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.

Fracture-based forming limit criteria for anisotropic materials in sheet metal forming

Park, Namsu,Huh, Hoon,Lim, Sung Jun,Lou, Yanshan,Kang, Yeon Sik,Seo, Min Hong Elsevier 2017 International journal of plasticity Vol.96 No.-

<P><B>Abstract</B></P> <P>This paper is concerned with modeling of fracture-based forming limit criteria for anisotropic materials in sheet metal forming to predict the sudden fracture in complicated forming processes. The Lou–Huh ductile fracture criterion is modified using the Hill's 48 anisotropic yield function instead of the von Mises isotropic yield function to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. For the derivation of an anisotropic ductile fracture criterion, the principal stresses (<I>σ</I> <SUB>1</SUB>,<I>σ</I> <SUB>2</SUB>,<I>σ</I> <SUB>3</SUB>) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress ( <SUB> η H </SUB> , <SUB> L P </SUB> , <SUB> σ ¯ H </SUB> ) based on the Hill's 48 yield function. Three different kinds of fracture-based forming limit criteria are suggested and investigated with an assumption that the stress state is under the plane stress condition with proportional loading. To determine the parameters of the model proposed, the 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 and the measurement results are investigated to identify the anisotropy effect on the equivalent plastic strain at the onset of fracture. This paper also discusses about a scaling method for a strain-based fracture forming limit criterion in order to capture the onset of fracture using a single forming limit curve for an anisotropic material. From the comparison of various forming limit criteria suggested, it is concluded that a polar effective plastic strain-based (PEPS) fracture forming limit diagram (FFLD) is suitable for prediction of the sudden fracture in AHSS sheets in complicated sheet metal forming processes on the basis of its path independence and simplicity of measuring strains in real forming processes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This paper is concerned with fracture-based forming limit criteria of DP980 sheets. </LI> <LI> The anisotropic ductile fracture criterion is suggested on the basis of the Lou–Huh ductile fracture criterion. </LI> <LI> The principal stresses (<I>σ</I> <SUB>1</SUB>,<I>σ</I> <SUB>2</SUB>,<I>σ</I> <SUB>3</SUB>) are expressed in terms of ( <SUB> η H </SUB> , <SUB> L P </SUB> , <SUB> σ ¯ H </SUB> ) . </LI> <LI> Three different kinds of fracture-based forming limit criteria are suggested for anisotropic materials. </LI> <LI> A fracture-based polar effective plastic strain FFLD is suitable for prediction of the sudden fracture of AHSSs. </LI> </UL> </P>

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 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.

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.-

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.

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.

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.

Anisotropic yield function based on stress invariants for BCC and FCC metals and its extension to ductile fracture criterion

Lou, Yanshan,Yoon, Jeong Whan Elsevier 2018 International journal of plasticity Vol.101 No.-

<P><B>Abstract</B></P> <P>It is essential to accurately model the anisotropic plastic deformation and ductile fracture of metals in order to guarantee the reliable numerical analysis and optimization of metal forming. For this purpose, the Drucker function is revisited. Effect of the third stress invariant in the Drucker function is analyzed and calibrated for metals with body-centered cubic (BCC) and face-centered cubic (FCC) crystal systems based on the yielding and plastic flow of both crystal plasticity and biaxial tensile experiments. The calibrated Drucker function is extended into an anisotropic form using a fourth order linear transformation tensor. The anisotropic flexibility is enhanced by two approaches: non-associate flow rule (non-AFR) and the sum of n-components of the anisotropic Drucker function. The proposed anisotropic Drucker function is applied to model the anisotropic behavior of both BCC and FCC metals. The predicted anisotropic behavior is compared with experimental results. The comparison demonstrates that the anisotropy is accurately modeled for both BCC and FCC metals by the anisotropic Drucker function. The anisotropic Drucker function is also implemented into numerical analysis of tension of specimens with a central hole to investigate its computation efficiency under spatial loading compared with the Yld2000-18p function. It is found that the proposed anisotropic Drucker function can reduce about 60% of computation time in case that the Yld2000-18p function is substituted by the anisotropic Drucker function in numerical computation due to its simplicity compared to the Yld2000-18p function. A ductile fracture criterion is also developed by coupling the Drucker function with the first stress invariant. The modified Drucker function is reformulated to investigate the effect of the stress triaxiality and the normalized third invariant on ductile fracture. Comparison of the modified Drucker fracture locus with the experimental results of AA2024-T351 demonstrates that the modified Drucker criterion accurately illustrates the fracture stress of the alloy in wide stress states with the stress triaxiality ranging from −0.5 in plane strain compression to 0.6 in tension of notched specimens. The modified Drucker fracture criterion is expected to be less sensitive to the change of strain path considering that the criterion describes fracture in the stress space. Accordingly, the anisotropic Drucker yield function and the pressure-coupled Drucker fracture criterion are suggested to model anisotropic plastic deformation and to predict the onset of failure for both BCC and FCC metals due to simple implementation in numerical analysis under spatial loading and computation efficiency with brick elements.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Drucker function is extended into anisotropy using linear transformation tensor. </LI> <LI> Anisotropic flexibility is enhanced by two approaches: non-AFR and sum of n-components of the anisotropic Drucker function. </LI> <LI> Comparison demonstrates that anisotropy is accurately modeled for BCC and FCC metals by the anisotropic Drucker function. </LI> <LI> The enhanced Drucker function can model anisotropy of BCC and FCC metals easily numerical analysis under spatial loading. </LI> <LI> Drucker function has been extended to ductile fracture criterion with high accuracy from compressive to tension. </LI> </UL> </P>

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.