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
27 Study on Dynamic Tensile Tests of Auto-body Steel Sheet at the Intermediate Strain Rate for Material Constitutive Equations (차체강판의 중변형률 속도에서의 동적 인장시험 및 물성 구성방정식에 관한 연구) 임지호  2005.11.29 26402
26 Analysis of Elasto-Plastic Stress Waves by a Time Discontinuous Variational Integrator of Hamiltonian with a Second-Order Integration Scheme of the Constitutive Model (해밀토니안의 시간 불연속 변분적분기와 구성방정식의 2차 정확도 적분법을 이용한 탄소 조상순  2008.12.15 22466
25 Forming Limit Diagram of Auto-body Steel Sheets at High Strain Rates for Sheet Metal Forming and Crashworthiness (박판성형 및 충돌성능 향상을 위한 고변형률속도에서의 차체강판 성형한계도) 김석봉  2010.07.13 22389
24 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 22219
23 A Study on the Tension/Compression Hardening Behavior of Auto-body Steel Sheets Considering the Pre-strain and the Strain Rate (초기 변형률 및 변형률 속도를 고려한 차체 강판의 인장/압축 경화 거동에 관한 연구) [1] 배기현  2011.01.11 21430
22 A Study on Material Properties of OFHC Copper Film at High Strain Rates using High-Speed Micro Material Testing Machine (고속마이크로재료시험기를 이용한 무산소동 박판의 고변형률속도 재료물성치 연구) 김진성  2010.07.13 21130
21 Finite Element Inverse Approach and Initial Guess Generation for Sheet Metal Forming Analysis of Complicated Auto-body Members (복잡한 차체부재의 박판성형공정을 위한 유한요소 역해석 및 초기추측치 계산) 김승호  2005.11.29 19876
20 A Study on the Dynamic Failure Model of a Spot Weld under Combined Loading Conditions for Auto-body Crash Analyses (차체용 부재의 충돌해석을 위한 복합하중조건에서 점용접부의 동적 파단모델 연구) [1] 송정한  2008.07.24 19553
19 Optimum Process Design in Sheet Metal Forming Processes using Finite Element Sensitivity Analysis (유한요소 민감도해석을 이용한 박판금속성형에서의 공정변수 최적설계) [1] 김세호  2005.11.29 19440
18 Strain-Rate Dependent Anisotropic Yield Criteria for Auto-body Steel Sheets (자동차용 강판의 변형률속도 의존 이방성 항복함수에 관한 연구) 허지향  2012.12.11 18994
17 Evaluation of a cast-joining process of dual metal crankshafts for heavy-duty engines with ductile cast iron and high strength forged steel(구상흑연주철과 고강도 단조강의 주조접합 이종금속을 이용한 중대형 엔진 크랭크샤프트의 평가) 한 신  2010.07.13 18647
16 Dynamic Formulation of Finite Element Limit Analysis for Impact Simulation of Structural Members (구조부재의 충돌해석을 위한 유한요소 극한해석의 동적 수식화) 김기풍  2005.11.29 18521
15 Measurement Uncertainty Evaluation for High Strain Rate Tensile Properties of Auto-body Steel Sheet (자동차용 강판 고속인장물성 데이터의 측정불확도 산출) 정세환  2012.12.10 18518
14 Development of a Nonlinear Degenerated Shell Element with the Drilling Degree of Freedom by the Cubic Polynomial Interpolation and the Assumed Strain Method (드릴링 자유도의 삼차 근사법과 대체변형률법을 이용한 비선형 감절점 쉘 요소의 개발) 이형욱  2005.11.29 18473
13 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 18143
12 Finite Element Simulation of 3-dimensional Superplastic blow forming with diffusion bonding (유한요소법을 이용한 초소성 재료의 삼차원 확산 접합 및 압력 성형 해석) 이기석  2005.11.29 17791
11 Microscopic investigation of the strain rate hardening for auto-body steel sheet(차체강판 변형률속도 경화의 미시적 관찰) 윤종헌  2010.07.13 17739
10 Crash Analysis of Auto-body Structures with an Explicit Finite Element Method ( 외연적 유한요소법을 이용한 차체 구조물의 충돌해석 ) 강우종  2005.11.29 17469
9 Shell Element Formulation for Limit Analysis of Thin-Walled Structures ( 박판부재의 붕괴거동해석을 위한 극한해석의 쉘요소 수식화 ) 김현섭  2005.11.29 15461
8 A Study on a Continuum Damage Yield Function to Predict Ductile Fracture of Materials (재료의 연성파단을 예측하기 위한 연속체 손상 항복 함수에 관한 연구) 고윤기  2012.12.10 15412