Nonlinear Elastic Unloading Coupled Damage Constitutive Model And Its Application In Springback Of X80 Pipeline Steel

Springback is a key problem in sheet metal forming process,which directly affects the shape and dimension precision of sheet metal forming.It is found that the stress distribution in sheet metal forming process and the elastic recovery strain in unloading process are the main factors affecting the springback calculation that is affected by the nonlinear changes of stress and strain in the process of damage and unloading of plastic deformation Damage and nonlinear elastic unloading have become a hot research topic in springback calculation at present.In this paper,the nonlinear elastic unloading behavior of two-phase structure(B+F)X80 pipeline steel was analyzed experimentally.At the same time,based on the Lemaitre continuum damage model and quasi-static nonlinear unloading constitutive QPE model,and considering the Bauschinger effect and hysteresis loop phenomenon in unloading process,the QPE-Damage coupling constitutive model is constructed and the corresponding numerical algorithm is developed.The results have been successfully applied to the springback analysis of pipe steel bending.The main contents of this paper are as follows:(1)Taking dual-phase structure(B+F)X80 high strength pipeline steel as the research subject,through uniaxial tensile test and cyclic loading-unloading experiment,The variation of elastic modulus and non-linear elastic unloading behavior during plastic deformation were studied in order to lay a foundation for the subsequent constitutive model.(2)In order to accurately describe the nonlinear elastic unloading behavior,damage behavior and Bauschinger effect,the nonlinear mixed harding constitutive model of QPE coupling damage was constructed.(3)The nonlinear mixed hardening constitutive model of QPE coupling damage wasrealized by finite element ABAQUS Standard/UMAT.Based on the fully implicit backward Euler constitutive integration algorithm,the numerical calculation formula of the constitutive model is derived.The cyclic loading process is calculated by ABAQUSStandard and Umat subroutine.The reliability of constitutive model was verified by comparing with the experimental data.(4)In order to analyze the effect of constitutive model on springback calculation,the three-point air bending springback process of two-phase structure(B+F)X80 high strength pipeline steel was taken as an example.The nonlinear mixed hardening QPE constitutive model,the nonlinear mixed strengthening Lemaitre damage constitutive modelis and the QPE-coupled damage nonlinear mixed strengthening constitutive model were respectively used to calculate the bending resilience.Compared with the experimental results of three-point air bending,the advantages of the QPE coupling damage nonlinear mixed harding constitutive model are verified.