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박남수(N. Park),허훈(H. Huh) 한국소성가공학회 2015 한국소성가공학회 학술대회 논문집 Vol.2015 No.10
This paper is concerned with modeling of a ductile fracture model considering the anisotropy of sheet metal to predict the sudden fracture of advanced high strength steel (AHSS) sheets in complicated forming processes. The Lou-Huh ductile fracture criterion is modified using the Hill`s 48 anisotropic plastic potential instead of the von Mises isotropic plastic potential to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. Two dimensional digital image correlation (2D-DIC) method is utilized to measure the strain histories on the surface of three different types of specimens during deformation. For the derivation of an anisotropic ductile fracture model, principal stresses (σ₁, σ₂, σ₃) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress (ηH, L, σ ̅H) based on the Hill`s 48 anisotropic plastic potential. It is quantitatively verified that the anisotropic ductile fracture suggested can predict the onset of the fracture over a wide range of loading paths based on the confirmation of damage accumulation, such that it can be utilized for complicated sheet metal forming processes to evaluate the forming limit of advanced high strength steel (AHSS) sheets.
다양한 하중경로에서의 DP980 강판의 파단변형률 예측에 관한 연구
박남수(N. Park),허훈(H. Huh) 한국소성가공학회 2014 한국소성가공학회 학술대회 논문집 Vol.2014 No.10
This paper is concerned with prediction of fracture strains for DP980 steel sheets at a wide range of loading paths. The usage of DP980 steel is significantly increasing in automotive industries for enhanced safety and high fuel efficiency. The material behavior of advanced high-strength steels (AHSSs) sheets show unpredictable and sudden fracture in sheet metal forming processes. A modified Lou-Huh ductile fracture criterion is utilized to predict the formability of AHSSs because the conventional forming limit diagram (FLD) constructed based on necking is unable to evaluate the formability of AHSSs. Fracture loci were extracted from three dimensional fracture envelopes by assuming the plane stress condition to evaluate equivalent plastic strains at the onset of fracture at a wide range of loading paths. Three different types of specimens such as pure shear, dog-bone and plane strain grooved specimens were utilized for tensile tests to calibrate the fracture model of DP980 steel sheets. Fracture strains of each loading path were evaluated to show that there is little deviation between fracture strains predicted from the fracture model and experimentally acquired ones. Through the comparison, it is clearly shown that the three dimensional fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets in complicated loading conditions from compressive loading, and shear to equi-biaxial tension.
FEGM을 이용한 자동차용 플라스틱의 진응력-변형률 선도 도출
박충희(C. H. Park),김진성(J. S. Kim),허훈(H. Huh),안창남(C. N. Ahn),최석진(S. J. Choi) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.10
The plastics are widely utilized in the inside of vehicles. The dynamic tensile characteristics of auto-body plastics are important in a prediction of deformation mode of the plastic component which undergoes the high speed deformation during car crash. This paper is concerned with the dynamic tensile characteristics of the auto-body plastics at intermediate strain rates. Quasi-static tensile tests were carried out at the strain rate ranged from 0.001/sec to 0.01/sec using the static tensile machine(Instron 5583). Dynamic tensile tests were carried out at the strain rate ranged from 0.1/sec to 100/sec using the high speed material testing machine developed. Conventional extensometry method is no longer available for plastics, since the deformation of plastics is accompanied with localized deformation. In this paper, quasi-static and dynamic tensile tests were performed using ASTM IV standard specimens with grids and images from a high speed camera were analyzed for strain measurement. True stress-strain relations and the actual strain rates at each deformation step were obtained by processing load data and deformation images, assuming the plastics to deform uniformly in each grid.