저자명 박남수 
년도 2017 

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.  

번호 제목 저자명 날짜 조회 수
7 A New Ductile Fracture Criterion for the Formability Prediction of Steel Sheets and Its Application to Finite Element Analysis (강판의 성형성 예측을 위한 새로운 연성 파괴 조건 및 유한 요소 해석에의 응용) [1] Yanshan Lou  2012.12.10 20811
6 Strain-Rate Dependent Anisotropic Yield Criteria for Auto-body Steel Sheets (자동차용 강판의 변형률속도 의존 이방성 항복함수에 관한 연구) 허지향  2012.12.11 17784
5 Rate Dependent Hardening Model for Pure Titanium Considering the Effect of Deformation Twinning (쌍정의 영향을 고려한 티타늄의 변형률속도 의존 경화 모델) 안광현  2012.12.12 16932
4 Dynamic Failure Criteria of a Laser-welded Region under Combined Loading Conditions for Auto-body Crash Analyses (차체용 부재의 충돌해석을 위한 복합하중조건에서 레이저 용접부의 동적 파단조건 연구) 하지웅  2013.04.14 7227
3 Rate-Dependent Hardening Model for Polymer-Bonded Explosives Considering a Wide Range of Strain Rates (광범위한 변형률속도를 고려한 복합화약의 변형률속도 의존 경화모델) 박충희  2013.08.30 3532
2 A Study on the Nonlinear Behaviors of Polymer-Bonded Explosives Considering Stress Softening Effect(응력연화현상을 고려한 복합화약의 비선형 거동에 관한 연구) 염기선  2013.09.08 4717
» Anisotropic Fracture Limit for the Failure Prediction of Advanced High-Strength Steel Sheets (고강도 강판의 파괴 예측을 위한 이방성 파괴 한계에 관한 연구) 박남수  2017.06.07 61