Yield Characteristic And Constitutive Model Of Anisotropic Aluminum Alloy Thin-walled Tubes In Overall Stress States

Tube hydroforming has become the mainstream technology for manufacturing lightweight structural parts of transportation equipment in aviation,aerospace,and automobile industries.The forming process of special-shaped integrated tubes involves complex stress states,such as the tension-tension(T-T)stress state,the tension-compression(T-C)stress state,and a special stress state in which the principal stress axes do not coincide with the material axes due to the presence of shear stress.Additionally,the tube blank is clearly anisotropic,making the forming process extremely complex.To date,studies of plastic deformation and constitutive models have been limited to the T-T stress state,resulting in intrinsic errors in numerical simulations of deformation.In this thesis,a new controllable biaxial stress loading test and a pure tube shear test were developed to systematically study the plastic deformation behavior of anisotropic tubes in complex stress states and establish an accurate constitutive model.The obtained results can provide fundamental guidance and technical support for the forming of complex thin-walled tubes.To study the plastic deformation characteristics of thin-walled tubes in complex stress states,a testing method was proposed for carrying out controllable loading of thin-walled tubes in overall stress states,including the T-T and T-C stress states.Then,the device for the implementation of the new testing method was developed.The newly developed method solved the problem of the inability of loading the thin-walled tube specimen in the T-C stress state in the previous methods,enabling an accurate application of arbitrary stress paths to thin-walled tubes in the T-T and T-C stress states.Additionally,the problem of insufficient accuracy in strain measurement and axial curvature radius measurement of the existing biaxial stress loading tests was solved.Therefore,the stress and strain data of tubes can be accurately and continuously measured over the whole deformation process.Using the developed controllable biaxial stress loading device,the plastic deformation characteristics of anisotropic AA6061-O tubes in the overall T-T and T-C stress states were studied experimentally.The accuracy of the Hill48,Barlat89,and Yld2000-2d anisotropic constitutive models that are commonly used in commercial finite element software was thoroughly verified in overall stress states.It was found that the yield behavior exhibited anisotropy in overall stress states,and the anisotropy is more pronounced in the T-T stress state.The plastic flow behavior of the AA6061-O tubes exhibits isotropic characteristics in the T-C stress state,while it is anisotropic in the T-T stress state.For the yield and plastic flow behavior in overall stress states,the Barlat89 and Yld2000-2d models based on the T-T data enable accurate predictions for the T-T stress state.Additionally,accurate predictions were achieved by all three constitutive models based on the T-C data in the T-C stress state.However,due to insufficient flexibility,none of the three models achieved accurate predictions of the plastic deformation behavior under the simultaneous application of the T-T and T-C stress states.To accurately predict the plastic deformation behavior of the tubes under the simultaneous application of the T-T and T-C stress states,a new anisotropic constitutive model was proposed.The new model solves the problem that these commonly used models cannot adapt to different kinds of yield loci and flow directions in the T-T and T-C stress states simultaneously.The accuracy and efficiency of the new model were verified via biaxial loading tests of the AA6061-O tubes.It was found that the new constitutive model can accurately predict the experimental yield loci and plastic flow directions of the AA6061-O tube under the simultaneous T-T and T-C deformation.In addition,the new constitutive model was implemented in ABAQUS using the VUMAT subroutine to simulate the tube hydroforming for a further verification of the accuracy of the new model in engineering practice.It was demonstrated that the new constitutive model can significantly improve the prediction accuracy of the axial profile and wall thickness distribution in tube hydroforming.To calibrate a constitutive model of anisotropic thin-walled tubes considering shear stress,a pure shear test method was proposed based on the V-notch tubular specimens.The feasibility of the designed specimen was verified by numerical simulation.The shear stress-strain relation of the AA6061-O tubes was obtained by the tube shear test,and the ratio of the shear stress to the axial yield stress was determined.Then,the results are used to calibrate the constitutive model of anisotropic thin-walled tube considering shear stress.By simulating the tensile behavior of notched tube specimens,it was verified that the calibrated model can accurately describe the plastic deformation behavior of tubes with shear stress.Based on the constitutive model of anisotropic thin-walled tube considering shear stress,a theory for determining the in-plane anisotropic parameters of tubes in arbitrary directions was developed that solves the problem of the inability of the existing testing methods to measure these parameters.The in-plane uniaxial yield stress and r-value of AA6061-O tubes were determined.In the range of 0～90°,the yield stress first increases and then decreases,reaching its maximum value at 50°.Meanwhile,the r-value shows the same evolution trend as the yield stress,reaching the maximum value of 0.680 at approximately 50°,which is 46.9% and 16.0%higher than the axial and circumferential values,respectively.