Study On The Establishment Of WLD And Its Influence Mechanism Of Wedge-shaped Specimen Drawing Under Complicated Boundary Conditions

With the rapid development of aerospace,aviation,defense,automotive and other manufacturing technology fields,the requirements for lightweight products,toughening and forming precision have become higher and higher.As one of the main defects in plastic forming of thin-walled components,wrinkling and instability has always been a key factor in forming regulation.In this paper,the establishment of Wrinkling Limit Diagram(WLD)of Yoshida Buckling Test(YBT)and its identification for wrinkling instability region are studied by combining numerical simulation and experimental methods.Because YBT cannot reflect the wrinkle instability regularity under complicated boundary conditions in the real forming process,this paper designed a wedge-shaped specimen with complicated boundary conditions of wrinkle tensile test.Taking it as the research object,combined with numerical simulation and experimental methods,the influence factors of shell WLD under complicated boundary conditions were systematically studied.The main research contents of this paper are as follows:(1)Using YBT under simple boundary conditions as a verification test,a numerical simulation model for the analysis of plastic sheet wrinkling instability is established.In this model,the buckling modes obtained from the eigenvalue buckling analysis of the specimens are added as the initial shape defects to the numerical simulation of the dynamic explicit of the plastic deformation of the specimens,so as to induce the real buckling morphology of the specimens.On this basis,the morphology and characteristics of WLD of YBT under non-uniform tensile load were investigated.Finally,the effectiveness of the WLD established in this chapter is verified by the non-contact full-field strain measurement system DIC,and the identification effect of WLD on the plastic wrinkling area of thin plate is discussed.(2)Aiming at the problem that the YBT cannot reflect the law of wrinkling instability of shells under complicated boundary conditions,a wedge-shaped specimen tensile wrinkle test was designed.Two kinds of forming conditions with and without planar normal constraints for wedge-shaped specimen were tested for tensile wrinkle,and the test results of these two conditions were analyzed and calculated by combining with DIC system and its post-processing software VIC-3D.And the online strain data of the wedge-shaped specimen tensile wrinkle test was obtained.Through the analysis of the test data,it is found that the wrinkle instability regularity under the complicated boundary conditions of the wedge-shaped specimen basically conforms to: the wrinkle instability starte from the middle of the top of the wedge-shaped specimen.As the deformation continued,the wrinkles gradually developed to both sides of the specimen.(3)Based on the finite element numerical simulation software ABAQUS,the numerical simulation method verified by YBT was used to simulate the tensile wrinkling instability of the wedge-shaped specimen under the two boundary conditions.The finite element simulation results are compared with the corresponding test results,and the stretching and wrinkling rules of wedge-shaped specimen with normal constraints are summarized.The results show that the simulation results on the shape,number and height of the wrinkling are in good agreement with the experiment.It is proved that the numerical simulation method based on the combination of eigenvalue buckling analysis and dynamic explicit algorithm is also accurate for simulating tensile wrinkling of wedge-shaped specimen under complicated boundary conditions.(4)Based on the numerical simulation results of the above wedge-shaped specimen,the wrinkle instability mechanism of the wedge-shaped specimen and the influencing factors of WLD were discussed: the wrinkle area of the wedge-shaped specimen under complicated boundary conditions is divided into regions.The main stress loading paths,stress-strain states,main stress ratios,and strain ratios in different regions were extracted and compared,and their effects on the shape and location of WLD were discussed.The conclusions are as follows: The stress state and stress path are different in different regions.The wrinkling occurs earlier in the region with the increase of main compressive stress.The direction of principal stress determines the direction of wrinkles.The regions with different stress characteristics have different WLD: in the same coordinate system,the higher the shear stress is,the higher the WLD slope is.It shows that the wrinkle resistance of the deformation element with shear stress is weaker than that of the deformation element with normal stress.In summary,the influence of stress characteristics of deformation element on WLD can be concluded as the influence of critical principal stress ratio on critical principal strain ratio.When the principal stress ratio of critical wrinkle element is larger,the critical wrinkle principal strain ratio is larger,that is,the higher the WLD slope is,the weaker the wrinkle resistance of the process is.Therefore,for the wrinkling region where the stress paths of each element approximately coincide,WLD can be highly linearly fitted.At this time,WLD is a straight line in the strain space.When the stress paths of each element in the wrinkling region are greatly different,the linear fitting accuracy of the critical principal strain ratio of each wrinkling element will decrease.For such a wrinkling region,WLD is no longer a straight line,but a region with upper and lower limits.