the straightening curvature-radius is firstly derived by using the classic unloading rule. In order to certify whether it is correct, we have done some dynamic simulations by FEA and the local experiments, the results and the comparisons with the classic theoretical method have shown that the model is correct and suitable for straightening thin-walled tubes. This work can also provide the basis for subsequent investigations.(2) Because of the plastic compressive and tensile instability occurring in thin-walled tubes straightening process, such as wrinkling on the compression side, the ovalization of the cross-section, and the thickness attenuation or the crack of the outer-lateral tube-wall, therefore, applying the general strain-displacement relations of the revolution shell and the J2 deformation theory, the plastic buckling critical bending-radius model of the thin-walled tube with initial bending curvature-radius under pure bending is derived by the Ritz method. Meanwhile, the limit bending-radius model for the prediction of diffuse plastic instability under pure bending is also derived by Swift’s criterion. The dynamic simulations and the experiments have done by simplified models, the results have shown that the models are approximate correct, the influence factors and the variation tendencies of the critical bending-radius and the limit bending-radius are analyzed by the comparisons of the results qualitatively, and the modal of the plastic instability for thin-walled tubes can be confirmed under different situations, and it is also confirmed that whether the thin-walled tube with single initial bending curvature-radius can be straightened and which is the final instable radius.(3) Aiming at thin-walled tubes with random initial bending curvature-radius, a technological plan is proposed to straighten thin-walled tubes by two cross-rolls straightener and multi cross-rolls straightener with equal curvature, which can confirm that how to chose the style of the roll space distribution, how to chose the numbers of the rolls, and how to optimize the bending curvature-radius for every roll, and it is executed by a series of programs which are proved to be effective by calculating the examples, and that is also used to decide whether the thin-walled tube with random initial bending curvature-radius can be straightened.(4) Because there is no special intermesh mathematical model for continuous straightening thin-walled tubes by multi cross-rolls straightener, therefore, firstly a new simplified cantilever unit model for calculating the intermesh is derived based on the structural features of the thin-walled tube and the multi-roll straightener, then the straightening force can be derived based on the principle of equivalence in the torque, the key parameter of the force can be got by the FEA, and then the bending-moment of the cross section in plane stress is subsequently obtained in the elastic zone and also in the elastic-plastic zone of the tube between the rolls, finally the mechanical model of the straightening intermesh is proposed using the principle of virtual work, and it is also shown how to solve by numerical method and how to develop calculating program synchronously. In order to certify whether the model is correct, we have done some dynamic simulations by FEA and also local experiments, the results have shown that the percent of pass is taken to 90%, up from 50%, which is indicated that the model is correct and suitable. So a series of technical parameter curves can be plotted by the datum calculated with the program, which can indicate the relationships between the intermesh, the bending-radius of the tube axis and the diameter, the wall thickness, the yield stress of the tube, that can provide the theoretical basis for adjusting the straightening intermeshes for the local production.(5) Bending of thin-walled tubes when continuous straightening induces ovalization to the tube cross-section, which is not only a difficult mechanical problem for theoretical calculation, but also an important influence factor to decide the ovality accuracy of the tubes and the quality of products, and it must be flattened for setting round subsequently. Therefore, the mechanical behaviour of the ovalization of the initially circular cross-section during bending is investigated, then the mathematical models of maximal and residual cross-sectional deformation are proposed based on the elastic-plastic theory and the principle of minimum potential energy. Subsequently the deformation process and the mechanism of flattening for setting round are analyzed, and then the mathematical model equation of the flattening intermesh for setting round is derived, the dynamic simulations and the experiments have shown that all the models are approximate correct. The results of the models indicate the relationships between the cross-sectional deformation, the flattening intermesh, and technological parameters, material properties, geometric parameters of a tube, which can provide the theoretical basis for adjusting the flattening intermeshes for the local production.This research focuses on the theoretical study and the mathematical modeling, proposes a series of reasonable, viable and general theoretical models to describe the mechanical behaviour and the key process parameters for continuous straightening thin-walled tubes, which can complete the theory of thin-walled tubes continuous straightening, also provide the basis for further investigating straightening mechanism and process of thin-walled tubes, and for actual production.