Study On Plastic Behavior Of Thin-Walled Metal Tubes Based On Multi-scale Plastic Yield Criterion

Due to the characteristics of hollow structure,lightweight,low energy consumption and impact absorbability,thin-walled metal pipes and tubular components have been greatly applied in the aerospace and shipping fields with the rapid industrial development.Higher precision in real manufacturing are required with the untilizaton rate increasing.Normally before plastic processing,pipes are produced after processing like drawing,extrusion,rolling and other methods,accompanied by the appearance of fiber organization and preferred crystallization which will lead to significantly anisotropic performance and greatly influence the the plastic deformation of pipes.In this paper,the basic tensile test and the rotary bending test of various pipes are firsty performed,based on which the corresponding forming theory is studied and finite element simulation results are analyzed under plastic isotropic assumption.Furtherly,the plasitic anisotropy through the wall thickness of closed-state pipes were directly characterized by contractile strain ratio(CSR)which was verified feasible and used to calibrate the plastic anisotric Hill48 yield criterion,to study the influence of the plastic model on the pipe deforming behavior.Finally,the anisotropic parameters of pipes and anistropic yield locus are calibrated by the crystal plasticity finite elment methods based on texture measurements.Combined with various hardening models,advanced multi-scale anisotropic yield criterions are established.Finite elment simulations of ultra low cycle fatigue test with repect to the multi-scale modeling are performed to study the capacity of the models.Influence of this yield criterion on plastic forming of thin-walled tube.The main research results are as follows:(1)The isotropic yield criterions of thin-walled pipes were calibrated by the basic mechanical uniaxial tensile test,and the rotary draw bending test of corresponding pipes and finite element simulation based on the isotropic hypothesis were carried out.According to the working way of the numerically controlled tube bender and taking the additional tensile force into account under plane strain assumption,the fomulas were derived to calculate the average principal stress in different directions,the equivalent stress,the variation of wall thickness,and the ratio of the minor axis to the original radius of outer contour.The corresponding experiments and simulation analysis were performed to compare with the calculated results,and it was revealed that the equivalent stress of the bending tube surface was nonuniform and the maximum equivalent stress localized in smaller deformation field around the bending terminating end.The consideration of the additional tensile force makes the equivalent stress in outer convex portion of tube remarkably greater than that in the inner concave portion,accelerating the thinning of tube wall and the ovalization of tube's outer contour.The approximate calculation formulae of additional tensile force was also derived and based on the deformation analysis;the proper adjustment of the additional tensile force could improve the quality of the NC bending tube.(2)The thick anisotropy along wall thickness of closed-state pipes was characterized by contractile contractile strain ratio(CSR)associated with the change of the inner and outer diameters before and after the deformation,and the feasibility of this characterization method was validated by relevant tests and theoretical analysis.Meanwhile,the plastic anisotropic yield criterion Hill48 was calibarated by the CSR and finite element simulations of uniaxial tensile test and rotary draw bening with respect to this yield criterion were respectively performed and compared with the isotropic simulation and the corresponding experimental results.The anisotropic yield criterion of calibrated by the CSR was revealed to better predict the plastic deformation of the thin-walled pipes.(3)Anisotropic yield locus using advanced multi-scale simulation based on texture measurements along wall thickness of the thin-walled steel pipe X65 were calibrated.The anisotropic yield locuses were used in combination with various hardening models to calibrate the of different multi-scale anisotropic yield criterions.The lab-scale ultra low cycle fatigue test on X65 steel pipe was simulated with the above multi-scaled finite element models,and the simulation results were compared with the isotropic assumed simulation and the corresponding experimental results to validate various material modeling assumptions.It was found that the advanced multi-scale plastic anisotropic yield criterion more accurately predicted the number of the cycles until failure,as well as the strain evolution during the plastic deformation.