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Anisotropic Fracture Limit for the Failure Prediction of Advanced High-Strength Steel Sheets (고강도 강판의 파괴 예측을 위한 이방성 파괴 한계에 관한 연구)
This thesis is concerned with developing anisotropic fracture forming limit criteria for the prediction of the material formability in sheet metal forming to predict the sudden fracture in complicated forming processes for advanced high-strength steel (AHSS) sheets. A new anisotropic ductile fracture criterion is proposed on the basis of the Lou--Huh ductile fracture criterion to evaluate the equivalent plastic strain at the onset of fracture over a wide range of loading states. In developing the anisotropic ductile fracture criterion, the Hill’s 48 criterion is employed to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. For the derivation of the anisotropic ductile fracture criterion, the principal stresses are expressed in terms of the stress triaxiality, the Lode parameter, and the effective stress based on the Hill’s 48 criterion to include the effect of directionality of typical loading states on the material orientation. For determination of the parameters of the fracture criterion proposed, the two-dimensional Digital Image Correlation (DIC) method is utilized to measure the strain histories on the surface of three different types of specimens: the pure shear; the uniaxial tension; and the plane strain tension. The measurement results are investigated to quantitatively identify the anisotropy effect on the equivalent plastic strain at the onset of fracture. From the proposed fracture criterion, three different kinds of fracture forming limit criteria are introduced: a strain-based Fracture Forming Limit Diagram (FFLD); a stress-based FFLD; and a Polar Effective Plastic Strain (PEPS) FFLD. These fracture forming limit criteria are obtained with an assumption of the proportional loading under the plane stress condition. A scaling method for a strain-based fracture forming limit criterion is also discussed in order to capture the onset of fracture using a single forming limit curve for an anisotropic material. Experimental validations are performed in the viewpoint of a structural and a specimen level through the square cup drawing and the tensile test at additional loading directions. In comparison of the experimental results with the ones predicted from the proposed fracture criterion, it is clearly shown that the fracture criterion proposed has a considerable potential in describing the equivalent plastic strain at the onset of fracture over a wide range of stress states in consideration of the material anisotropy as well as non-directionality of the equi-biaxial fracture strain. Comparison of the strain-based FFLD and the PEPS FFLD reveals that the PEPS FFLD is able to predict the fracture initiation regardless of the strain path change whereas the performance of the strain-based FFLD can be valid only for the case that the material point is under the proportional loading condition. Consequently, the PEPS FFLD can play a key role in predicting the fracture initiation during complicated sheet metal forming processes of AHSS sheets in addition to its path independence and simplicity of measuring strains in real forming processes.