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RISS 인기검색어
- 검색키워드
- 저자 : F. Barlat (검색결과 49 건)
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Lee, J.Y.,Lee, J.W.,Lee, M.G.,Barlat, F. Pergamon Press ; Elsevier Science Ltd 2012 International journal of solids and structures Vol.49 No.25
In this work, a constitutive model based on anisotropic hardening was used in the finite element (FE) simulations of springback and its performance was compared with that of conventional hardening laws. The homogeneous yield function-based anisotropic hardening (HAH) model (Barlat et al., 2011), considered in this work, describes a partial distortion of the yield surface under plastic loading. Although it does not use the concept of kinematic hardening, the HAH model was able to predict the complex material behavior upon load reversals such as the Bauschinger effect, transient hardening and permanent softening. For the application to springback, FE simulations were conducted for U-draw/bending of base (as-received) and pre-strained DP780 steel sheets, which was recently proposed as one of the Numisheet 2011 benchmark problems. The predictions with the HAH model, combined with a non-quadratic anisotropic yield function and a plastic strain-dependent unloading modulus, were in good agreement with experimental results for both as-received and pre-strained DP780 sheets.
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Characterization of the post-necking strain hardening behavior using the virtual fields method
Kim, J.H.,Serpantie, A.,Barlat, F.,Pierron, F.,Lee, M.G. Pergamon Press ; Elsevier Science Ltd 2013 International journal of solids and structures Vol.50 No.24
The present study aims at characterizing the post-necking strain hardening behavior of three sheet metals having different hardening behavior. Standard tensile tests were performed on sheet metal specimens up to fracture and heterogeneous logarithmic strain fields were obtained from a digital image correlation technique. Then, an appropriate elasto-plastic constitutive model was chosen. Von Mises yield criterion under plane stress and isotropic hardening law were considered to retrieve the relationship between stress and strain. The virtual fields method (VFM) was adopted as an inverse method to determine the constitutive parameters by calculating the stress fields from the heterogeneous strain fields. The results show that the choice of a hardening law which can describe the hardening behavior accurately is important to derive the true stress-strain curve. Finally, post-necking hardening behavior was successfully characterized up to the initial stage of localized necking using the VFM with Swift and modified Voce laws.
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Modeling of forming limit for multilayer sheets based on strain-rate potentials
Kim, D.,Kim, H.,Kim, J.H.,Lee, M.G.,Kim, K.J.,Barlat, F.,Lee, Y.,Chung, K. Pergamon Press ; Elsevier Science Ltd 2015 International journal of plasticity Vol.75 No.-
In order to evaluate the forming limits of planar anisotropic multilayer sheet materials, the Marciniak-Kuczynski (M-K) model was formulated based on the strain-rate potential. A common practice to predict the forming limits of sheet materials has been based on yield stress potentials defined in the stress field. As an alternative, a method based on plastic stain-rate potentials, which are especially convenient to apply for rigid-viscoplasticity, was considered in this work. The formulation based on the strain-rate potential facilitates the modeling of forming limits for multilayer sheet materials because the number of unknown variables is significantly reduced by assuming the iso-strain condition for each layer without delamination. As for the strain-rate potential, Srp2003-2d, which is the pseudo-conjugate of the yield stress potential Yld2000-2d, was applied along with Hill's 1948 strain-rate potential for comparison. In the approach proposed, rigid-viscoplasticity was formulated according to the incremental deformation theory based on the minimum plastic work path. The rotation of anisotropic symmetry axes in the groove region was also properly accounted for in the formulation. For verification purposes, the predicted forming limit criteria such as the strain-based forming limit diagram (FLD), the stress-based forming limit diagram (FLSD) and the effective strain-based forming limit diagram (x-EPS) were experimentally validated for a monolithic aluminum alloy (AA5182-O) sheet and a three-layer AA5182-O/polypropylene/AA5182-O (AA/PP/AA) sandwich sheet, which confirmed good agreement. In addition, the path-sensitivity of the strain-based FLD and the path-insensitivity of the FLSD and x-EPS were numerically proven based on the M-K model for the three-layer sandwich sheet. Finally, the M-K model was compared with a maximum force model most recently developed.
