Yield loci evaluation of some famous yield criteria with experimental data

Yanshan Lou,Hoon Huh(허훈) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

Effect of pyrolysis temperature on the char micro-structure and reactivity of NO reduction

Yanshan Yin,Jun Zhang,Changdong Sheng 한국화학공학회 2009 Korean Journal of Chemical Engineering Vol.26 No.3

A phenol-formaldehyde resin (PFR) and a bituminous coal (SH) were pyrolyzed at various temperatures. The structure and the char-NO reactivity were analyzed in order to examine the effect of pyrolysis temperature on the micro-structure of the resulting char and further on the reactivity towards NO. Micro-structure of the char samples was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. It was indicated that the micro-structure of PFR char and coal char experienced remarkable changes during pyrolysis, which resulted in the decrease of phenolic OH, aromatic hydrogen and more ordered structure. The pyrolysis temperature showed a weak impact on the reactivity of PFR char but comparatively remarkable impact on t

Formability Prediction of Steel Sheets based on a Modified Oh Ductile Fracture Criterion

Yanshan Lou,Hoon Huh(허훈),Younki Ko 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5

This paper modifies the Oh ductile fracture criterion by adding an exponent to the weight function. The modified Oh criterion is applied to predict forming limit diagrams (FLDs) of several steel sheets. Two material constants in the modified Oh criterion are evaluated by the experimental data of the materials. The mechanical properties of the materials are characterized by the Hollomon strain hardening law and the Yld89 yield function with the normal anisotropy. FLDs are predicted by the modified Oh criterion and several other famous ductile fracture criteria. FLDs predicted by ductile fracture criteria are compared with experimental data. The comparison demonstrates that FLDs predicted with the modified Oh criterion match the experimental data with higher accuracy than other ductile fracture criteria especially in the biaxial tension region.

Prediction of Fracture Forming Limit for DP780 Steel Sheet

Yanshan Lou,Sung Jun Lim,허훈 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.4

This paper is concerned with modeling of fracture strains of DP780 using a newly proposed micro-mechanism-motivated ductile fracture criterion (Lou et al., 2012) and its application to predict limit dome heights (LDH) for nine hemispherical punch-stretch tests. Dog-bone specimens are tested to characterize strain hardening behavior. Five arc-shaped specimens and four square-shaped specimens are drawn until fracture to construct a fracture forming limit diagram (FFLD) using circle grid analysis. Fracture strains are approximated from constructed FFLD in uniaxial, plane strain and balanced biaxial tension. The approximated fracture strains are employed to calculate material constants of the proposed criterion as well as six conventional criteria. FFLDs predicted by these criteria are compared with experimental results. The comparison demonstrates that only the proposed criterion describes FFLD perfectly from uniaxial tension to balanced biaxial tension. All criteria are implemented into ABAQUS/Explicit to predict LDHs of punch-stretch tests. Numerical results indicate that LDHs are severely underestimated for the square-shaped specimens by conventional criteria while the proposed criterion predicts LDHs with good agreement for nine tests with strain paths between uniaxial tension and balanced biaxial tension. Thus, the proposed criterion is recommended to access formability from uniaxial tension to balanced biaxial tension.

Parameter study on quasi-statically axial crush of frusta with small semi-apical angles using finite element method

Yanshan Lou,Chunghee Park,Hoon Huh 한국자동차공학회 2008 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-

Effect of the Yield Stress and r-value Distribution on the Earing Profile of Cup Drawing with Yld2000-2d Yield Function

Yanshan Lou,Gihyun Bae,Changsoo Lee,Hoon Huh 한국소성가공학회 2010 기타자료 Vol.2010 No.6

This paper deals with the effect of the yield stress and r-value distribution on the earing in the cup drawing. The anisotropic yield function, Yld2000-2d yield function, is selected to describe the anisotropy of two metal sheets, 719B and AA5182-O. The tool dimension is referred from the Benchmark problem of NUMISHEET’2002. The Downhill Simplex method is applied to identify the anisotropic coefficients in Yld2000-2d yield function. Simulations of the drawing process are performed to investigate the earing profile of two materials. The earing profiles obtained from simulations are compared with the analytical model developed by Hosford and Caddell. Simulations are conducted with respect to the change of the yield stress and r-value distribution, respectively. The correlation between the anisotropy and the earing tendency is investigated based on simulation data. Finally, the earing mechanism is analyzed through the deformation process of the blank during the cup deep drawing. It can be concluded that ears locate at angular positions with lower yield stress and higher r-value while the valleys appear at the angular position with higher yield stress and lower r-value. The effect of the yield stress distribution is more important for the cup height distribution than that of the rvalue distribution.

