Research On The Influence Of Cross-Section Compression On Stress Homogenization And Shape Accuracy Of Bent Tube

Ultra high strength steel(UHSS)tubular components with bending axis and complex cross-section have been widely used in vehicle body structure because they can meet the urgent demand of lightweight in automobile manufacturing.However,the UHSS has large springback value,and the axis springback and shape springback exist in the tubular components simultaneously.The composite influence of material and structure increases the forming difficulty of such components,and hinders the application of UHSS in the vehicle body.The traditional control method has some disadvantages such as low efficiency(springback compensation method)and crosssection distortion(stretch bending method).In order to overcome those problems,based on the theoretical innovation that the tube/shell can undergo stable compression deformation under the normal force support,this paper proposes a method to solve the axis/cross-section springback defect of this kind of components through the crosssection compression method to achieve stress homogenization.Through theory analysis,finite element simulation and experiment verification,the plastic deformation behavior of the material during the cross-section compression was systematically studied,the mechanical principle of springback control was revealed,and the change rule of axial/circumferential stress was obtained.Finally,the instability model and residual stress distribution characteristics after compression were given,which provided theoretical support for the application of the process.Based on the elastic-plastic theory,a mathematical model of axial/circumferential stress varying with the compression displacement was established,and the mechanical principle of the cross-section compression method to control the axis/cross-section springback was revealed.The key point is the homogenization of axial/circumferential stress.For the shape springback,the circumferential compression(parallel to the bending direction of the cross-section)is applied,which the compressive stress is superimposed and the tensile stress is canceled in the bending deformation area,eliminating the problem of uneven bending stress in the inner/outer layers and reducing the bending moment.For the axis springback,circumferential compression deformation(perpendicular to the axis bending direction)is applied to make each element in the bending deformation area to enter the subsequent yield.Based on the plane strain theory,the tensile stress is transformed into the compressive stress and the compressive stress value is reduced by half in the bending deformation area.Finally,the axial stress difference between the compressive stress on the inner layer and the tensile stress on the outer layer is changed into the uniform compressive stress,which eliminates the mechanical conditions of the axis springback.The influence of pre-bending process on critical internal pressure was studied.Considering the different cold work-hardening and thickness distribution of the inner/outer layers caused by pre-bending,the change rule between the critical internal pressure and the circumferential strain was reconstructed based on the energy method.It is found that the degree of cold work-hardening has a significant effect on the change rule.Then,an analytical model of stress-strain was established,and the circumferential strain distribution of the pre-bending under cross-section compression was obtained.It is pointed out that the thickness difference caused by pre-bending plays a leading role in the circumferential strain distribution.Finally,the critical internal pressure prediction model under pre-bending condition is established.The critical internal pressure is determined by the outer straight,and the critical internal pressure required is larger than that of the straight tube.The reduction of bending radius and friction coefficient will make the effect of pre-bending on critical internal pressure more significant.The experimental results show that the prediction accuracy of critical internal pressure can be improved by more than 74%.The plastic deformation behavior and axial stress change mechanism of the bent tube in the cross-section compression process were studied,and the change rule between the axial springback and the compression displacement was given.Prebending makes the thickness and cold work-hardening degree of the inner/outer layer in the bending deformation zone different,resulting in the subsequent yield and axial stress changes of the inner/outer layer during in the cross-section compression process.The critical compression displacement required to control the axis springback is given.With the decrease of bending radius,the difference of workhardening degree and thickness between the inner layer and outer layer increases,resulting in the growth of critical compression displacement.Because the crosssection compression method belongs to the stress-state change method,its springback control effect is not affected by the material properties.The experimental results showed that the axis springback of DP600 and DP800 bent tube can be reduced by more than 95% by cross-section compression.The plastic deformation behavior of the closed cross-section and the change mechanism of the circumferential stress during the compression process were studied,and the relationship between the springback and compression displacement was obtained.During cross-section compression,the inner layer of the bending deformation zone maintains the compressive stress,and the outer layer undergoes the unloading reverse loading from the tensile stress to the compressive stress.The change rule of the bending moment with the compression displacement was given.Based on this,the mechanical judgment condition of stress homogenization was established,and the critical compression displacement required to control the shape springback was given.It was pointed out that the basic reason for the concave deformation of the straight wall is that the curvature of the corner is not changed but the radian is increased after the stress is homogenized.After the section is compressed,the residual tensile stress(280MPa)on the outer surface was firstly eliminated,and under the structural constraint of the closed section,the residual compressive stress(-40MPa)was finally formed on the outer surface.The influence of section shape(fillet radius r,wall thickness t and length to height ratio a(L/h))on residual stress distribution is mainly studied,and the influence of wall thickness t is the largest.Better surface morphology can be obtained by cross-section compression than by shot peening.