| Sheet metal buckling characteristic belongs to the stamping formability. Sheet metal buckling is not only a familiar objection during the stamping process ,but also is the important cause of stamping waster .Yoshida Buckling Test(YBT)which has been developed by Japanese famous scholar Yoshida is a notable test for appraising the sheet stamping formability. The thesis mainly performs the FEM simulation of Al-Mg-Si based alloy auto body sheets using Marc/Mentat software, analyses and investigates buckling performance, buckling cause and buckling mechanism on new type Al-Mg-Si based alloy auto body sheets and the macro mechanical properties which effect the buckling characteristic in order to search the new scheme for optimizing properties of Al-Mg-Si based alloy auto body sheets and at the same time to provide the method foundation of the numerical simulation for investigating and innovating the virtual stamping formability test in the future .First, basic theory for the analysis of sheet metal forming is expounded, and constitutive equation for large elastoplastic deformation is introduced in this paper.Some basic problems of FEM, such as principles and algorithm for nonlinear problems, convergence criterion, contact algorithm and friction model, are analyzed and introduced in this paper. Moreover, the numerical model of YBT is created and the definition method of boundary condition and calculation parameters are put forward.The total fields of displacement, stress and strain in biaxial diagonal tension deformation process and the variation regularities of stress and strain are analyzed and investigated at critical buckling instability, and buckling causes on new type Al-Mg-Si based alloy auto body sheets are showed using Marc/Mentat software. The results found that: (1)the transverse corner field resulting from the uneven geometrical width of YBT model leads to the non-uniform spreads and distributions of the active lengthwise tensile stress, the stretchy strain and the passive transverse pressure stress in the principal axis; (2)the center of the YBT model is the most possible critical buckling instability point; (2)the buckling performance of the YBT model not only possess both properties of elasticity and plasticity, but also the interlaced distributions possible appear on the elastic and plastic stress points; (4)buckling instability of the total model not only is correlated with the non-uniform stress and strain on eigenaxis or eigensection, but also is the result of the total fields of stress and strain which spreadand distribute non-uniformly.Based on the YBT model and using Marc/Mentat software, the relationships of mechanical properties of Al-Mg-Si based alloy auto body sheets and buckling eigenvalues of the YBT model are searched by means of supposing sheet mental isotropy and changing modulus of elasticity, hardening coefficient and yield limit. The results show that: it is different effects to increase modulus of elasticity, hardening coefficient and yield limit for different buckling eigenvalues, but they will increase probability of elastic buckling instability or induced elastic instability. Therefore, only based on single factor, it couldn't be too high to ensure fitability and fixability of Al-Mg-Si based alloy auto body sheets for modulus of elasticity, hardening coefficient and yield limit.Buckling characteristic of Al-Mg-Si based alloy auto body sheets is comparing with those of low carbon steel sheet and high intensity steel sheet using the YBT modle. The results show that: ﹊f the sheet mental has an excessively high yield limit, it will induce the lengthwise tensile strain focus in the ends of Y-axis. The focus reduces the tensile strain of the main-axis center, as yet the center easily becomes the dangerous point of buckling instability. It is why high intensity steel sheet appears easier buckling than low carbon steel sheet and Al-Mg-Si based alloy auto body sheets under the un-uniform tensile stress; ﹖he main causes of buckling instability are relative of the un...
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