Research On Forming Limits Of Sheet Metal Basing On Continuum Damage Mechanics

Sheet metal forming processes are among the most important metal-working operations. In recent years, more and more advanced materials have been used in metal industry, so it is indispensable to study their formability thoroughly. In this dissertation, theoretical method was applied to analyzed sheet metal forming limit and its damage process, as well as testing investigation. The damage-coupled anisotropic plasticity models were presented, to estimate the damage and limit strains of sheet metal during stamping process. A comparision of theoretical results with testing measures was carried out.First of all, classic theory of plasticity was summarized, along with several anisotrpical yielding functions. A general anisotropic constitutive model was founded. Moreover, comparision of anisotropic plasticity with isotropic flow theory was carried out. Then, the foundational concepts and theory of Continuum Damage Mechanics were introduced, including the formulas deduction and comprehensiveanalysis on two kinds of theories--Lemaitre damage model and a general damagetheory in the framework of continuum thermodynamics.As to the experiment research in this dissertation, the principle and methods of measure of FLD (or FLDs, short for Forming Limit Diagrams) were introduced,especially for the key part -- strain measurement, including analysis method and silk screening technique. Then by regarding the change of sheet metal's density as the damage variable, testing study of damage was carried out, on one kind of advanced sheet metal -IF steel which has been broadly used in forming industry production recently. According to the results obtained in the experiments in two cases of forming path - uniaxial tension and equivalent bi-axial tension, the forming mechanism of IF sheet metal has been discussed.Furthermore, basing on the thermodynamics fundamentals, anisotropic plasticity flow theory and damage evolution functions were deduced, together with a simplified damage-coupled isotropic plastic constitutive model. For the experimental materials in this study - some IF and 03A1 sheet metal, Lemaitre and Chaboche isotropic damage model was applied to predict their forming limits. The result has shown a great difference of the FLD predicted from those obtained by experiment. 'For the case of isotropic damage assumed in this dissertation, and by using the damage dissipation potential function presented by Lemaitre, a damage coupled anisotropic plasticity model was founded. Then for giving proportional loading condition, damage evolutional functions were obtained, as well as the curves of the damage variable changing with acumulation plastic strains by numerical analysis. Comparision of the curves predicted with those tested shows quite accordance in tendency, especially for the case of uniaxial tension.Assuming the propotional loading condition, with the damage coupled anisotropic plasticity model , the limit strains expressions of sheet metal were obtained. Then the limit strain curves of two types of sheet metal were calculated. Comparision of the FLD predicted with forming limit experiments measured has been carried out, which shows a good accordence in the left half region of FLD.At last, through numerical calculation the influence of plane strain ratio, material anisotropic parameters on stress function was discussed, as well as that of material parameters on stress triaxial factor and forming limit curves.