Springback Characteristic Of Reciprocating Bending For Stable Metal Materials And Its Engineering Application

Reciprocating bending of stable metal materials is a kind of plastic deformation in which the material is subjected to mutiple alternating positive bending springback and reverse bending springback.In recent years,scholars at home and abroad have done a lot of research and made great progress about the prediction and control of once bending springback,but there are few reports on the study of the reciprocating bending springback.In the application of the reshape engineering,the roller type straightening process and setting round process can achieve the effect of uniform curvature by reciprocating bending.The former is to unify the different initial curvatures of each section in the length direction of the profile or plate to zero;the latter is to unify the different initial curvatures of each section in the circumferential direction of the pipe to the nominal curvature.These two engineering practices have proved that the difference of initial curvature can be eliminated by reciprocating bending,and the curvature of each section can be unified into the same direction and the same value.However,there is no existing principle or theorem to describe it.Firstly,the plane curved beam is taken as the research object,the deformation process of uniform curvature of reciprocating bending is studied by graphic method,the unified curvature theorem of reciprocating bending is presented.A linear simple kinematic hardening constitutive model and a mathematical induction method are adopted,the curvature equation and uniform curvature equation after reciprocating bending springback are established,and the correctness of the unified curvature theorem of reciprocating bending is proved theoretically.The greater the ratio of the plastic modulus to the elastic modulus,the slower the speed of uniform curvature,the more the required bending times.The required bending times of the experiment is more than the theoretical solution.The "pendulum type" experiment device for reciprocating bending is designed to verify the correctness of the theorem.These work not only expands the springback theory of small curvature plane bending,reveals the mechanism of the roller type straightening process,and provides a theoretical basis for the parameter optimization and equipment design of roller-type straightening process and setting round process.The cyclic tension-compression test of low cycle is carried out by the symmetric constant strain amplitude control,the macroscopic performance and the effect of pre-strain on the stable hysteresis loop on the stable metal materials under cyclic tension-compression loading are studied,the variation laws of elastic modulus,yield stress and plastic modulus with cycle times are investigated,the suitable fitting functions are given for mathematical description,and then a linear variable parameter constitutive model is established,which is based on the three factors of deformation strengthening,Bauchinger effect and cyclic softening.It improves the analytical accuracy of reciprocating bending springback.In the research of the symmetrical three-roller setting round process,the static equilibrium conditions,the physical relationships of elastic-plastic deformation,the deformation compatibility conditions,and the constraint conditions are analyzed comprehensively,the mechanical model of three-roller setting round process is established by applying the discrete method and the load increment method of variable step size.The mechanism of three-roller setting round process is expounded.When the elastic area ratio reaches 20%,the residual ovality can be controlled at about 0.35%,the upper roller load is greatly improved with the decrease of the residual ovality,the prediction of the reduction is achieved,which provides a theoretical guidance for the engineering application of the symmetric three-roller setting round.In the research of the two-roller straightening process,the change process of the curvature of the section from different values to zero is analyzed by using the graphic method and the mathematical induction method,the mechanism of two-roller straightening process is expounded.The one-way three-curvature roller gap model is take as an example,the stress,strain,straightening force and straightness are analyzed,and the variation law of elastic area ratio,residual curvature and residual deflection with bending times is quantitatively analyzed.The results show that the maximum residual deflection of the bar is 0.92 mm/m,and the bending times of roller shape design are much more than the theoretical analysis.Finally,three kinds of new process and their equipment are developed,including the roller type global setting round process,the roller type continuous setting round process and the roller type continuous straighting and setting round process,which can not only promote the technical innovation of the relevant machinery industry,but also improve the machining efficiency and the quality of pipe.