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Bok, H.H.,Choi, J.W.,Suh, D.W.,Lee, M.G.,Barlat, F. Elsevier 2015 International journal of plasticity Vol.73 No.-
<P><B>Abstract</B></P> <P>Elastically driven shape change, or springback, in a press-hardened U-channel part made from a tailor-welded blank (TWB) was simulated using a fully coupled thermo-mechanical–metallurgical finite element (FE) method. The TWB consists of boron steel and high-strength low-alloy steel, which have significantly different hardenabilities. A combined implicit–explicit three-step simulation consisting of air cooling, forming and die quenching, and springback was used for computational efficiency. All the required material models such as the modified phase-transformation kinetics and phase-transformation-related stress-update scheme were implemented in the FE software ABAQUS with the user-defined subroutines UMAT, VUMAT, and HETVAL. The developed FE procedure, including the material models, satisfactorily predicted the experimentally measured shape changes of the TWB part. Here we present an in-depth analysis of the residual stress development during forming and die quenching using different material modeling schemes. It should be noted that the stress evolution of the two materials with high and low hardenabilities were significantly different depending on the phase transformation kinetics during forming and quenching. Moreover, in order to enhance the prediction capability of the press-hardening simulations, it was essential to include the phase-transformation-related strains in the material model.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Continuum based phase transformation model was implemented in FE analysis for press hardening application. </LI> <LI> Enhanced phase transformation kinetics by diffusion and displacive mechanisms were proposed. </LI> <LI> The FE model satisfactorily predicted experimentally measured shape changes, hardness and phase volume fractions. </LI> <LI> A quantitative analysis on the development stress in the press-hardening process was provided. </LI> </UL> </P>
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Properties controlling the bend-assisted fracture of AHSS
Lee, J.,Kim, J.H.,Lee, M.G.,Barlat, F.,Zhou, C.,Chen, Z.,Wagoner, R.H. Pergamon Press ; Elsevier Science Ltd 2015 International journal of plasticity Vol.75 No.-
Bend-assisted fracture, also commonly called shear fracture, is the splitting of metal sheets during forming in tight-bending regions. It has been shown to be predominantly a result of plastic localization for most advanced high strength steels (AHSS). Such fractures are poorly predicted by typical industrial methods involving finite element modeling (FEM) and forming limit diagrams (FLDs). In order to understand the source of the problem, the sensitivity of simulated shear-fracture formability to material and process parameters was determined using FEM in conjunction with a realistic range of constitutive models, element sizes, and friction coefficients. Two types of shear fracture process were simulated. (1) Draw-bend fracture (DBF) tests are laboratory analogs of industrial forming conditions producing shear fracture; they offer the opportunity of experimental validation but introduce complexity because of varying strain state and unavoidable transitions between shear fracture and tensile fracture. (2) Plane-strain (PS) draw-bend fracture simulations correspond more closely to industrial forming conditions; they simplify the modeling (fixed strain state, no transitions) but no corresponding full-scale laboratory experiments currently exist. The DBF test was found to be sensitive to every material and process parameter tested, with the largest factors being the form of 1-D hardening law and the yield function. Varying these quantities in ranges representing what practical measurements would produce showed variations in predicted formability of up to 80%. The PS simulations, which represent industrial practice more closely, showed large variations in predicted formability only for two variables: 1-D hardening law and friction coefficient. All other parameters were insignificant, except for thermo-mechanical effects, which were important for high-rate tests only. These results show why it is difficult or impossible to predict shear fracture using standard industrial techniques designed for traditional steels. They suggest ways to modify such techniques to accommodate advanced high strength steels. The results also give guidance to alloy designers in terms of which constitutive parameters are most important in inhibiting shear fracture, and which are relatively insignificant.
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Kim, H.,Lee, J.,Barlat, F.,Kim, D.,Lee, M.G. Elsevier Science 2015 Acta materialia Vol.97 No.-
<P>The effects of the stress state and temperature on the martensitic phase transformation behavior in a TRIP-assisted steel (TRIP780) were investigated using multi-axial experimental techniques. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equi-biaxial tension, plane strain tension and simple shear. A range of temperatures from room to 100 degrees C for each stress state condition except the simple shear test were investigated. In particular, for the equi-biaxial tension data in warm conditions, a specially designed hydraulic bulge experiment was adopted. In situ magnetic measurements were performed to monitor the evolution of the martensitic content throughout each experiment. A stress state and temperature dependent transformation kinetics law was proposed, which incorporates a non-linear function of the stress triaxiality, Lode angle parameter and temperature. This new model captures the measured martensitic phase transformation kinetics of TRIP780 steel over a wide range of stress states and temperature reasonably well. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
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A Novel Multi-objective Genetic Algorithms-Based Calculation of Hill’s Coefficients
Hariharan, K.,Chakraborti, N.,Barlat, F. d.,Lee, M. G. Springer Science + Business Media 2014 Metallurgical and Materials Transactions A - Physi Vol.45 No.6
<P>The anisotropic coefficients of Hill's yield criterion are determined through a novel genetic algorithms-based multi-objective optimization approach. The classical method of determining anisotropic coefficients is sensitive to the effective plastic strain. In the present procedure, that limitation is overcome using a genetically evolved meta-model of the entire stress strain curve, obtained from uniaxial tension tests conducted in the rolling direction and transverse directions, and biaxial tension. Then, an effective strain that causes the least error in terms of two theoretically derived objective functions is chosen. The anisotropic constants evolved through genetic algorithms correlate very well with the classical results. This approach is expected to be successful for more complex constitutive equations as well.</P>
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