Forming Limit Calculation based on the M-K Model with Yld96 yield function

Yanshan Lou,Hoon Huh,Seokbong Kim,Changsoo Lee 한국자동차공학회 2009 한국자동차공학회 학술대회 및 전시회 Vol.2009 No.11

This paper deals with the forming limit diagrams (FLDs) of DP590 steels based the Marciniak-Kuczynski model (M-K model) using Barlat’s Yld96 yield function. Hill48 yield function is also used in the M-K model for the purpose of comparison. Uniaxial tension tests are carried out for the parameter characterization in the yield functions. The yield stress of the balance biaxial tension is referred from Lee et. al’s work. The Downhill Simplex method proposed by Nelder and Mead is used for calculating the anisotropic coefficients because of the high nonlinearity of the equations. Johnson-Cook (J-C) model is adopted to consider the dynamic material properties. The initial inhomogeneous factor in the M-K model is adjusted so that the predicted limit strain of the plane strain condition (FLD?) is the same as that from experiments. The yield loci, the directionalities of the Rvalues and the yield stresses based on Hill48 and Yld96 yield criteria are compared with experimental data. Yld96 yield criterion predicts the precise R-values and the yield stresses in rolling, diagonal and transverse direction and the yield stress of balanced biaxial tension condition. Hill48 yield criterion overestimates the FLDs and Yld96 yield function predicts the FLDs better comparing with hill48 yield criterion. However, the predicted FLDs using Yld96 yield criterion still overestimates the formability of the steel. This discrepancy might be caused by the slightly strain path change in the experiment, which is validated by the finite element computation.

Comparative Study of the Ductile Fracture criteria on the Prediction of FLDs for Aluminum Alloys

Yanshan Lou,Hoon Huh,Younki Ko,Jiwoong Ha 한국자동차공학회 2010 한국자동차공학회 부문종합 학술대회 Vol.2010 No.5

This paper evaluates the performance of ductile fracture criteria on the prediction of forming limit diagrams (FLDs) of aluminum alloys. The ductile fracture criteria in this study include Cockcroft-Latham criterion (1968), Brozzo criterion (1972), Oh criterion (1979), Oyane-Sato criterion (1980), Clift criterion (1990) and Ko-Huh criterion (2007). The material properties and the experimental FLDs of 30 kinds of aluminum alloys are collected from researchers. Hosford79 yield function with normal anisotropy is applied. The Hollomon equation is utilized to describe the stress-strain relation of aluminum alloys. The experimental FLDs on the left hand side (LHS) are fitted linearly and the fitted straight line is applied to determine the limited strains of uniaxial tension and plane strain conditions. The material constants in the ductile fracture criteria are evaluated with the limited strains of uniaxial tension and plane strain tension conditions. The FLDs are predicted with these ductile fracture criteria with linear strain increment. The predicted FLDs are compared with experimental data. The results show that the FLDs predicted from Oyane-Sato and Ko-Huh ductile fracture criteria match the experimental data well for the LHS FLDs while other ductile fracture criteria overestimate the limited strain of LHS FLDs. For the right hand side FLDs, all these ductile fracture criteria do not perform well compared with experimental FLDs.

Application of Ductile Fracture Criteria for the FLD Prediction of Steel Sheets

Yanshan Lou,Soekbong Kim,Younki Ko,Hoon Huh 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.10

This paper applies ductile fracture criteria for the formability prediction of various types of steel sheets to evaluate the accuracy of ductile fracture criteria on the prediction of forming limit diagrams (FLDs). The ductile fracture criteria studied in this paper include the Cockcroft criterion (1968), the Brozzo criterion (1972), the Oh criterion (1979), the Oyane-Sato criterion (1980), the Clift criterion (1990) and the Ko-Huh criterion (2007). The material properties and the experimental FLDs of 24 kinds of steel sheets are collected from researchers. Hosford79 yield function with normal anisotropy is utilized to model the yield surfaces of these steel sheets. The Hollomon equation is employed to describe the strain hardening behaviors. The material constants in the ductile fracture criteria are evaluated with the limited strains in the uniaxial tension state for Cockcroft criterion, Brozzo criterion, Oh criterion, Clift criterion and Ko-Huh criterion while the limit strains in both the uniaxial tension and plane strain tension conditions are used for the material constant evaluation in Oyane-Sato criterion. The FLDs are predicted with these ductile fracture criteria with linear strain paths. The predicted FLDs are compared with experimental data. The results show that the FLDs predicted from Oyane-Sato and Ko-Huh ductile fracture criteria match the experimental data well for the LHS FLDs while other ductile fracture criteria overestimate the limited strain in plane strain tension conditions. For the right hand side FLDs, all these ductile fracture criteria do not perform well compared with experimental FLDs.

Application of Ductile Fracture Criteria to Fracture Prediction of Punch-Stretch Tests

Yanshan Lou,Hoon Huh(허훈),Gihyun Bae(배기현),Younki Ko(고윤기),Seokbong Kim(김석봉) 한국자동차공학회 2011 한국자동차공학회 부문종합 학술대회 Vol.2011 No.5

This paper applies various ductile fracture criteria into finite element analysis (FEA) to predict the fracture initiation of punch-stretch tests. Punch-stretch tests are carried out to construct forming limit diagrams (FLDs) of SPCC (Steel plated cold-rolled commercial quality). Limit dome heights (LDHs) are measured from the fractured specimens in punch-stretch tests. The limit strains in uniaxial tension, plane strain and equibiaxial tension conditions are measured in FLDs and utilized to evaluate the material constants in ductile fracture criteria. To implement ductile fracture criteria into FEA, user subroutines are developed for the commercial software ABAQUS/Explicit with the Hill48 yield function. Simulations are carried out to predict fracture initiation of punch-stretch tests by ductile fracture criteria. Fracture locations predicted by ductile fracture criteria are compared with fractured specimens in experiments. LDHs are measured from simulations and compared with experimental results. The comparison demonstrates that LDHs predicted by various ductile fracture criteria match the experimental results with higher accuracy for arc-shape specimens while the Lou-Huh criterion predicts more accurate LDHs than other ductile fracture criteria for square-shaped specimens.