Prediction Of Bending Behavior And Measurement Of Forming Limit For Stainless Steel Thin-walled Tube

Metal tubes have a hollow structure and exhibit advantages such as light weight,high specific strength,and good formability,and have become important lightweight materials in fields such as automobiles,rail transportation,aerospace,and construction.Bending forming is one of the important processes in plastic processing of tubes.The bending deformation process of tubes is relatively complex.Cracking,wrinkling,cross-sectional shape distortion and springback are the main defects in tube bending forming process.With the help of finite element tools,accurately predicting material flow and springback behavior during tube bending process and reasonably controlling strain range of materials,and provide basis for design of plastic bending process for tubes.The premise of finite element simulation is to accurately obtain constitutive model and forming limit of tubes.The preparation process of metal tubes mainly includes rolling,drawing,extrusion or welding.Similar to plates,the mechanical properties of tubes usually exhibit obvious anisotropy.In the subsequent bending forming process,the anisotropy of tubes determines the flow behavior of materials in different orientations and significantly affects the wall thickness change rate and cross-sectional shape distortion degree.Tube bending springback is a reverse loading process and belongs to strain path changing work hardening behavior.Bauschinger effect and elastic modulus attenuation need to be considered.Therefore,it is very necessary to accurately obtain anisotropic yield criterion and kinematic hardening model for tubes.Compared with the planar structure of plates,the spatial curved surface structure of tubes increases the measurement difficulty of their constitutive models.Based on this,this paper proposes measurement methods for anisotropic yield criterion and kinematic hardening model for 06Cr19Ni10 stainless steel tubes,and obtains constitutive model of tubes through parameter identification.Finite element simulation is carried out for tube bending forming process based on constitutive model,and verified by actual process test.At the same time,a measurement method for forming limit of tube under tensile side with core shaft is proposed,and experimental measurement and numerical prediction are carried out.The research work and main conclusions are as follows:(1)Establish Hill48 anisotropic yield criterion for tubes based on yield stress.Introduce plane strain state tensile yield stress to replace traditional equibiaxial tensile state yield stress.Derive theoretical formula for anisotropic parameters.Identify Hill48 yield criterion parameters through axial tensile,circumferential tensile and other tests combined with optimization algorithm.Adopt inverse strategy based on tension-compression test and finite element simulation for parameter identification of Y-U hardening model.Complete parameter calibration of variable elastic modulus model through cyclic tensile-unloading test.Accurate calibration of constitutive model parameters lays foundation for establishing reliable finite element model.(2)Finite element simulation is carried out for tube bending forming process and springback behavior,and compared with actual process test.The results show that Hill48 yield criterion can accurately describe anisotropic characteristics and plastic deformation behavior of tubes.Compared with Mises yield criterion,prediction accuracy of wall thickness change rate and cross-sectional shape distortion degree by Hill48 anisotropic yield criterion is increased by 10.7% and 5.8%,respectively.Adopting Y-U hardening model significantly improves prediction accuracy of springback angle.Compared with isotropic hardening model,prediction accuracy is increased by an average of 17.6%,proving that considering Bauschinger effect and change of elastic modulus is particularly important for predicting tube bending springback.(3)Measure forming limit curve of tube under tensile side with core shaft based on tensile test of different notch specimens.Adopt DIC test device to measure limit strain of tube from uniaxial tension to plane strain state in tension-compression zone.Simulate tensile process of different notch specimens through finite element simulation and predict forming limit of tube based on different instability judgment criteria.The results show that the limit strain predicted by maximum load judgment criterion is lower than experimental value,while the result predicted by strain rate change judgment criterion is closer to experimental result.